Hawkins Electrical Guide v. 04 (of 10)
by N. Hawkins · English · Gutenberg #49512 · 505,198 chars · public domain (US)
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Title: Hawkins Electrical Guide v. 04 (of 10)
Author: N. Hawkins
Release date: July 23, 2015 [eBook #49512]
Most recently updated: October 24, 2024
Language: English
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*** START OF THE PROJECT GUTENBERG EBOOK HAWKINS ELECTRICAL GUIDE V. 04 (OF 10) ***
[Illustration: THE THOUGHT IS IN THE QUESTION THE INFORMATION IS IN THE
ANSWER
HAWKINS
ELECTRICAL GUIDE
NUMBER
FOUR
QUESTIONS
ANSWERS
&
ILLUSTRATIONS
A PROGRESSIVE COURSE OF STUDY
FOR ENGINEERS, ELECTRICIANS, STUDENTS
AND THOSE DESIRING TO ACQUIRE A
WORKING KNOWLEDGE OF
ELECTRICITY AND ITS APPLICATIONS
A PRACTICAL TREATISE
by
HAWKINS AND STAFF
THEO. AUDEL & CO. 72 FIFTH AVE. NEW YORK]
COPYRIGHTED, 1914,
BY
THEO. AUDEL & CO.,
NEW YORK.
Printed in the United States.
TABLE OF CONTENTS GUIDE NO. 4
DISTRIBUTION SYSTEMS 697 to 720
=Classification=—series system—danger in series arc light
system—constant current system—parallel system—arrangement
of feeder and mains in parallel system—series-parallel
system—center of distribution—=Edison three wire
system=—evolution of the three wire system—balanced
three wire system—balancing of three wire system—=copper
economy in three wire systems=—Dobrowolsky three wire
system—modifications of three wire system—three
wire storage battery system—three wire double dynamo
system—three wire bridge system—three wire three brush
dynamo system—Dobrowolsky three wire system—three
wire auxiliary dynamo system—three wire compensator
system—=extension of the three wire principle=—five wire
system—=dynamotor=—connections of balancing set—=balancing
coils=—distribution by dynamo motor sets—=boosters and their
uses=—=auxiliary bus bar.=
WIRES AND WIRE CALCULATION 721 to 764
Preliminary considerations—=various wires=—copper
wire—iron wire—German silver wire—=standard of copper wire
resistance=—relative conductivity of different metals and
alloys—conductors—cable for elevator annunciators—covered
conductors—rubber covered conductors—rubber
insulated telephone and telegraph wires—weather proof
conductors—twisted weather proof wires—precautions in
using weather proof conductors—slow burning wire; where
used—slow burning weather proof wire; where used; how
installed—miscellaneous insulated conductors—=safe
carrying capacity of wire=—pothead wires—=circular
mils=—square mils—mil foot—lamp foot—ampere foot—=center
of distribution—wire gauges—=B. & S. standard wire
gauge—micrometer screw gauge—calculating gauge—=table of
various wire gauges=—table of lamp feet for rubber covered
wires—lamp table for weather proof wires—symmetrical
and unsymmetrical distribution—=wiring table for light
and power circuits=—table of wire equivalents; how to
use—table of cable capacities—=incandescent lamps on
660 watt circuits=—"tree" and "modified tree" system of
wiring—distribution with sub-feeders—wrong and right methods
of loop wiring—table, of amperes per motor; of amperes per
dynamo—calculations for three wire circuit—=three wire
circuit panel board=—size of the neutral wire.
INSIDE WIRING 765 to 798
The term "wiring"—=open or exposed wiring=—selection
of wires—=installation=—disadvantages of open
wiring—=splicing=—pitch of wires—crossing of wires—wiring
across pipes—=practical points relating to exposed
wiring=—methods of carrying wires, through floors; on
walls—protecting exposed wiring on low ceilings—=various
porcelain knobs and cleats=—wires used in mouldings—standard
wooden moulding—=kick box=—usual character of moulding
work—practical points relating to wiring in mouldings—tapping
outlets—=arc light wiring=—arc lamps on low pressure
service—circular fixture block—=concealed knob and tube
wiring=; objections; method of installation—arrangement of
switch and receptacle outlet in knob and tube wiring—switch
boxes—=rigid conduit wiring=; advantage—types of rigid
conduit; requirements—-conduit box—disadvantages of unlined
conduit—=flexible conduit wiring=—Greenfield flexible steel
conduit—"fishing"—insulating point—canopy insulator—fish
plug and method of insertion—=method of installing conduits
in fireproof buildings=—service entrance to rubber conduit
system—condulet outlet to arc lamp—=hickey=—methods of
bending large conduits—=armoured cable wiring=; features;
installation.
OUTSIDE WIRING 799 to 824
=Materials for outside conductors=—tensile strength of
copper wire—=pole lines=—pole constructing tools—wooden
poles—preservation of wooden poles—preservation
processes—methods of setting wooden poles in unsuitable
soil—reinforced concrete poles—cross arms—lineman's
portable platform—poles for light and power wires—spacing
of poles—=erecting the poles=—guy anchors—method of
raising a pole—method of pulling anchor into place—=guys
for poles=—head and foot guying—guying corner poles—guy
stubs and anchor logs—climbers—=wiring the line=—pay out
reels—method of stringing wires—"come alongs"—tension
on wires—=sag table=—lineman's block and fall with
"come alongs"—attaching wire to insulator—=splicing;
American wire joint=; McIntire sleeve and sleeve
joint—=transpositions—insulators—overhead cable
construction=—petticoat insulator—Clark's "antihum"—service
connections and loops—method of making series, and parallel
service connections—=joint pole crossing=—service wires.
UNDERGROUND WIRING 825 to 844
=City conditions=—underground systems—=various
conduits=—vitrified clay pipe conduit—vitrified clay
or earthenware trough conduit—joints in multiple-duct
vitrified clay conduit—concrete duct conduits—=methods of
laying conduit=—method of laying single duct vitrified clay
conduit—method of laying multiple duct clay conduit—wooden
duct conduits—=objection to use of wood;= remedy; adaptation
wooden built-in conduits; method of installation—wrought iron
or steel pipe conduits; method of installation—porcelain
bridgework or carriers—cast iron pipe and trough
conduit; advantages—fibre conduits; joints—=Edison tube
system=—underground cables—metal sheaths on underground
cables—=pot heads=—General Electric manhole junction
box—=pot head connections.=
WIRING OF BUILDINGS 845 to 864
=Preliminary considerations=—electrician's
instructions—=location of receptacles=—ceiling
buttons—hallway wiring—=selection of wiring system=—three
wire convertible system—method of wiring—=location
of panel boards=—current required on each floor of
building—=arrangement of feeders=—installation of
motors—largest size of feeder permissible—=method of cutting
pocket in floor=—outlet baseboard—completed pocket—how
to examine partition interiors—house plan of conduit
wiring—attachment of mains to knobs—=precautions in making
joints=—wiring for heating appliances—wiring with combination
of moulding, flexible tubing or conduit in non-fire proof
building—feeder system for large hotel.
SIGN FLASHERS 865 to 884
=Classification=—Carbon flashers—wiring diagram for Dull's
carbon flashers—brush flashers—knife flashers—flasher
transmission gearing—simple on and off flashers—=flash
system of gas lighting=—high speed flashers—lighting
flashers—wiring diagram for flags—diagram showing method
of wiring for high speed effects on single lines—method of
wiring for a torch—=wiring diagram for high speeds=—Dull's
lightning—=script breakers=—chaser flashers—thermo
flashers—=carriage calls=—monogram for carriage calls—wiring
diagrams for sign illumination—-National carriage call
operating keyboard—clock monogram—Bett's clock mechanism for
operating electric monogram time flasher—=talking signs=—two
way thermal flasher.
LIGHTNING PROTECTION 885 to 892
Lightning rods; =why sharp points are used=; erection—diagram
showing principle of air gap arrestor—=variable gap arc
breaker=—location of lightning arrestor—carbon arresters with
fuses for telephone lines—=ground connection.=
STORAGE BATTERY 893 to 968
Early experiments—=theory=—description of storage
cell—electrolyte—effect of current passing through
electrolyte—types of storage batteries—=Plant
cells=—Willard plates; wood separator—parts of "Autex"
automobile cells—=Faure or pasted type=—comparison
of Plant and Faure plates—=the electrolyte;= kind
generally used; preparation; test; mixing acid and water;
kind of vessel used;—=specific gravity table=—effect
of deep vessel—density of electrolyte—=hydrometer
syringe=—impurities in electrolyte—=tests for impurities=;
chlorine, nitrates, acetic acid, iron, copper, mercury,
platinum—=old electrolyte=—voltage of a secondary
cell—=charging=—connections for charging—charging;
first time; period; regulation of voltage—Edison cell
data—=frequency of charging=—cadmium test—emergency
connections for weak ignition battery—portable testing
instruments—=charge indications=—two methods of charging from
a direct current lighting system—colors of the plates—how
best results are obtained in charging—=charge voltage=—two
ways of charging—diagram of charging connections—how
to keep charging current constant—=tests while charging;
after charging=—charge indications—behavior of electrolyte
during discharge—lead burning outfit—"=boiling=";
causes—hydrogen gas generator for lead burning—=quick
charging=—charging through the night—charging period for new
battery—Willard underhung battery box for automobiles—=high
charging rates=—"National" instructions for taking voltage
readings—=mercury arc rectifier=—capacity—table of capacity
variation for different discharge rates—how to increase
the capacity—=discharging=; too rapidly rating; maximum
rate—=Edison alternating current rectifier=—attention
after discharging—=the battery room=—battery
attendants—points on care and management—=placement of
cells=—how to avoid leakage—precautions when unpacking
cells—=assembling cells=—connections—precaution in joining
terminals—=battery troubles=—short circuiting; indication;
location—overdischarge; buckling—sulphation of plates—data
on National cells; American cells; Autex cells—action in idle
cell—lack of capacity—=how to prevent lead poisoning=—low
specific gravity without short circuits; causes—=treatment
of weak cells=—pole testing paper—disconnecting
cells—=sulphuric acid specific gravity table=—how to take
a battery out of commission—Witham charging board—putting
batteries into commission—cleaning jars—table of voltage
charge as affected by discharge rate—=condensed rules for the
proper care of batteries=.
STORAGE BATTERY SYSTEMS 969 to 996
=Uses of storage batteries=; their importance in power
plants—load curve showing use of storage battery as aid to
the generating machinery—parallel connection of battery and
dynamo—"=floating the battery on the line="—diagram showing
effect of battery in regulating dynamo load—=connections and
circuit control apparatus=—diagram showing action of battery
as a reservoir of reserve power—three wire system with battery
and dynamo—methods of control for storage batteries—diagram
of connections for ignition outfit—variable resistance—end
cell switches—diagram of connection of battery equipment for
residential lighting plant—end cell switch diagram—features
of end cell switch construction—=end cell switch
control=—circuit diagram for charging battery in two parallel
groups and discharging in series—reverse pressure cells;
regulation—Holzer-Cabot dynamotor—=boosters=—application
of series booster system—Bijur's battery system—load
diagram—characteristics of series booster—shunt boosters;
with battery—Entz' carbon pile booster system—=application
of shunt booster=—circuit diagram for non-reversible shunt
booster and battery system—=compound boosters=; their
connections—method of charging battery at one voltage and
supplying lights at a different voltage—connections of one
form of differential booster—=differential boosters=; with
compensating coil; adaptation.
CHAPTER XXXVI
DISTRIBUTION SYSTEMS
The selection of the system of transmission and distribution of
electric energy from the generating plant to lamps, motors, and other
devices, is governed mainly by the cost of the metallic conductors,
which in many electrical installations, is a larger item than the cost
of the generating plant itself. This is especially true in case of long
distance transmission, while in those of the lighting plants, the cost
of wiring is usually more expensive than that of the boilers, engines,
and generators combined.
The principal distribution systems, are classed as:
1. Series;
2. Parallel;
3. Series-parallel;
4. Parallel-series.
=Ques. What is the characteristic feature of each class?=
Ans. In the series systems the current is constant, but the voltage
varies. In the parallel systems, the voltage is constant, but the
current varies.
=Series System of Distribution.=—A series system affords the simplest
arrangement of lamps, motors, or other devices supplied with electric
energy. The connections of such a system are shown in fig. 783. The
current from the terminal of the dynamo passes through the lamps, L,
L, L, L, one after the other and finally returns to the terminal. The
current remains practically constant, but the voltage falls throughout
the circuit in direct proportion to the resistance, and the difference
in pressure between any two points in the circuit is equal to the
current in amperes multiplied by the resistance in ohms included
between them.
For example. Each open arc lamp requires about 50 volts. In
the system shown in fig. 783, the pressure measured across
the brushes of the dynamo is assumed to be 1,000 volts. As
this current flows through the circuit 45 volts will be
actually lost in each lamp, and as the drop on the line wire
is usually about 10 per cent. of the total voltage, there will
be a drop of 5 volts on the conductor between any two lamps.
In the circuit shown, there are twenty lamps, therefore, the
difference in pressure between either terminal of the dynamo
and middle point A of the circuit will be 10 lamps × 50 volts =
500 volts. Likewise, the difference in pressure between any two
points on the circuit will be equal to 50 volts multiplied by
the number of lamps included between them.
[Illustration: FIG. 783.—Series system of distribution. This is
a constant current system, so called because the current remains
practically constant. It is used chiefly for arc lighting.]
=Ques. Describe the danger in a series arc light system?=
Ans. Since the total voltage of the system is equal to the sum of the
volts consumed in all of the lamps, it is high enough to be dangerous
to personal safety.
This is illustrated in fig. 783. If the line be grounded at B
owing to defective insulation, the pressure of the circuit at
that point will be zero, and therefore, a man standing on the
ground could touch that point without receiving a shock, but
if he should touch the line at the point C, he will receive a
slight shock of 150 volts, as there are three lamps between the
point C, and the ground connection B. Therefore, the danger of
touching the circuit increases directly with the resistance
between the point touched and the ground connection, so that
if a man touch the circuit at the point D, he will receive a
dangerous shock of 16 × 50 = 800 volts. In practice, sixty
lamps are often placed on a single arc lighting circuit, so
that its total pressure is about 3,000 volts, thus greatly
increasing the danger of the system.
=Ques. What is a constant current system?=
Ans. The series system is a constant current system, and is so called
because the current remains practically constant, while the voltage
falls throughout the circuit in direct proportion to the resistance.
=Ques. What are the principal applications of the series system?=
Ans. For arc lighting, and telegraphic circuits.
=Ques. What are the advantages of the series system?=
Ans. In the case of telegraphic circuits only one wire is required, and
for lighting and power transmission and distribution, only two wires;
therefore, it is simpler and cheaper than any other system.
=Ques. What is the disadvantage of the series system?=
Ans. The danger due to the high voltage in installations such as arc
lighting circuits.
=Parallel System.=—Parallel or multiple systems are usually more
complicated than series systems, but since the voltage can be
maintained nearly constant by various methods, practically all
incandescent lamps, electric motors, and a large proportion of arc
lamps are supplied by parallel systems.
The general principle of the parallel system is shown in fig.
784. With six lamps on the circuit, each requiring one-half
ampere of current, at 110 volts, the dynamo will have to
supply a current of 3 amperes at a pressure of 112 volts, and
this current will flow through the circuit and distribute
itself as shown on account of the lesser resistance of the
wire relatively to that of the lamps. At the first lamp, the 3
amperes will divide, ½ ampere flowing through the lamp and
the remaining 2½ amperes passing on to the next lamp and so
on through the entire circuit. The reduction of pressure from
112 volts across the brushes to 110 volts at the last lamp is
due to the resistance of the conducting wires.
=Ques. What three effects are due to this drop in pressure?=
Ans. 1, All the lamps or motors in the circuit receive a lower voltage
than that at the dynamo, 2, some lamps or motors may receive a lower
voltage than the others, and 3, the voltage at some lamps or motors may
vary when the others are turned on or off.
[Illustration: FIG. 784.—Parallel system of distribution. This is
a constant voltage system and is used principally for incandescent
lighting and electric motor circuits.]
The first is the least harmful and may be counteracted by
running the dynamo at a little higher voltage; but the second
and third are very objectionable and difficult to overcome.
They are counteracted successfully in practice, however, by
various methods of regulation, the use of _boosters_, and the
operation of dynamos in parallel.
=Ques. What are the principal applications of parallel or constant
pressure systems?=
Ans. They are used on practically all incandescent lamp and electric
motor circuits, and on some arc lamp circuits.
=Ques. Why is it specially applicable to incandescent lamp circuits?=
Ans. Incandescent lamps cannot be made to stand a pressure much over
220 volts, and therefore have to be operated on low voltage systems.
=Ques. What is the principal disadvantage of a parallel system as
compared with a series system?=
Ans. The greater cost of the copper conductors.
[Illustration: FIG. 785.—Arrangement of feeder and mains in parallel
system. By locating the feeder at the electrical center, less copper
is required for the mains. The cut does not show the fuses which in
practice are placed at the junction of feeder and main.]
=Ques. What is the usual arrangement of parallel systems?=
Ans. Conductors known as _a feeder_ run out from the station, and
connected to these are other conductors known as _a main_ to which in
turn the lamps or other devices are connected as shown in fig. 785.
=Ques. In what two ways may feeders be connected?=
Ans. They may be connected at the same end of the mains, known as
_parallel feeding_, or they may be connected at the opposite end of the
main, called _anti-parallel feeding_.
The main may be of uniform cross section throughout, or it may
change in size so as to keep the current density approximately
constant. The above condition gives rise to four possible
combinations:
1. Cylindrical conductors parallel feeding, fig. 786;
2. Tapering conductors, parallel feeding, fig. 787;
3. Cylindrical conductors, anti-parallel feeding, fig. 788;
4. Tapering conductors, anti-parallel feeding, fig. 789.
[Illustration: FIGS. 786 to 789.—Various parallel systems. Fig. 786,
cylindrical conductors parallel feeding; fig. 787, tapering conductors
parallel feeding; fig. 788, cylindrical conductors anti-parallel
feeding; fig. 789, tapering conductors anti-parallel feeding. The term
"tapering" is here used to denote a conductor made up of lengths of
wire, each length smaller than the preceding length, the object of
such arrangement being to avoid a waste of copper by progressively
diminishing the size of wire so that the relation between circular
rails and amperes is kept approximately constant. In an anti-parallel
system, the current is fed to the lamp from opposite ends of the
system.]
=Series-Parallel System.=—This is a combination of the series and
parallel systems, and is arranged as indicated in fig. 790. Several
lamps are arranged in parallel to form a group, and a number of such
sets are connected in series, as shown. It is not necessary for the
groups to be identical, provided they are all adapted to take the
same current in amperes, which should be kept constant, and provided
the lamps of each set agree in voltage. For example, on the ordinary
10-ampere arc circuit, one group might consist of 5 lamps, each
requiring 2 amperes at 50 volts; the next might be composed of 10
lamps, each taking 1 ampere at 100 volts, and so on.
[Illustration: FIG. 790.—Series-parallel system of distribution. It
consists of groups of parallel connected receptive devices, the groups
being arranged in the circuit in series.]
=Parallel-Series System.=—In this method of connection, one or more
groups of lamp are connected in series and the groups in parallel as
shown in fig. 791.
[Illustration: Fig. 791.—Parallel-series system of distribution. It
consists of groups of series connected receptive devices, the groups
being arranged in the circuit in parallel.]
=Ques. When is a parallel-series system used?=
Ans. When it is desired to operate a number of lamps or motors on a
line where voltage is several times that required to operate a single
lamp or motor.
The parallel-series system is employed chiefly in the lighting
circuit on electric traction lines; here, usually five 110
volt lamps are connected to the source of supply which has a
pressure of 550 volts.
=Center of Distribution.=—It is important to determine the point at
which the feeders should be attached to the mains in order to minimize
the amount of copper required. The method employed is similar to that
used in determining the best location of a power plant as regards
amount of copper required. The center of distribution may be called
the electrical center of gravity of the system, and is found by
separately obtaining the center of gravity of straight sections and
then determining the total resultant and point of application of this
resultant of the straight sections.
Feeders (feeding cables or conductors) are run from the source
of supply to the distributing centers, and, as these feeders
are in many cases of considerable length, a substantial loss of
pressure generally occurs in them. The pressure at the source
of supply, however, is so regulated as to compensate for the
drop in the feeders, and the pressure at the distributing
centers is thus kept constant; or the same result is obtained
by the use of regulating devices in the feeders. The
essential condition in most systems is that the pressure at
the distributing centers shall be kept practically constant,
irrespective of the load.
=Edison Three Wire System.=—In electric lighting systems used up to
about 1897, it was not considered practicable to use incandescent lamps
requiring a pressure exceeding 120 volts. This limited the operating
voltage of parallel systems, and necessitated the use of conductors
of large size and weight, especially where the current had to be
transmitted a considerable distance.
The effect of this limiting voltage is more apparent when it is clearly
understood that the size of wire required to carry a current depends
upon the amperes and not upon the volts.
A wire capable of carrying a current of 10 amperes at 20
volts, can carry 10 amperes at 20,000 volts or any other
voltage. Therefore, since the amount of electric energy or
power transmitted through a conductor is equal to the amperes
multiplied by the volts, it is clear that by increasing the
voltage, the power transmitting capacity of a current can be
almost indefinitely increased without increasing the size of
the conducting wire. This is the reason why considerations of
economy dictate the use of the highest voltages possible in
long distance transmissions. The voltage of the current is
determined, however, by the requirements of the apparatus to be
operated.
Incandescent lamps usually require a pressure of 110 volts, and
the current required by a 16 candle power lamp at that voltage
is about ½ ampere. Therefore if the lamp be designed for
a pressure of 220 volts, the current will be reduced to ¼
ampere, and the same size of wire could be used to feed twice
as many lamps.
[Illustration: FIGS. 792 and 793.—Evolution of the three wire system.
Fig. 792 shows two dynamos supplying two independent circuits. These
may be connected in series as in fig. 793, thus operating the two
circuits of fig. 792 with two wires instead of four. To balance the
system in case of unequal loading, a third or _neutral wire_ is used as
shown in fig. 794.]
The saving of copper is the sole merit of the three wire system, and
the object which led to its invention was to effect this economy with
the use of 110 volt lamps.
=Principle of the Three Wire System.=—In fig. 792, two dynamos A and
B are shown supplying two independent incandescent lighting circuits,
each circuit receiving 3 amperes of current at a pressure of 110 volts.
It is evident that the dynamos could be connected with each other in
series, and the lamps connected in series with two each, as shown in
fig. 793, thus making the two wires K and L of the two independent
circuits unnecessary, as the pressure will be increased to 220 volts
while the current will remain at 3 amperes, and each lamp will require
¼ ampere.
[Illustration: FIG. 794.—Balanced three wire system. The middle
conductor, known as the _neutral wire_, keeps the system balanced in
case of unequal loading, that is, a current will flow through it, to or
from the dynamos, according to the preponderance of lamps on the one
side or the other. These current conditions are shown in fig. 797.]
The amount of copper saved will be 100 per cent., but this arrangement
is open to the objection, that when one of the lamps is turned off, or
burned out, its companion will also go out. This difficulty is avoided
in the three wire system by running a third wire N, from the junction
O, between the two dynamos, as shown in fig. 794, thus providing a
supply or return conductor to any one of the lamps, and permitting
any number of lamps to be disconnected without affecting those which
remain. If the system be exactly balanced, no current will flow through
the wire N, because the pressure _toward_ the - terminal of the dynamo
A, will be equal to the pressure _from the_ + terminal of dynamo B,
thus neutralizing the pressure in the wire. For this reason the middle
wire of a three wire system is called the _neutral wire_, and is
usually indicated by the symbol O or ± the latter meaning that it is
positive to the first wire and negative to the second. If the system
be unbalanced, a current will flow through the neutral wire, to or
from the dynamos, according to the preponderance of lamps in the upper
or lower sets. When the number in the lower set is the greater, the
current in the neutral wire will flow _from_ the dynamos as shown in
fig. 797, and _toward_ the dynamos under the reverse condition.
In the case represented in fig. 797, there are five lamps in
circuit, requiring 2½ amperes of current at a pressure
of 110 volts. The two lamps in the upper set will require 1
ampere, and the three lamps in the lower set, 1½ amperes.
Since a pressure of 110 volts can force only a current of one
ampere through resistance of the two lamps in the upper set,
it is evident, that the additional ½ ampere required by the
three lamps in the lower set will have to be supplied through
the neutral wire, as shown.
=Balancing of Three Wire System.=—In practice it is impossible to
obtain an exactly balanced system, as the turning on and off of lamps
as required results in a preponderance of lamps in the upper or lower
sets, and furthermore, even when the number of lamps in the two sets
are equal, they may be located irregularly, thereby causing the
currents to flow for short distances in the neutral line. Therefore,
the larger the number of lamps in the circuit, the easier it will be to
keep the system in a balanced condition.
=Copper Economy in Three Wire Systems.=—Theoretically, the size
of the neutral wire has to be only sufficient to carry the largest
current that will pass through it. A large margin of safety, however,
is allowed in practice so that its cross section ranges from about
one-third that of the outside line, in large central station systems,
to the same as that of each outside line in small isolated systems.
If the neutral wire be made one-half the size of the outside
conductor, as is usually the case in feeders, the amount of
copper required is 5/16 of that necessary for the two wire
system. For mains it is customary to make all three conductors
the same size increasing the amount of copper to ⅜ of that
required for the two wire system.
[Illustration: FIG. 795.—Dobrowolsky three wire system with
self-induction coil. It consists of an ordinary direct current
dynamo, the armature A and pole pieces N and S of which are shown.
A self-induction coil D, is connected to two diametrically opposite
points of the winding of the armature A. The coil D may be carried by
and revolve with the armature; but in the construction represented,
it is stationary, being connected to the armature winding through the
brushes CC, rings and wires JJ. The middle point of the self-induction
coil D, is connected to the neutral conductor O of the three wire
system, the outside conductors + and - being supplied from the brushes
BB in the usual manner. The pressure at the terminals of the coil D
is alternating; hence the latter, on account of its self-induction,
does not act as a short circuit to the armature. Furthermore, the
inductances of the two halves of the coil D being equal, the pressure
of the neutral wire O is kept midway between the pressures of the
outside wires + and -. When the two sides of the system are unbalanced
in load, the difference in current carried in one direction or the
other by the neutral wire passes freely through the coil D, since the
current is steady, or varies slowly, and is therefore unimpeded by the
self-induction. It is evident that the ohmic resistance of D should be
as low and its self-induction as high as possible, in order that the
loss of energy and the difference in voltage on the two sides of the
system shall be as small as possible under all conditions.]
=Modifications of the Three Wire System.=—By the employment of
suitable arrangements, it is possible to operate a three wire system
with only one dynamo. Some of the various arrangements which have
been used or proposed in this connection may be briefly mentioned as
follows:
=Three Wire Storage Battery System=, in which a storage battery
is connected between the two outside wires, and the pressure of
the neutral wire varied to balance the system by shifting the
point at which it is connected to the battery.
=Three Wire Double Dynamo System=, in which a double dynamo
having two armature windings upon the same core, connected to
two separate commutators, is used in the same manner as two
separate dynamos connected in series.
=Three Wire Bridge System=, in which a resistance is connected
across the two outside wires, and the neutral wire is brought
to a point on the resistance through a movable switch. The
pressures on the two sides of the circuit are equalized by
adjusting the arm of the switch for any change of load.
[Illustration: FIG. 796.—Three wire compensator system. A and
B are the compensators or equalizers. They consist of auxiliary
dynamos coupled together and connected to the system as shown.
D is the main dynamo, and E, a booster.]
=Three Wire Three Brush Dynamo System=, in which the neutral
wire is connected to a third brush on the dynamo.
=Dobrowolsky Three Wire System=, in which a self-induction
coil is connected to two diametrically opposite points of the
armature of an ordinary direct current dynamo. The principle of
this system is illustrated in fig. 795.
=Three Wire Auxiliary Dynamo System=, in which the neutral wire
is connected to an auxiliary dynamo which supplies a pressure
one-half as great as that of the main dynamo. The auxiliary
dynamo is usually belt driven by the main dynamo, and acts
as a dynamo when the load is greater on the negative side of
the circuit, and as a motor when the excess of load is on the
positive side.
=Three Wire Compensator System=, in which two auxiliary dynamos
A and B called _compensators or equalizers_, are coupled
together and connected to the system as shown in fig. 796. Each
compensator generates one-half as much pressure as the main
dynamo D, and serves to equalize the pressure and the load, the
compensator on the lightly loaded side operating as a motor
and driving the other as a dynamo. When the system is exactly
balanced, both compensators run as motors under no load,
therefore, consume very little energy. In this arrangement only
one booster E, is required for both sides of the system, as
the compensators are connected to the outside wires at a point
beyond the boosters, and therefore, sub-divide the increased
difference of pressure equally between the two sides of the
system.
[Illustration: FIG. 797.—Three wire double dynamo system having two
separate windings on the same core and separate commutators A and B as
shown.]
=Extension of the Three Wire Principle.=—In order to attain still
greater economy in copper, the principles of the three wire system may
be extended to include four, five, six, and seven wire systems. The
comparative weights of copper required by such systems are as follows:
Two wire system 1.000
Three " " all wires of equal size .370
Three " " neutral wire one-half size .313
Four " " all wires of equal size .222
Five " " " " " " " .156
Seven " " " " " " " .096
The four wire system requires about two-ninths as much copper, and the
seven wire system about one-tenth as much copper, as an equivalent two
wire system; but neither is desirable, as their operation involves too
much inconvenience, too many unavoidable complications, and create a
possibility of accident, which more than offsets the saving in copper.
[Illustration: FIG. 798.—Diagram showing dynamotor connections when
used as an equalizer in the three wire system. DM, dynamotor; G,
generator side; M, motor side.]
=The Five Wire System.=—This system is employed advantageously in many
places in England and Europe, but has not as yet been introduced to any
extent in America. It is very probable that in the future the three
wire 440 volt system will be selected in preference to the five wire
system.
=Dynamotor.=—This is a combination of dynamo and motor on the same
shaft, one receiving current and the other delivering current, usually
of different voltage, the motor being employed to drive the dynamo
with a pressure either higher or lower than that received at the motor
terminals.
The dynamotor in the direct current circuit corresponds to the
transformer in the alternating current circuit.
[Illustration: FIG. 799.—Diagram showing connections of balancing set
in three wire one dynamo system. The set consists of a motor and dynamo
connected, and its operation is practically the same as a dynamotor.]
=Ques. How is the dynamotor used as an equalizer in the three wire
system?=
Ans. When thus used, the machine is connected as in fig. 798. When
both sides of the system are balanced, there will be no current in the
neutral lead N, and a small current will pass through the two armature
windings of the dynamotor in series, both armatures acting as motors.
If the load on one side of the system become larger than the load on
the other side, there will be a greater drop in the leads connected to
the overloaded side and consequently a lower voltage will exist over
the larger load than exists over the smaller load. The armature winding
of the dynamotor connected to the higher voltage will act as a dynamo,
whose pressure will tend to raise the voltage of the more heavily
loaded side.
The direction of the currents in an unbalanced three wire
system that is being supplied with energy from a main dynamo is
shown in the figure. The commutator at G is connected to the
dynamo winding of the dynamotor and is supplying current to the
upper or larger load, and the lower commutator is connected to
the motor winding of the dynamotor and is taking current from
the lightly loaded side.
=Motor-Dynamo or Balancing Set.=—A balancing set or balancer consists
of a motor mechanically connected to a dynamo used to balance a three
wire system. The operation of such a combination is practically the
same as the dynamotor just described. The balancer is connected as
shown in fig. 799.
[Illustration: FIG. 800.—Holzer-Cabot type M motor-dynamo set. This
combination is known as a booster, and is used to raise or lower the
voltage on feeders. The motor is series wound and connected in series
with one leg of the feeder. Thus, the voltage which the booster will
add to the line will be directly in proportion to the current flowing
in the feeder. The regulation is therefore automatic.]
When an unbalanced load comes on, the voltage on the lightly loaded
side rises and on the heavily loaded side drops. The machine on the
light side then takes power from the line and runs as a motor driving
the machine on the heavy side as a dynamo, supplying the extra current
for that side. This action tends to bring the voltage back to normal
and gives good regulation.
In some cases the field of each machine is connected to the
opposite side of the system which gives a quicker action. This
regulation is automatic and the set takes care of unbalanced
loads in either direction without adjustment.
=Balancing Coils.=—Another method of balancing a three wire system
which does away with any additional rotating machines makes use of
balance coils.
=Ques. Describe the type of dynamo used with balancing coils?=
[Illustration: FIG. 801.—Diagram showing connections of balancing coil
system. The dynamo used in this system is provided with both commutator
and collector rings.]
Ans. The regular two wire dynamo is used supplying power to the outside
wires, but there are collector rings connected to the armature. These
rings are much lighter than they would be for a converter as they carry
only about ⅛ of the dynamo load. These rings being light are usually
placed at the end of the commutator and are connected directly to the
commutator bars.
=Ques. How are the balancing coils constructed?=
Ans. They are built of standard transformer parts, and are placed in
cases similar to those of ordinary small transformers.
[Illustration: FIGS 802 and 803.—Distribution by dynamo-motor sets.
Fig. 802, sets in parallel; fig. 803, sets in series. In fig. 802,
current produced by the main dynamo G, is carried to the machines by
the conductors A and B to which the motor portions M are connected
in parallel. These motors are provided with shunt wound field coils
which may be connected to the primary or to the secondary circuit,
consequently the machines run at a practically constant speed. The
dynamo portions D of the transformers are connected to the secondary
circuits which supply the lamps, etc., L, as indicated. The field
magnets of these dynamos may also be fed by the main circuit AB, or
they may be self-excited by shunt or compound winding. In fig. 803, the
motors M are all connected in series with the main dynamo G, and the
dynamo elements D of the transformers, connected to the lamps, etc.,
L. If the current be kept constant (the dynamo G having a regulator
like a series arc dynamo), and the motors M are simple series wound
machines, they will exert a certain torque, or turning effort, which
will be constant. It follows, therefore, that if the dynamos D be
also series wound, each will generate a certain current which will
be constant. If lamps or other devices, designed for that particular
current, be connected in series on the secondary circuits, the dynamos
D will always maintain that current, no matter how many lamps there
may be. When lamps are added, the resistance of the local circuit is
raised, and the current in it decreases, so that the dynamo increases
its speed until it generates sufficient pressure to produce practically
the same current as before. Hence this constitutes a system which is
self-regulating, when lamps, etc., are cut in or out of the secondary
circuits. No harm results even if the secondary be short circuited,
since only the normal current can be generated. But if the secondary
circuit be opened, then the machine will race, and probably injure
itself by centrifugal force, because the torque of the motor M has its
full value, and there is no load upon the dynamos D. To guard against
this danger, some automatic device should be provided to short circuit
the field or armature of the motor when its speed or reverse voltage
rises above a certain point.] The coil has a straight continuous
winding, both ends and a connection from the middle point of the
winding being brought out of the case.
=Ques. How are the coils connected to the dynamo?=
Ans. Two coils are used and are connected to the collector rings as
shown in fig. 801, one coil across each phase. The connections from the
middle points of the coils are connected together and to the neutral
wire of the system.
[Illustration: FIG. 804.—Diagram to show correctness of balancing coil
connection. In the figure, AE, BF, CG, and DH represent the balance
coil and its connection for different positions of the armature of a
bipolar machine.]
=Ques. What is the action of the coils in equalizing the load?=
Ans. On balanced load, the coils take a small alternating exciting
current from the collector rings as any transformer does when connected
to an alternating current line with its secondary open. When an
unbalanced load comes on, the current in the neutral divides, half
going to each coil. This enters the coil at the middle point and half
flows each way through the coil and the slip rings into the armature
winding. The unbalanced current is thus fed back directly into the
dynamo armature continuously.
The coils are small and can be placed back of the switchboard
or below the floor, as they require no attention. The current
flowing to each slip ring is 25% of the direct current in the
neutral wire with the small exciting current taken by the coil
added.
The coils are usually built to take care of current in the
neutral equal to 25% of full load current of the dynamo with a
voltage regulation not to exceed 2 per cent.
=Ques. Upon what does the operation of the balancing coil system
depend?=
Ans. It depends on the following points: First, the _impedance_[1] of
the coils keeps the exciting current which they take from the collector
rings down to a small value as it is alternating current. At the same
time the current from the neutral wire flows through the four half
coils in parallel, and being direct current is impeded only by the
ohmic resistance of the coils, which is low, giving only a slight loss
in the coils. The common point to which the neutral wire is connected
must at all times be neutral to the - and + direct current brushes.
[1] NOTE.—The term _impedance_ means the total opposition in an
electric circuit to the flow of an alternating current, being made up
of the actual or ohmic resistance and the apparent resistance due to
self-induction, or if the circuit contain also capacity, the resultant
apparent resistance due to self-induction and capacity.
That this common point is at all times neutral is readily
shown. Referring to fig. 804, let AE, BF, CG and DH represent
the balance coil and its connection for different positions
of the armature of a bipolar machine. Let O be the tap to the
middle point of the winding.
Take the instant when the balance coil taps are directly
under the direct current brushes as shown at position AE. It
is evident that since the point O is the middle point of the
coil, it is neutral between A and E. When the armature turns so
that the balance coils take the position BF, the voltage drop
between A and E may be divided into 4 parts, AB, BO, OF and FE.
As in the first instance, O is neutral between the ends of the
coil, and the voltage drop over OF equals that over OB.
Since the space AB includes the same number of armature coils
as space FE and they are in fields of equal strength, the
voltages across the two spaces will be equal, and the voltage
over AB equals that over FE. Then adding equals: AB + BO = FE +
FO and O is neutral between A and E as in the first case.
In the same way it can be shown that O is neutral between the
direct current brushes for any position of the balance coil
taps. One coil will operate the system, but two coils, giving
four points spaced 90 electrical degrees apart, give better
distribution of the current to the armature winding and better
regulation of the voltage.
=Boosters.=—A booster may be defined as, _a dynamo inserted in a
circuit at a point when it is necessary to change the voltage_. A
booster is generally driven by a motor, the two armatures being
directly coupled, although boosters are sometimes driven from the
engine or line shaft.
[Illustration: FIG. 805.—Crocker Wheeler motor-dynamo set. There are
numerous cases where such a combination is useful for furnishing a
circuit with a voltage different from that of the main plant or with a
voltage that can be varied independently. For storage battery charging
and electrolytic work, where constant current is desirable, it forms a
simple means of voltage regulation. Where a circuit of special voltage
is required, the set not only supplies current at the desired pressure,
but insulates the special circuit, which may be subject to more severe
requirements than the main system. The advantage of the three wire
distribution can be obtained from any two wire dynamo by means of a
small rotary balancer or balancing transformer, which consists of two
direct current machines of the same voltage, mechanically connected
together with their armatures in series. Multiple voltage systems for
speed regulation can also be obtained by a similar arrangement.]
=Ques. Explain the use of a booster?=
Ans. When a number of feeders run out from a station, the longest
and those carrying the heaviest loads will have so much drop on the
line that the pressure at distant points is too low. It is therefore
necessary to raise the pressure to compensate for the drop and this is
done by inserting a booster in the circuit.
It would not be economical to raise the voltage on all the
lines by supplying current from the main dynamo at higher
pressure, hence the voltage is raised only on the lines which
need it by means of the booster working in series with the main
dynamo.
[Illustration: FIG. 806.—Diagram showing use of auxiliary bus bar. In
order to avoid the necessity for boosters, some stations have an extra
bus bar, which is kept at a higher pressure than the main bus, and to
this are connected the feeders that have an extra large drop.]
=Ques. For what other service are boosters employed?=
Ans. They are used in connection with storage battery plants for
the purpose of raising the voltage of the bus bars to the pressure
necessary for charging storage batteries.
=Ques. What is an auxiliary bus bar?=
Ans. An extra bus bar which is kept at higher pressure than the main
bar.
=Ques. What is the object of an auxiliary bus bar?=
Ans. It is used in place of a booster as shown in fig. 806. One or more
dynamos maintain the pressure between the auxiliary bar and the common
negative bar. The feeders which need boosting are connected to the
common negative bar and the auxiliary bar as shown.
CHAPTER XXXVII
WIRES AND WIRE CALCULATIONS
The wireman who is called upon to plan and install a system of wiring
will find it necessary first to have a knowledge of the various kinds
of wire so as to select the one best suited for the work, and to be
able to make simple calculations in order to determine the proper sizes
of wire for the various circuits.
Wires are generally made of circular cross section. The process
of manufacture consists in drawing the material through steel
dies, when its properties permit this treatment. In the case of
some substances, as for instance, tin and lead, difficulties
arise in the drawing process, and these are therefore
"squirted."
The metals most extensively used for wires are copper and
iron; German silver, tin and lead are also employed, but only
at points where it is desirable to have a comparatively high
resistance in the circuit.
=Copper Wire.=—Copper is used in nearly all cases of wiring because it
combines high electrical conductivity with good mechanical qualities
and reasonable price. In conductivity it is only surpassed by silver,
but the cost of the latter of course prohibits its use for wiring
purposes.
Copper wire is used for electric light and power lines, for most
telephone and some telegraph lines, and for all cases where low
resistance is required at moderate cost.
Hard drawn copper wire is ductile, and has a high tensile
strength; these properties allow it to be bent around corners
and drawn through tubes without injury.
Pure annealed copper has a specific gravity of 8.89 at 60°
Fahr. One cubic inch weighs .32 pound; its melting point is
about 2,100° Fahr.
Good hard drawn copper has a tensile strength of about three
times its own weight per mile length. Thus, a number 10 B. &
S. gauge copper wire, weighing 166 lbs. per mile, will have a
breaking strength equal to approximately 3 × 166 = 498 lbs.
=Iron Wire.=—This kind of wire is largely used for telegraph and
telephone lines, although it is rapidly being replaced by copper in
long lines.
There are three grades of iron wire:
1. =Extra best best (E. B. B.)= which has the highest
conductivity and is the nearest to being uniform, in quality,
being both tough and pliable;
2. =Best best (B. B.)=, which varies more in quality, is not so
tough, and is lower in conductivity. _It is frequently sold as_
=E. B. B.=;
3. =Best (B.)=, which is the poorest grade made, being more
brittle, and lowest in conductivity. Iron wire should be well
galvanized.
=German Silver Wire.=—German silver is an alloy consisting of 18 to
30% nickel, and the balance about four parts copper to one part zinc.
It is very largely used as a resistance material in making resistance
coils, and is sold in the form of wire, and strip. The resistance of
this wire varies with its composition.
The resistance of the 18% alloy at 25° C. is 18 times that of
copper, and of the 30% alloy about 28 times that of copper.
The safe carrying capacity of the wire in spirals in open air
for continuous duty is such that the circular mils per ampere
varies from about 1,500 in No. 10 wire to about 475 in No. 30.
For intermittent duty the capacity is twice as great.
=Standard of Copper Wire Resistance.=—Matthiessen's standard for
resistance of copper wire is as follows: _A hard drawn copper wire
one meter long, weighing one gramme, has a resistance of .1469 B. A.
unit at 32° Fahr._ Relative conducting power: silver, 100; hard or
un-annealed copper, 99.95; soft or annealed copper, 102.21.
A committee of the Am. Inst. Electrical Engineers recommends the
following form of Matthiessen's standard, taking 8.89 as the specific
gravity of pure copper: _A soft copper wire one meter long and one
millimeter in diameter has an electrical resistance of .02057 B. A.
unit at 0°C._[2] From this the resistance of a soft copper wire one
foot long and .001 in. in diameter (mil-foot) is 9.72 B. A. units at
0°C.
[2] NOTE.—The international ohm ÷ B. A. ohm = 1 ÷ .9866. The B. A. ohm
÷ International ohm = 1 ÷ 1.0136. Hence, to reduce British Association
ohms to International ohms, divide by 1.0136. or multiply by .9866.
For every degree Fahr., the resistance of copper wire increases
.2222%. Thus a piece of copper wire having a resistance of 10
ohms at 32°, would have a resistance of 11.11 ohms at 82°.
Relative Conductivity of Different Metals and Alloys.
(According to Lazare Weiler.)
Pure silver 100
Pure copper 100
Alloy, ½ copper, ½ silver 86.65
Telephonic siliceous bronze 35
Pure zinc 29.9
Brass with 35% zinc 21.5
Swedish iron 16
Pure platinum 10.6
Copper with 10% nickel 10.6
Pure lead 8.88
Pure nickel 7.89
Phosphor-bronze, 10% tin 3.88
=Conductors.=—Copper is used more than any other metal for
transmitting electrical energy, and for interior wiring it is used
exclusively. Copper conductors should be of the highest commercial
conductivity, not less than 97%.
For conductors up to sizes as large as No. 8 B. & S. gauge, single
conductors may be used, but for larger sizes the necessary conductivity
should be obtained by conductors made up of strands of smaller wires.
The size of these strands depend upon the size of the conductors and
the conditions under which they are to be used.
Where conductors are very large (as for instance dynamo leads),
and where it is essential that they should be as flexible as
possible, strands as small as No. 20 or 22 B. & S. gauge may be
used.
Conductors for flexible cords, pendants, fixtures, etc.,
should also consist of very fine strands, so that they may be
perfectly pliable and flexible.
The individual strands for instance, for a No. 16 B. & S. gauge
flexible cord should be as fine as No. 30.
[Illustration: FIG. 807.—Elevator cable for annunciators. This type
of cable is designed for connecting the movable elevator car with the
signal buttons upon the different floors, and is constructed so as to
secure strength and flexibility.]
=Covered Conductors.=—For most conditions of service, wires are
protected with an insulating covering. Wires used in interior circuits
should have a covering which shall act both as an electrical insulator
and as a mechanical protection. In some instances, however, the
insulating qualities are of secondary importance.
The various forms of covering now in use commercially for wires are:
1. Rubber;
2. Weather proof;
3. Slow burning;
4. Slow burning weather proof;
5. Armoured.
=Rubber Covered Conductors.=—This class of conductor consists of a
tinned copper wire with a rubber covering, protected by an outside
braiding of cotton saturated with a preservative compound.
=Ques. What are the advantages of rubber insulation for conductors?=
Ans. It is waterproof, flexible, fairly strong, and has high insulating
qualities.
[Illustration: FIG. 808.—Rubber insulated telephone and telegraph
wires. The inner coat of rubber should be free from sulphur or other
substances liable to corrode the copper.]
=Ques. What are the disadvantages of rubber insulation?=
Ans. It deteriorates more or less rapidly and is quickly injured by
temperatures above 140° Fahr.
=Ques. For what service are rubber covered conductors adapted?=
Ans. For interior wiring.
=Ques. Is pure rubber used?=
Ans. No. The covering should be made from a compound containing from 20
to 35 per cent. of pure rubber.
It would be difficult to place pure rubber on a wire,
and moreover a covering made of pure rubber would not be
durable and would deteriorate very rapidly, particularly at
temperatures above 120° Fahr. Accordingly, it is mixed with
other materials, such as French chalk, silicate of magnesia,
sulphur, red lead, etc.
=Weather Proof Conductors.=—In this class of conductor, the wire is
protected from the weather by a waterproof covering, consisting usually
of braided cotton of two or three thicknesses saturated with a moisture
resisting insulating compound.
=Ques. Where are weather proof conductors used?=
Ans. In places subject to dampness, such as cellars, tunnels, open
sheds, breweries, etc.
[Illustration: FIG. 809.—Twisted weather proof wires. The insulation
consists of two or three thicknesses of braided cotton saturated with a
moisture resisting insulating compound.]
=Ques. What are the advantages of weather proof conductors?=
Ans. The insulation is cheap, very durable, and does not deteriorate
unless exposed to high temperatures such as will melt the compound.
=Ques. State the disadvantages.=
Ans. The covering is more or less inflammable and is not very efficient
as an insulator.
=Ques. What precaution should be taken in using weather proof
conductors?=
Ans. On account of the inflammable character of the covering, care
should be taken in wiring at points where any considerable number of
conductors are brought together, or where there is much woodwork or
other combustible material.
=Ques. For what use are weather proof conductors especially adapted?=
Ans. For outside wiring where moisture is certain and where fireproof
quality is not necessary.
Obviously conductors of this class should not be used in
conduits, nor in fact, in any way except exposed on glass or
porcelain insulators.
=Slow Burning Wire.=—This class of conductor is defined as: _one
that will not carry fire_. The covering consists of layers of cotton
or other thread, all the interstices of which are filled with the
fireproofing compound, or of material having equivalent fire resisting
and insulating properties. The outer layer is braided and specially
designed to withstand abrasion. The thickness of insulation must not
be less than that required for slow burning weather proof wire and
the outer surface must be finished smooth and hard.—_Underwriters'
requirements._
[Illustration: FIG. 810.—Slow burning wire, formerly known as
Underwriter's Wire. The insulation is triple braided, saturated with a
white fireproof compound. Solid conductor.]
=Ques. Where should slow burning wires be used?=
Ans. In hot dry places, where ordinary insulations would be injured,
and where wires are bunched, as on the back of a large switchboard or
in a wire tower.
A slow burning covering is considered good enough when the
wires are entirely on insulating supports. Its main object is
to prevent the copper conductors coming into contact with each
other or anything else.
=Ques. What must be done before using weather proof wire?=
Ans. Permission to use the wire must first be obtained from the local
Inspection Department.
=Slow Burning Weather Proof Wire.=—The covering of this type wire
is a combination of the underwriters and weather proof insulations.
The fireproof coating comprises a little more than half of the total
covering. When the fireproof coating is placed on the outside, the wire
is called "slow burning weather proof."
[Illustration: FIG. 811.—Slow burning weather proof wire. The
insulation is composed of two braids thoroughly saturated with a fire
proof composition, over which is a highly polished weather proof third
braid. This wire was formerly known as "fire and weather proof" wire.]
=Ques. How does slow burning weather proof wire compare with weather
proof wire?=
Ans. It is less inflammable and less subject to softening under heat.
=Ques. Where should slow burning weather proof wire be used?=
Ans. In places where the wires are to be run exposed and where moisture
resisting quality is desired, also where at the same time it is
desirable to avoid an excess of inflammable covering.
=Ques. How should it be installed?=
Ans. It should be set on glass or porcelain insulators.
=Miscellaneous Insulated Conductors=
[Illustration: FIG. 812.—Armoured submarine cable. This type of cable
is insulated with a rubber compound containing not less than 30% of
pure Para rubber. The following specifications have been adapted by
various telegraph companies and the United States Government for
general use.]
+————————————-+————————————+————————-+——————————————+——————————+——————————-+
| No. of | Gauge of | No. of | Gauge of | Outside | Weight |
| Conductors | Conductors | Armour | Armour Wires | Diameter | per 1,000 |
| | (B. & S.) | Wires | B. W. G. | (inch) | Feet |
+————————————-+————————————+————————-+——————————————+——————————+——————————-+
| | | | | | |
| 1 | 14 | 12 | 8 | ⅞ | 1,150 |
| 2 | 14 | 16 | 8 | 1-31/32 | 1,675 |
| 3 | 14 | 14 | 6 | 1¼ | 2,400 |
| 4 | 14 | 16 | 6 | 1-5/16 | 2,750 |
| 5 | 14 | 19 | 6 | 1⅜ | 3,100 |
| 6 | 14 | 21 | 6 | 1½ | 3,500 |
| 7 | 14 | 21 | 6 | 1½ | 3,600 |
| 10 | 14 | 22 | 4 | 1⅞ | 4,600 |
| | | | | | |
+————————————-+————————————+————————-+——————————————+——————————+——————————-+
NOTE.—The above specifications refer only to river and harbor
cables. Ocean cables are of an entirely different character,
and consist of "shore end," "intermediate" and "deep sea" types.
[Illustration: FIG. 813.—Gas engine ignition cable. This is a special
cable made to stand the hard service necessary on automobiles. The
conductor is composed of 36 strands of No. 27 tinned copper wire, equal
to No. 14 in capacity, which gives it necessary flexibility. About this
conductor are woven two layers of cotton thread. Over this are woven,
in opposite directions, several layers of specially prepared tape which
has been given two coatings of fine insulating varnish. Two strong
braids of cotton form the outside covering, and each of these different
braids is passed through a bath of insulating liquid and baked in a
steam heated oven. With three layers of tape the cable will stand a
test of 18,000 to 20,000 volts, and with five layers, 30,000 volts.]
[Illustration: FIG. 814.—Paper insulated lead encased telephone
cable.]
=Ques. For what service is slow burning weather proof wire not suited?=
Ans. It is not adapted to outside work.
=Safe Carrying Capacity of Wire.=—All wires will heat when a current
of electricity passes through them. The greater the current or the
smaller the wire, the greater will be the heating effect. Large wires
are heated comparatively more than small wires because the latter have
a relatively greater radiating surface.
[Illustration: FIG. 815.—Pothead wires. The standard wire for pothead
work is either No. 19, 20 or 22 B. & S. gauge, either single conductor
or twisted pair, insulated to a diameter of 3/32 inch over rubber,
without any outer braid or protection. In the case of twisted pairs
one conductor is usually made of a differently colored rubber than the
other, so as to distinguish between them.]
The temperature of a wire increases approximately as the square of the
current, and inversely as the cube of the diameter of the wire.
The elevation in temperature of a wire carrying a current represents so
much lost energy.
From these considerations it must be clear that it is important not to
overload conductors in order to secure efficient working, and to avoid
risk of fire on inside installations.
The Board of Underwriters specifies that the carrying capacity
of a conductor is safe when the wire will conduct a certain
current without becoming painfully hot.
In the following table of carrying capacity, prepared by
the underwriters, a wire is assumed to have a safe carrying
capacity when its temperature is not increased by the given
current over 30° Fahr. above that of the surrounding air.
SAFE CARRYING CAPACITIES OF WIRES
(Maximum amperes allowed by the Underwriters.)
+————————+——————————————-+————————————+————————————+
| Brown | | Rubber | Other |
| and | | insulation | insulations|
| Sharpe | Circular mils | —————— | —————— |
| Gauge | | Amperes | Amperes |
| | | | |
+————————+——————————————-+————————————+————————————+
| | | | |
| 18 | 1,624 | 3 | 5 |
| 16 | 2,583 | 6 | 8 |
| 14 | 4,107 | 12 | 16 |
| 12 | 6,530 | 17 | 23 |
| 10 | 10,380 | 24 | 32 |
| 8 | 16,510 | 33 | 46 |
| 6 | 26,250 | 46 | 65 |
| 5 | 33,100 | 54 | 77 |
| 4 | 41,740 | 65 | 92 |
| 3 | 52,630 | 76 | 110 |
| 2 | 66,370 | 90 | 131 |
| 1 | 83,690 | 107 | 156 |
| 0 | 105,500 | 127 | 185 |
| 00 | 133,100 | 150 | 220 |
| 000 | 167,800 | 177 | 262 |
| 0000 | 211,600 | 210 | 312 |
| | 200,000 | 200 | 300 |
| | 300,000 | 270 | 400 |
| | 400,000 | 330 | 500 |
| | 500,000 | 390 | 590 |
| | 600,000 | 450 | 680 |
| | 700,000 | 500 | 760 |
| | 800,000 | 550 | 840 |
| | 900,000 | 600 | 920 |
| | 1,000,000 | 650 | 1,000 |
| | 1,100,000 | 690 | 1,080 |
| | 1,200,000 | 730 | 1,150 |
| | 1,300,000 | 770 | 1,220 |
| | 1,400,000 | 810 | 1,290 |
| | 1,500,000 | 850 | 1,360 |
| | 1,600,000 | 890 | 1,430 |
| | 1,700,000 | 930 | 1,490 |
| | 1,800,000 | 970 | 1,550 |
| | 1,900,000 | 1,010 | 1,610 |
| | 2,000,000 | 1,050 | 1,670 |
| | | | |
+————————+——————————————-+————————————+————————————+
The lower limit is specified for rubber covered wires to
prevent gradual deterioration of the high insulations
by the heat of the wires, but not from fear of igniting
the insulation. The question of drop is not taken into
consideration in the table on page 731.
The carrying capacity of Nos. 16 and 18 B. & S. gauge wire is
given, but no smaller than No. 14 is to be used, except as
allowed under rules for fixture wiring.—_Underwriters' Rules._
=Circular Mils.=—The unit of measurement in measuring the cross
sectional area of wires is the _circular mil_; it is the area of a
circle one mil (.001 in.) in diameter.
_The area of a wire in circular mils is equal to the square of the
diameter in mils._
[Illustration: FIG. 816.—Diagram illustrating circular mils. The
circular mil is used as a unit of cross sectional area in measuring
wires. It is equal to the area of a circle .001 in. diameter; its value
is .0000007854 square inch. In the figure the sum of the areas of the
nine small circles equals the area of the large circle. This is evident
from the following: Take the diameter of the small circles as unity,
then the diameter of the large circle is three. Hence, the sum of the
area of the small circles × (¼ π × 1²) × 9 = .7854 × 9 =
7.0686; area of the large circle = ¼ π × 3² = .7854 × 9 =
7.0686. Therefore since the area of the large circle equals the sum of
the areas of the small circles, the area of a wire in circular mils is
equal to _the square of its diameter expressed in mils_.]
Thus a wire 2 mils in diameter (.002 in.) has a cross sectional
area of 2 × 2 = circular mils. Accordingly to obtain the area
of a wire in circular mils, _measure its diameter with a
micrometer which reads directly in mils or thousandths of an
inch, and square the reading_.
The circular mil (abbreviated C.M.) applies to all _round_
conductors, and has a value of .7854 times that of the square
mil, that is, 1 circular mil = .7854 square mil. If the
diameter be expressed as a fraction of an inch, as for instance
⅓ in., the circular mil area may be found as follows: Reduce
the fraction ⅓ to the decimal of an inch, multiply the result
by 1,000 to express the diameter in mils, and square the
diameter so expressed, thus: ⅓ = 1,000 ÷ 3 = .333. .333 ×
1,000 = 333 mils; 333 × 333 = 110,889 circular mils.
The diameter of any wire may be found when its circular mil
area is known by extracting the square root of the circular mil
area.
=Square Mils.=—For measuring conductors of square or rectangular cross
section, such as bus bars, copper ribbon, etc., the square mil is used.
A square mil is the area of a square whose sides are one mil (.001 in.
long) and is equal to .001 × .001 = .000001 square inch.
[Illustration: FIG. 817.—Diagram illustrating square mils. A square
mil is a unit of area employed in measuring the areas of cross sections
of square or rectangular conductors. It is equal to .000001 square
inch. One square mil equals 1.2732 circular mils. The figure shows
an area of nine square mils; this is equal to 9 × 1.2732 = 11.4588
circular mils.]
EXAMPLE.—A copper ribbon for a field coil measures ⅝ inch by
⅛ inch. What is its area in square mils? What is its area in
circular mils?
⅝ = .625 in., or 625 mils; ⅛ = .125 in., or 125 mils.
Area in square mils = 625 × 125 = 78,125.
Area in circular mils= { 78,125 ÷ .7854 }
{or 78.125 × 1.2732} = 99,469.
=Mil Foot.=—This unit is used as a basis for computing the resistance
of any given wire. A mil foot means _a volume one mil in diameter and
one foot long_.
_The resistance of a wire of commercially pure copper one mil in
diameter and one foot long is taken as a standard in calculating the
resistance of wires, and has been found to be equal to 10.79 ohms at
75° Fahr._
The calculation is made according to the following rule:
_The resistance of a copper wire is equal to its length in feet,
multiplied by the resistance of one mil foot (10.79 ohms) and divided
by the number of circular mils, or the square of its diameter._
Expressed as a formula:
length of wire in ft. × 10.79
resistance in ohms = —————————————————————————————— (1)
circular mils
EXAMPLE. What is the resistance of a copper wire 1,500 feet
long and having a transverse area of 10,381 circular mils?
Substituting these values in formula (1)
1,500 × 10.79
resistance = ———————————————————— = 1.559 ohms.
10,381
_The transverse area of a copper wire is found by multiplying the
resistance of a mil foot (10.79) by its length in feet and dividing the
result by its resistance in ohms._
This is obtained directly from the formula (1) by solving the equation
for circular mils, thus:
length of wire in ft. × 10.79
circular mils = ——————————————————————————————- (2)
resistance in ohms
EXAMPLE. What is the circular mil area of a wire 1,500 feet
long and having a resistance of 1.559 ohms?
Substituting the values in equation (2)
1,500 × 10.79
circular mils = ————————————- = 10,381
1.559
[Illustration: FIGS. 818 and 819.—Diagrams illustrating the meaning
of the term lamp foot, and how to apply it in calculating a circuit.
As defined, _one 16 candle power lamp at a distance of one foot from
the fuse block or point of supply is called a lamp foot_; this is
equivalent to one 8 candle power lamp at a distance of 2 feet, or one
32 candle power lamp one-half foot from the fuse block. In fig. 819,
there are four 8 candle power lamps, and the distance to center of
distribution is 10 feet. The circuit then contains 4 ÷ 2 × 10 = 20 lamp
feet.]
=Lamp Foot.=—This unit facilitates laying out wiring and calculating
the drop. A lamp foot is defined as _one 16 candle power lamp at a
distance of one foot from the point of supply_. Accordingly the number
of lamp feet in any circuit is equal to the number of 16 candle power
lamps (or equivalent in other sizes) in the circuit multiplied by the
distance in feet from the fuse block to the center of distribution.
When no point is specified, the feet are always measured from
the supply point to the center of distribution. When other than
16 c.p. lamps are in the circuit they must be reduced to 16
c.p. lamps. Thus two 8 c.p. lamps would be counted one 16 c.p.
lamp, one 32 c.p. lamp would be counted two 16 c.p. lamps, etc.
=Ampere Foot.=—From the foregoing explanation of _lamp foot_, the
significance of _ampere foot_ is easily understood—the two terms are
in fact self-defining.
An ampere foot may be defined as _the product of one ampere multiplied
by one foot_.
The unit ampere foot is used in figuring motor circuits or currents
designed to carry a mixed load.
[Illustration: FIG. 820.—The center of distribution of a circuit
coincides with the geometrical center of the group of lamps when the
lamps are of uniform size and spaced equal distances apart. The center
of distribution is here indicated by the dotted line A B.]
The ampere feet of a main are found by _multiplying the maximum load in
amperes by the distance from the fuse block to the electrical center of
the load_.
Thus if the center of distribution be 50 feet from the fuse
block and the maximum load is 9 amperes, the number of ampere
feet is equal to 9 × 50 = 450.
=Electrical Center of Distribution.=—The electrical center of a
circuit depends upon the distances between the lamps and the fuse
block; also the relative sizes of the lamps.
It may be defined as _the sum of the lamp feet for each section divided
by the number of 16 candle power lamps in the circuit_.
If the lamps be of uniform capacity, and placed at equal distances
apart, the center of distribution will coincide with the geometrical
center of the group of lamps. However, if the lamps vary in size, and
be irregularly spaced, the electrical center will not coincide with the
geometrical center unless the lamps be symmetrically arranged so as to
compensate for the difference in sizes and spacing.
[Illustration: FIG. 821.—Diagram of an irregular circuit illustrating
method of finding the center of distribution. Rule: _Divide the sum of
the lamp feet for each section by the number of 16 candle power lamps
or equivalent in the circuit_; the quotient gives the distance in feet
from the fuse block to the center of distribution.]
In such cases, as shown in fig. 821, the electrical center can
be determined by adding together the lamp feet of the several
sections A, B, C, etc., of the main and dividing the result by
the 16 c.p. units. Thus the lamp feet of
Section A = 10 lamps × 10 feet= 100
" B = 9 " × 5 " = 45
" C = 7 " × 6 " = 42
" D = 6 " × 4 " = 24
" E = 5 " × 5 " = 25
" F = 4 " × 10 " = 40
" G = 2 " × 5 " = 10
——-
which added together gives a total of 286 lamp feet.
This when divided by the ten 16 c.p. units comprising four 16
c.p. lamps and three 32 c.p. lamps, gives a little over 28½
feet as the distance from the fuse block to the center of
distribution, the position of which is shown by the line M N in
fig. 821, while that of the geometrical center is shown by the
line K L.
When the center of distribution is at a considerable distance
from the supply circuit, and it becomes advisable to divide
the wiring into two distinct elements—a feeder and one or
more mains, the junction of the feeder and the mains should
be located at the electrical center of the mains whenever
possible. When this is done, it is obvious that the wire size
of only one half the main needs to be calculated, as both
halves of the main are identical.
[Illustration: FIG. 822.—Brown and Sharpe (B. & S.), or American
Standard wire gauge. This gauge was adopted by the brass manufacturers
Jan., 1858. The cut is full size, and therefore, shows the actual sizes
corresponding to the gauge numbers.]
=Wire Gauges.=—For the purpose of facilitating the measurement
of wire, a number of gauges have been designed by various wire
manufacturing concerns. The principal gauges used in the United States
are the American or Brown & Sharp's gauge; the English standard or
Birmingham gauge; Washburn & Moen's standard gauge; Imperial wire
gauge; Stubs' steel wire gauge, and the U. S. Standard wire gauge.
The several gauges are here given with explanation of their use.
=The American Standard or Brown and Sharp's Gauge.=—This gauge
is commonly designated as A. W. G. or B. & S., and has been
adopted by brass manufacturers and is used mostly in measuring
brass, copper, silver, German silver, and gold in both wire and
plate.
=Birmingham or Stub's Wire Gauge (B. W. G.).=—Old English
Standard and Iron Wire Gauge. _Birmingham or Stubs' Iron Wire
Gauge is not the same as Stubs' Steel Wire Gauge_. A table of
Stub's Steel Wire Gauge is given on page 741.
[Illustration: FIG. 823.—Micrometer screw gauge. It consists
essentially of a screw whose thread is accurately turned to a pitch
of some convenient fraction of an inch or centimetre. When the screw
is screwed home, the surfaces of A and B meet, and the instrument
should then read zero on both the straight and the circular scale. If
this be not so, there is a zero error which must be either allowed
for, or corrected by means of the screw provided for that purpose. If
the former course be adopted, the reading of the instrument is taken
when the faces A and B are in contact, and this number added to or
subtracted from the final reading according to whether the error makes
the wire apparently smaller or greater than its real size. The surfaces
A and B are now screwed apart and then, after the wire to be measured
(which should be clean and straight) has been introduced between them,
they are screwed together to lightly grip the wire. If the gauge be
screwed up too tightly the value of the measurement is destroyed, since
a copper wire can easily be crushed, and in addition the accurate
screw may be permanently damaged. To avoid the possibility of this
happening, screw gauges are provided with a ratchet which prevents an
excessive force being applied to the screw. If the pitch of the screw
in the gauge be ½0th of an inch, and the circular scale consist of
50 divisions, then for each revolution of the screw, the surface B
will travel a distance equal to the pitch, that is ½0th of an inch.
The graduations on an instrument of this kind are generally ⅒th of
an inch on the straight scale, with shorter lines to mark the half
divisions. The thickness of a wire on the straight scale can therefore
be read to the nearest ½0th inch. Each division of the circular scale
represents 1/50th of a revolution of the screw, which corresponds to a
change in distance between A and B, of 1/50 of 1/20 = 1/1,000 in. If
then the reading on the straight scale be 1 and on the circular scale
35, the distance between A and B is .1 + .035 = .135 inch.]
=Washburn and Moen's Standard Wire Gauge.=—Commonly designated
as W. & M. G. has been adopted by the U. S. Steel Corporation
in making their wire.
=New British Standard (N. B. S.).=—British Imperial English
Legal Standard and Standard Wire Gauge, and is variously
abbreviated by S. W. G. and I. W. G.
=Roebling Gauge.=—Washburn Moen, American Steel & Wire Co.'s
Iron Wire Gauge.
[Illustration: FIGS. 824 and 825.—U. S. wireman's calculating gauge;
views showing both sides. On the side shown in fig. 824, set the
required number of feet on the small circle opposite the required
number of amperes on the large circle, then set the small pointer at
the required voltage and loss. Then on the other side (fig. 825) the
large pointer will indicate the required size of wire in B. & S. gauge,
and will also indicate the safe carrying capacity, while the wire may
be gauged by slot A (fig. 824).]
=U. S. Standard Wire Gauge.=—This gauge is used for measuring
sheet and plate iron, and steel, by the U. S. Government in
assessing duties, and in making requisitions for supplies.
=Old English Standard Wire Gauge.=—The old English gauge is
the same as the Birmingham or Stubs' standard gauge, commonly
designated as B. W. G. It is used chiefly for measuring sheet
iron and steel, also soft steel and iron wire.
=London Gauge.=—Old English (_not Old English Standard_).
From the foregoing it is seen that great confusion exists with
such a multiplicity of gauges and emphasizes the importance of
specifying the gauge and of knowing what gauge to use.
In using the gauges known as Stubs' Gauges, there should be
constantly borne in mind the difference between the Stubs' Iron
Wire Gauge and the Stubs' Steel Wire Gauge. The Stubs' Iron
Wire Gauge is the one commonly known as the English Standard
Wire, or Birmingham Gauge and designates the Stubs' _soft_ wire
sizes. The Stubs' Steel Wire Gauge is the one that is used in
measuring drawn steel wire or drill rods of Stubs' make and is
also used by many makers of American drill rods.
=STUBS' STEEL WIRE GAUGE=
——————-+————————-++——————+————————-++——————+————————-++——————+———————-
| Size of || | Size of || | Size of || | Size of
Letter.| Letter ||No. of| Number ||No. of| Number ||No. of| Number
| in || Wire | in || Wire | in || Wire | in
|Decimals.||Gauge.|Decimals.||Gauge.|Decimals.||Gauge.|Decimals.
——————-+————————-++——————+————————-++——————+————————-++——————+———————-
Z | .413 || 1 | .227 || 28 | .139 || 55 | .050
Y | .404 || 2 | .219 || 29 | .134 || 56 | .045
X | .397 || 3 | .212 || 30 | .127 || 57 | .042
W | .386 || 4 | .207 || 31 | .120 || 58 | .041
V | .377 || 5 | .204 || 32 | .115 || 59 | .040
U | .368 || 6 | .201 || 33 | .112 || 60 | .039
T | .358 || 7 | .199 || 34 | .110 || 61 | .038
S | .348 || 8 | .197 || 35 | .108 || 62 | .037
R | .339 || 9 | .194 || 36 | .106 || 63 | .036
Q | .332 || 10 | .191 || 37 | .103 || 64 | .035
P | .323 || 11 | .188 || 38 | .101 || 65 | .033
O | .316 || 12 | .185 || 39 | .099 || 66 | .032
N | .302 || 13 | .182 || 40 | .097 || 67 | .031
M | .295 || 14 | .180 || 41 | .095 || 68 | .030
L | .290 || 15 | .178 || 42 | .092 || 69 | .029
K | .281 || 16 | .175 || 43 | .088 || 70 | .027
J | .277 || 17 | .172 || 44 | .085 || 71 | .026
I | .272 || 18 | .168 || 45 | .081 || 72 | .024
H | .266 || 19 | .164 || 46 | .079 || 73 | .023
G | .261 || 20 | .161 || 47 | .077 || 74 | .022
F | .257 || 21 | .157 || 48 | .075 || 75 | .020
E | .250 || 22 | .155 || 49 | .072 || 76 | .018
D | .246 || 23 | .153 || 50 | .069 || 77 | .016
C | .242 || 24 | .151 || 51 | .066 || 78 | .015
B | .238 || 25 | .148 || 52 | .063 || 79 | .014
A | .234 || 26 | .146 || 53 | .058 || 80 | .013
| || 27 | .143 || 54 | .055 || |
——————-+————————-++——————+————————-++——————+————————-++——————+———————-
The following table gives the diameters, in decimal parts of an inch,
of the various sizes of wire corresponding to the number of gauge
numbers of the different standard wire gauges used in the United
States.
=TABLE OF VARIOUS WIRE GAUGES=
=In decimal parts of an inch=
————————+————————-+————————-+——————————+——————-+——————-+——————-+——————-
Number | Ameri- | Birming-| Wash- | Tren- |G.W. | Old | Brit-
of | can, or | ham, or | burn & | ton | Pren- | Eng- | ish
Wire | Brown & | Stubs | Moen | Iron | tiss, | lish, | Stan-
Gauge | Sharpe | (B.W.G.)| Mfg. Co.,| Co., | Holy- | From | dard
| (B.&S.) | | Worces- | Tren- | oke, | Brass | (S.W.-
| | | ter, | ton, | Mass. | Mfrs' | G.)
| | | Mass. | N.J. | | List |
————————+————————-+————————-+——————————+——————-+——————-+——————-+——————-
0000000 | | | | | | | .500
000000 | | | .460 | | | | .464
00000 | | | .430 | .450 | | | .432
0000 | .46000 | .454 | .393 | .400 | | | .400
000 | .40964 | .425 | .362 | .360 | .3586 | | .372
00 | .36480 | .380 | .331 | .330 | .3282 | | .348
0 | .32486 | .340 | .307 | .305 | .2994 | | .324
1 | .28930 | .300 | .283 | .285 | .2777 | | .300
2 | .25763 | .284 | .263 | .265 | .2591 | | .276
3 | .22942 | .259 | .244 | .245 | .2401 | | .252
4 | .20431 | .238 | .225 | .225 | .2230 | | .232
5 | .18194 | .220 | .207 | .205 | .2047 | | .212
6 | .16202 | .203 | .192 | .190 | .1885 | | .192
7 | .14428 | .180 | .177 | .175 | .1758 | | .176
8 | .12849 | .165 | .162 | .160 | .1605 | | .160
9 | .11443 | .148 | .148 | .145 | .1471 | | .144
10 | .10189 | .134 | .135 | .130 | .1351 | | .128
11 | .090742 | .120 | .120 | .1175 | .1205 | | .116
12 | .080808 | .109 | .105 | .1050 | .1065 | | .104
13 | .071961 | .095 | .0920 | .0925 | .0928 | | .0920
14 | .064084 | .083 | .0800 | .0800 | .0816 | .08300| .0800
15 | .057068 | .072 | .0720 | .0700 | .0726 | .07200| .0720
16 | .050820 | .065 | .0630 | .0610 | .0627 | .06500| .0640
17 | .045257 | .058 | .0540 | .0525 | .0546 | .05800| .0560
18 | .040303 | .049 | .0470 | .0450 | .0478 | .04900| .0480
19 | .035890 | .042 | .0410 | .0400 | .0411 | .04000| .0400
20 | .031961 | .035 | .0350 | .0350 | .0351 | .03500| .0360
21 | .028462 | .032 | .0320 | .0310 | .0321 | .03150| .0320
22 | .025347 | .028 | .0280 | .0280 | .0290 | .02950| .0280
23 | .022571 | .025 | .0250 | .0250 | .0261 | .02700| .0240
24 | .020100 | .022 | .0230 | .0225 | .0231 | .02500| .0220
25 | .017900 | .020 | .0200 | .0200 | .0212 | .02300| .0200
26 | .015940 | .018 | .0180 | .0180 | .0194 | .02050| .0180
27 | .014195 | .016 | .0170 | .0170 | .0182 | .01875| .0164
28 | .012641 | .014 | .0160 | .0160 | .0170 | .01650| .0148
29 | .011257 | .013 | .0150 | .0150 | .0163 | .01550| .0136
30 | .010025 | .012 | .0140 | .0140 | .0156 | .01375| .0124
31 | .008928 | .010 | .0130 | .0130 | .0146 | .01225| .0116
32 | .007950 | .009 | .0120 | .0120 | .0136 | .01125| .0108
33 | .007080 | .008 | .0110 | .0110 | .0130 | .01025| .0100
34 | .006305 | .007 | .0100 | .0100 | .0118 | .00950| .0092
35 | .005615 | .005 | .0095 | .0095 | .0109 | .00900| .0084
36 | .005000 | .004 | .0090 | .0090 | .0100 | .00750| .0076
37 | .004453 | | .0085 | .0085 | .0095 | .00650| .0068
38 | .003965 | | .0080 | .0080 | .0090 | .00575| .0066
39 | .003531 | | .0075 | .0075 | .0083 | .00500| .0052
40 | .003145 | | .0070 | .0070 | .0078 | .00450| .0048
41 | | | | | | | .0044
42 | | | | | | | .0040
————————+————————-+————————-+——————————+——————-+——————-+——————-+——————-
NOTE.—The sizes of wire are ordinarily expressed by an
arbitrary series of numbers. Unfortunately there are several
independent numbering methods, so that it is always necessary
to specify the method or wire gauge used. The above table gives
the numbers and diameters in decimal parts of an inch for the
various wire gauges in general use.
=Wiring Terms.=—The various members of a complex wiring installation
are designated feeders, sub-feeders, mains, branches, and taps.
A _feeder_ is a stretch of wiring to which no connection is made except
at its two ends.
A _sub-feeder_ is of the same class as a feeder, but is distinguished
either by being one of two or more connecting links between the end of
a single feeder and several distributing mains, or by constituting an
extension of a feeder.
[Illustration: FIG. 826. Circuit diagram illustrating names of the
various parts. A circuit may consist of the following parts as defined
in the accompanying text: 1, feeder, 2, sub-feeders, 3, mains, 4,
branches, 5, taps. It is well to clearly distinguish between these
divisions because the terms are constantly used in wiring.]
A _main_ is a stretch of wiring supplied from one or more feeders or
sub-feeders and distributing current to a number of taps, or else to a
number of branches.
A _branch_ distributes current among a number of lamps, etc.
A _tap_ almost invariably delivers current to a single lamp or other
device.
Reference to fig. 826 will make these definitions clearer. This
diagram is intended merely to illustrate the above definitions
and does not represent any special plan of wiring.
[Illustration: FIGS. 827 and 828. Simplest forms of circuit, consisting
of a main with one or more lamps at the end. The smallest size wire
allowed (No. 14 B.&S. gauge) will generally be found amply large for
such circuits. Note carefully the difference between a main and a
branch by comparison with fig. 826. A main begins from a fuse block,
while a branch is an offset from a main without any fuse block.]
The simplest possible wiring installation is one in which a
single lamp or compact cluster of lamps is located at the
end of a main, as shown in figs. 827 and 828. In such cases
calculations are almost always unnecessary, for the reason that
No. 14 wire, the smallest size allowed by the underwriters,
will supply several lamps at a long distance (as interior
wiring goes) with a very moderate drop. For example, if the
three lamps shown at the end of the main in fig. 828, be of 16
candle power each, and the voltage of the supply circuit be
110 volts, a main of No. 14 wire would supply the lamps at a
distance of 135 feet from the fuse block with a drop of only 1
per cent.
When the lamps are strung along the main, however, as in fig.
826, it is sometimes necessary to choose the size of wire with
regard to the drop, and in order to do this the main must be
measured for either "ampere feet" or "lamp feet."
=Wire Calculations.=—The problem of calculating the size of wire will
be presented here in as simple a form as possible, with explanation of
the various steps so that any one can understand how the formula is
derived.
In determining the size of wire, there are four known factors which
enter into the calculation, viz.:
1. Length of circuit in feet;
2. Maximum current in amperes;
3. Drop or volts lost in the circuit, _in % of the impressed voltage_;
4. Heating effect of the current.
The calculation is based on the _mil foot_, which as previously
explained, is a foot of copper wire one mil in diameter and whose
resistance is equal to 10.79 ohms at 75° Fahr.
[Illustration: FIG. 829.—Wiring for lights requiring unusually long
feeders.]
The first step is to find an expression for the resistance of the wire
which may be later substituted in Ohm's law formula. Accordingly, the
resistance of any conductor is equal to _its length in feet multiplied
by its resistance per mil foot and the product divided by its area in
circular mils_, thus:
length in feet × resistance per mil foot
resistance in ohms = —————————————————————————————————————————
circular mils
feet × 10.8
or ohms = ————————————- (1)
circular mils
(calling the resistance per mil foot 10.8 instead of 10.79 to
facilitate calculation).
LAMP TABLE FOR RUBBER COVERED WIRES
Showing the maximum number of 16 candle power 110 to 240 volt
lamps in parallel that may be carried by the various sizes of
wire without violating the underwriters' rules.
+——————————+———————+——————————————+————————————————+————————————-+
|Wire size |Amperes|3.1-watt lamps|3.5-watt lamps. |4-watt lamps.|
|B. & S. | +——————————————+————————————————+————————————-+
| gauge | | At 110 220 | At 110 220 | 220 230 240 |
| | | volts V. | volts V. | V. V. V. |
+——————————+———————+——————————————+————————————————+—————————————+
|0000 | 210 | 462 924 | 412 825 | 722 754 787 |
| 000 | 177 | 389 778 | 347 695 | 608 636 663 |
| 00 | 150 | 330 660 | 294 589 | 515 539 562 |
| | | | | |
| 0 | 127 | 279 558 | 249 499 | 436 456 476 |
| 1 | 107 | 235 470 | 210 420 | 367 384 401 |
| 2 | 90 | 197 396 | 176 353 | 309 323 337 |
| | | | | |
| 3 | 76 | 167 334 | 149 298 | 261 273 285 |
| 4 | 65 | 143 286 | 127 255 | 223 233 243 |
| 5 | 54 | 118 237 | 106 212 | 185 194 202 |
| | | | | |
| 6 | 46 | 101 202 | 90 180 | 158 165 172 |
| 8 | 33 | 72 145 | 64 129 | 113 118 123 |
| 10 | 24 | 52 105 | 47 94 | 82 86 90 |
| | | | | |
| 12 | 17 | 37 74 | 33 66 | 58 61 63 |
| 14 | 12 | 26 52 | 23½ 47 | 41 43 45 |
| 16[3] | 6 | 13 .. | 11 .. | 20 21 22 |
+——————————+———————+——————————————+————————————————+—————————————+
[3] This size can be used only in the shape of flexible cord.
Now, according to Ohm's law,
volts = amperes × ohms (2)
hence, substituting in (2) the value for the resistance in ohms, as
obtained in (1):
feet × 10.8
volts = amperes × ————————————-
circular mils
or using the usual symbols
feet × 10.8
E = I × ————————————- (3)
circular mils
or expressed in words, formula (3) means that the volts lost or
_drop_ between the beginning and end of a circuit is equal to the
current flowing through the circuit multiplied by the product of the
conductors' length in feet multiplied by the resistance of one mil foot
of wire, divided by the area of the conductor in circular mils.
LAMP TABLE FOR WEATHER PROOF WIRES
Showing the maximum number of 16 candle power 120 to 240 volt
lamps in parallel that may be carried by various sizes of
weather proof wire without violating the underwriters' rules.
+———————+————————+———————————————+———————————————+———————————————-+
|Wire |Amperes |3.1-watt lamps.|3·5-watt lamps.| 4-watt lamps. |
|size | +——————————————-+——————————————-+————————————————|
|B. & S.| | 110 220 | 110 220 | 220 230 240 |
|gauge | | V. V. | V. V. | V. V. V. |
+———————+————————+———————+———————+————————+——————+———————————————-+
| 0000 | 312 | 686 | 1372 | 612 | 1225 | 1072 1121 1170 |
| 000 | 262 | 576 | 1152 | 514 | 1029 | 900 941 982 |
| 00 | 220 | 484 | 968 | 432 | 864 | 756 790 825 |
| | | | | | | |
| 0 | 185 | 407 | 814 | 363 | 726 | 636 665 693 |
| 1 | 156 | 343 | 686 | 306 | 612 | 536 560 585 |
| 2 | 131 | 288 | 576 | 257 | 514 | 450 470 491 |
| | | | | | | |
| 3 | 110 | 242 | 484 | 216 | 432 | 378 395 412 |
| 4 | 92 | 202 | 404 | 180 | 361 | 316 330 345 |
| 5 | 77 | 169 | 338 | 151 | 302 | 264 276 288 |
| | | | | | | |
| 6 | 65 | 143 | 286 | 127 | 255 | 223 233 243 |
| 8 | 46 | 101 | 202 | 90 | 180 | 158 165 172 |
| 10 | 32 | 70 | 140 | 62 | 125 | 110 115 120 |
| | | | | | | |
| 12 | 23 | 50 | 101 | 45 | 90 | 79 82 86 |
| 14 | 16 | 35 | 70 | 31 | 62 | 55 57 60 |
+———————+————————+———————+———————+————————+——————+———————————————-+
Now, since the length of the circuit is given as the "run" or distance
one way, that is, one half the total length of wire in the circuit,
formula (3) must be multiplied by 2 to get the total drop, that is:
feet × 10.8 X 2 I × feet × 21.6
E = I × ——————————————- = ———————————————- (4)
circular mils circular mills
Solving the last equation for the unknown quantity, the following
equation is obtained for size of wire:
[Illustration: FIGS. 830 and 831.—Symmetrical and unsymmetrical
distribution. When a main is supplied by a feeder, the junction of the
two, if practicable, is located at the electrical center of the main,
as indicated in fig. 830, so that the distribution is symmetrical, that
is, the ampere feet each way from the junction are the same. This is
nearly always practicable in surface wiring, and when it is practiced
it is only necessary to calculate the wire size for one-half of the
main, as the other half is identical. In fig. 830 there are four lamps
on each side of the junction, J; the center of each group is at a
distance, M, so that the lamp feet in each half of the main are 5 × M.
The lamp feet of the feeder would be 10 × N, N being the distance from
the feeder fuse block to the junction, J. In concealed work, however,
it does not always happen that a feeder can be made to join a main at
its electrical center; when this is not practicable, each end of the
main should be figured separately. In fig. 831, for instance, the main
has five lamps on one side and two on the other, and the distances from
the junction to the centers of the two groups are at unequal distances
S and S'. If the distance S be 14 feet, and the lamps, 16 c. p., the
lamp feet in the left hand main equals 5 × 14 = 70, while in the main
to the right, taking S' at 10 feet, there are only 2 × 10 = 20 lamp
feet. Hence what appears to be one continuous main in this case would
have to be treated as two mains, and each part figured separately.]
I × feet × 21.6 amperes × feet × 21.6
circular mils = ———————————————- = ————————————————————— (5)
E "drop"
The following practical example is given to illustrate the application
of the formula just obtained:
EXAMPLE.—What size wire should be used on a 250 volt circuit
to transmit a current of 200 amperes a distance of 350 feet to
a center of distribution with a loss of three per cent. under
full load?
The volts lost or drop is equal to 250 × .03 = 7.5 volts.
=PROPERTIES OF COPPER WIRE=
+————————+————————+——————————+—————————————————————————————+———————————————————————-+
| Number |Diameter| Area in | Weight in pounds | Resistance at 68° Fahr.|
|of gauge| in | circular |——————————————————————————————————————————————————————|
|B. & S. | mils | mils | 1,000 feet| mile |feet per| 1,000 feet | mile |
| | | | | | pound | | |
+————————+————————+——————————+———————————+—————————————————+———————————————————————-+
| 0000 | 460 | 211,600 | 640.5 |3,381 | 1.561 | .04893 | .2583 |
| 000 | 409.6 | 167,800 | 508 |2,682 | 1.969 | .06170 | .3258 |
| 00 | 364.8 | 133,100 | 402.8 |2,127 | 2.482 | .07780 | .4108 |
| 0 | 324.9 | 105,500 | 319.5 |1,687 | 3.130 | .09811 | .5180 |
| | | | | | | | |
| 1 | 289.3 | 83,690 | 253.3 |1,337 | 3.947 | .12370 | .6531 |
| 2 | 257.6 | 66,370 | 200.9 |1,062 | 4.977 | .1560 | .8237 |
| 3 | 229.4 | 52,630 | 159.3 | 841.1 | 6.276 | .1967 | 1.0386 |
| 4 | 204.3 | 41,740 | 126.4 | 667.4 | 7.914 | .2480 | 1.3094 |
| | | | | | | | |
| 5 | 181.9 | 33,100 | 100.2 | 529.0 | 9.980 | .3128 | 1.6516 |
| 6 | 162.0 | 26,250 | 79.46 | 419.5 | 12.580 | .3944 | 2.0824 |
| 7 | 144.3 | 20,820 | 63.02 | 332.7 | 15.87 | .4973 | 2.6257 |
| 8 | 128.5 | 16,510 | 49.98 | 263.9 | 20.01 | .6271 | 3.3111 |
| | | | | | | | |
| 9 | 114.4 | 13,090 | 39.63 | 209.2 | 25.23 | .7908 | 4.1754 |
| 10 | 101.9 | 10,380 | 31.13 | 166.0 | 31.82 | .9972 | 5.2652 |
| 11 | 90.74| 8,234 | 24.93 | 131.6 | 40.12 |1.257 | 6.6370 |
| 12 | 80.81| 6,530 | 19.77 | 104.4 | 50.59 |1.586 | 8.374 |
| | | | | | | | |
| 13 | 71.96| 5,178 | 15.68 | 82.79| 63.79 |2.000 | 10.560 |
| 14 | 64.08| 4,107 | 12.43 | 65.63| 80.44 |2.521 | 13.311 |
| 15 | 57.07| 3,257 | 9.858 | 52.05|101.4 |3.179 | 16.785 |
| 16 | 50.82| 2,583 | 7.818 | 41.28|127.9 |4.009 | 21.168 |
| | | | | | | | |
| 17 | 45.26| 2,048 | 6.200 | 32.74|161.3 |5.055 | 26.690 |
| 18 | 40.30| 1,624 | 4.917 | 25.96|203.4 |6.374 | 33.655 |
| 19 | 35.89| 1,288 | 3.899 | 20.59|256.5 |8.038 | 42.440 |
| 20 | 31.96| 1,022 | 3.092 | 16.33|323.4 |10.14 | 53.540 |
+————————+————————+——————————+———————————+—————————————————+———————————————————————-+
Substituting the given value in formula (5)
350 × 200 × 21.6
circular mils = —————————————————— = 201,600.
7.5
Diameter = 2 √201,600 = 449 circular mils or .449 in.
From the table (on page 731 or on page 742) the nearest
(=larger=) size of wire is 0000 B. & S. gauge.[4]
[4] =CAUTION.=—The size thus obtained should be compared with the table
of carrying capacity of wires as given on page 731 to see if the wires
would have to carry more than the allowable current.
WIRING TABLE FOR LIGHT AND POWER CIRCUITS
===================+=======================================================
VOLTS | PERCENTAGE OF LOSS
===================+=======================================================
2000 | 1.7 1.5 1.4 1.2 1.1 1.0 0.75 0.5
1000 | 3.4 2.9 2.7 2.4 2.2 2.0 1.5 1.0
500 | 6.5 5.7 6.2 4.8 4.3 3.9 2.9 2.0
220 | 13.7 12.0 11.0 10.3 9.3 8.3 6.5 4.4
110 | — — 20.0 18.5 17.0 15.4 12.0 8.4
52 | — — — — — — 22.4 16.1
===================+=======================================================
ACTUAL VOLTS LOST
=========+=========+=======================================================
Carrying | |
Capacity | Size | 35 30 27.5 25 22.5 20 15 10
Amperes. | B. & S. |
=========+=========+=======================================================
300 | 0000 | 345800 296400 271700 247000 222300 197600 148200 98800
245 | 000 | 274400 235200 215600 196000 176400 156800 117600 78400
215 | 00 | 217525 186450 170912 155375 139837 124300 93225 62150
190 | 0 | 172550 147900 135575 123250 110925 98600 73950 49300
160 | 1 | 136850 117300 107525 97750 87975 78200 58650 39100
135 | 2 | 108500 93000 85250 77500 69750 62000 46500 31000
115 | 3 | 86100 73800 67650 61500 55350 49200 36900 24600
100 | 4 | 68250 58500 53625 48750 43875 39000 29250 19500
90 | 5 | 54250 46500 42625 38750 34875 31000 23250 15500
80 | 6 | 43050 36900 33825 30750 27675 24600 18450 12300
60 | 8 | 26965 23130 21202 19275 17347 15420 11565 7710
40 | 10 | 16975 14550 13337 12125 10912 9700 7275 4850
30 | 12 | 10675 9150 8388 7625 6862 6100 4575 3050
22 | 14 | 6720 5760 5280 4800 4320 3840 2880 1920
5[5] | 16 | 4235 3630 3328 3025 2723 2420 1815 1210
————————-+————————-+——————————————————————————————————————————————————————-
===================+=====================================================
VOLTS | PERCENTAGE OF LOSS
===================+=====================================================
2000 | 0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05
1000 | 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1
500 | 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2
220 | 3.9 3.5 3.1 2.7 2.2 1.8 1.4 0.9 0.45
110 | 7.6 6.8 6.0 5.2 4.4 3.5 2.7 1.8 0.9
52 | 14.7 13.3 11.8 10.3 8.8 7.1 5.5 3.7 1.9
===================+=====================================================
ACTUAL VOLTS LOST
=========+=========+=====================================================
Carrying | |
Capacity | Size | 9 8 7 6 5 4 3 2 1
Amperes. | B. & S. |
=========+=========+=====================================================
300 | 0000 | 88920 79040 69160 59280 49400 39520 29640 19760 9880
245 | 000 | 70560 62720 54880 47040 39200 31360 23520 15680 7840
215 | 00 | 55935 49720 43505 37290 31075 24860 18645 12430 6215
190 | 0 | 44370 39440 34510 29580 24650 19720 14790 9860 4930
160 | 1 | 35190 31280 27370 23460 19550 15640 11730 7820 3910
135 | 2 | 27900 24800 21700 18600 15500 12400 9300 6200 3100
115 | 3 | 22140 19680 17220 14760 12300 9840 7380 4920 2460
100 | 4 | 17550 15600 13650 11700 9750 7800 5850 3900 1950
90 | 5 | 13950 12400 10850 9300 7750 6200 4650 3100 1550
80 | 6 | 11070 9840 8610 7380 6150 4920 3690 2460 1230
60 | 8 | 6939 6168 5397 4626 3855 3084 2313 1542 771
40 | 10 | 4365 3880 3395 2910 2425 1940 1455 970 486
30 | 12 | 2745 2440 2135 1830 1525 1220 915 610 305
22 | 14 | 1728 1536 1344 1152 960 768 576 384 192
5[5] | 16 | 1089 968 847 726 605 484 363 242 121
————————-+————————-+————————————————————————————————————————————————————-
[5] NOTE.—In case a larger loss than any given in the table is
required, proceed as follows:—Divide the ampere feet by 10 and then
refer to column of Actual Volts Lost divided by 10, from which the size
of wire is found as before.
RULE.—_Multiply current in amperes by single distance and refer to the
nearest corresponding number under column of actual volts lost, to find
size of wire._ It should also be noted that the underwriters prohibit
the use of wire smaller than No. 14 B. & S. gauge, except as allowed
for fixture work and pendant cord.
TABLE OF WIRE EQUIVALENTS
GAUGE NUMBER OF WIRES
B. & S. 2 4 8 16 32 64 128 256 512 1024 2048 4096 8192 16384
0000 0 3 6 9 12 15 18 21 24 27 30 33 36 39
000 1 4 7 10 13 16 19 22 25 28 31 34 37 40
_____
00 2 5 8 11 14 17 20 23 26 29 32 35 38| 0+6
0 3 6 9 12 15 18 21 24 27 30 33 36 39| 1+7
1 4 7 10 13 16 19 22 25 28 31 34 37 40| 2+8
_____|
2 5 8 11 14 17 20 23 26 29 32 35 38|3+9 4+6
3 6 9 12 15 18 21 24 27 30 33 36 39|4+10 5+7
4 7 10 13 16 19 22 25 28 31 34 37 40|5+11 6+8
_____|
5 8 11 14 17 20 23 26 29 32 35 38|6+12 7+9
6 9 12 15 18 21 24 27 30 33 36 39|7+13 8+10
7 10 13 16 19 22 25 28 31 34 37 40|8+14 9+11
_____|
8 11 14 17 20 23 26 29 32 35 38|9+15 10+12
9 12 15 18 21 24 27 30 33 36 39|10+16 11+13
10 13 16 19 22 25 28 31 34 37 40|11+17 12+14
_____|
11 14 17 20 23 26 29 32 35 38|12+18 13+15
12 15 18 21 24 27 30 33 36 39|13+19 14+16
13 16 19 22 25 28 31 34 37 40|14+20 15+17
____|
14 17 20 23 26 29 32 35 38|15+21 16+18
15 18 21 24 27 30 33 36 39|16+22 17+19
16 19 22 25 28 31 34 37 40|17+23 18+20
____|
17 20 23 26 29 32 35 38|18+24 19+21
18 21 24 27 30 33 36 39|19+25 20+22
19 22 25 28 31 34 37 40|20+26 21+23
____|
20 23 26 29 32 35 38|21+27 22+24
21 24 27 30 33 36 39|22+28 23+25
22 25 28 31 34 37 40|23+29 24+26
____|
23 26 29 32 35 38|24+30 25+27
24 27 30 33 36 39|25+31 26+28
25 28 31 34 37 40|26+32 27+29
____|
26 29 32 35 38|27+33 28+30
27 30 33 36 39|28+34 29+31
28 31 34 37 40|29+35 30+32
____|
29 32 35 38|30+36 31+33
30 33 36 39|31+37 32+34
31 34 37 40|32+38 33+35
____|
32 35 38|33+39 34+36
33 36 39|34+40 35+37
34 37 40|36+39
___|
35 38|37+39
36 39|38+40
37 40|
=Ques. If the calculated size of wire be larger than any in the table
how is the required area obtained?=
Ans. By using two or more smaller wires in parallel, whose combined
area is equal to the required area.
To facilitate finding the equivalent sizes the above table of
wire equivalents has been prepared.
=Ques. How is the table of wire equivalents used?=
Ans. To use the table, find in the vertical column at left the size
of conductor desired; then follow along horizontally until the size
of wire that is desired to use for the strands, and the corresponding
number at top of column will give the number of strands of that size
wire required.
[Illustration: FIG. 832.—Break down switch for use on three wire
circuit, enabling it to be operated break down fashion with the two
outers connected together and the neutral wire serving as one side
of the resultant two wire circuit. Such circuits must be figured as
two wire installations of one half the three wire voltage. The size
of the neutral wire of a three wire circuit depends on conditions of
operation. Three wire circuits for occasional two wire working, must
have a neutral wire whose cross section is equal to the combined cross
sections of the two outer wires. This plan is useful for buildings
supplied from a central station, as it will be satisfactory for two
wire operation in emergencies, and for three wire, two phase or three
phase distribution should the central station ultimately be changed
over to either of those alternating current systems. The expense for
the extra copper in the beginning will not be nearly so great as that
entailed by a change in the wiring later on should developments require
it. It is permissible, however, to make the cross section of the
neutral wire smaller than that of each outer wire, if one be reasonably
sure that there will never be any changes such as those just mentioned,
and if the drop in the two outer wires do not exceed 1½ per cent.
Under such conditions, it will be found a very good rule to calculate
the neutral wire of a principal feeder for a maximum unbalancing of 25
per cent, that is, a condition under which the current in one outer
wire will be 75 per cent of the current in the other one, the current
in the neutral being 25 per cent of that in the heavier loaded outer
wire.]
=Ques. What is the significance of the zig-zag line?=
Ans. The figures below this line give the gauge numbers of two wires
which will have the same conductivity as the corresponding conductor in
left hand column.
TABLE OF CABLE CAPACITIES
[Illustration: FIG. 833.—Diagram showing capacities of cables for both
open and concealed work as allowed by the underwriters.]
=Incandescent Lamps on 660 Watt Circuits.=—The standard incandescent
lamp is rated as equivalent to the light given by 16 candles, and may
consume, according to type and make, from 50 to 56 watts, or from 3.1
to 3.5 watts per candle power. Therefore, a 660 watt circuit will carry
thirteen 16 candle power 49.6 watt lamps, or eleven 56 watt lamps.
[Illustration: FIG. 834.—Diagram showing symmetrical and unsymmetrical
distribution. The two 5 lamp centers are located at equal distances
from the distributing pocket or cabinet, P, so that the sub-feeders,
A and A', have equal values of lamp feet. The sub-feeders, B, B',
have equal lengths, but as one supplies 10 lamps, and the other 16,
the lamp feet are different, and each sub-feeder must therefore be
figured separately. The main, G, should be considered as a part of
the sub-feeder, B, in order to avoid the necessity for a fuse at the
junction of the two. As it is symmetrically divided, only one-half of
it would be considered. Thus, if the sub-feeder, B, were 50 feet long,
and the main, G, 30 feet long, B would have 16 × 50 = 800 lamp feet and
one-half of the main would have 8 × 15 = 120 lamp feet (assuming all
16 c. p. lamps). Hence 800 + 120 = 920 lamp feet should be taken as
the load length and the proper size wire used for that figure, making
the sub-feeder, B, of the same size as the main, G. The same procedure
applies to the sub-feeder B' and main G'; also to the sub-feeder, E,
and the main, F. The proper sizes of wire for the different circuits is
easily found from the lamp feet table, after having calculated the lamp
feet assigned the drop.]
[Illustration: FIGS. 835 and 836.—The "tree" and "modified tree"
systems of wiring. The tree system consists of a feeder reducing in
size and supplying mains for each floor, the general arrangement
resembling the trunk and branches of a tree. Since fuses must be
inserted on each floor where the size of the feeder is reduced, the
system requires a large number of joints, and in the event of a fuse
blowing it could not be quickly located. The tree system is not to be
recommended, as it results in considerable drop, and at full load the
lamps nearest the point of supply will either burn too brightly or
those more remote will not give the rated candle power. In the modified
tree system, fig. 836, the size of the feeder is not reduced. With
this arrangement the losses are considerably reduced owing to the much
smaller losses on the feeder between those centers farthest away from
the point of supply.]
The proper size of wire for a 660 watt circuit will depend upon
the voltage for which the lamps are made. For example: a 16
candle power lamp which consumes 56 watts on 110 volt circuit
will take, 56 ÷ 110 = .5 or ½ ampere of current, while the
same lamp, if made for 220 volts, will take only 55 ÷ 220 = .25
or ¼ ampere. Therefore, eleven 16 candle power 56 watt lamps
will require a current of 5½ amperes at 110 volts, or 2¾
amperes at 220 volts.
According to the laws of resistance, the resistance of a round
wire is inversely proportional to the square of the diameter,
and if the circuit be taken at 100 feet, and the allowable
percentage of drop at 1 volt, then according to formula, (5)
on page 748, the wire required for a circuit carrying eleven
16 candle power 56 watt 110 volt lamps, will have a cross
sectional area of,
5.5 × 100 × 21.6
———————————————— = 11,880 circular mils.
1
while the same number of lamps on a 220 volt circuit will
require wire having a cross sectional area of,
2.75 × 100 × 21.6
————————————————- = 5,940 circular mils.
1
In order to conform to the underwriters' requirements, No. 8.
B. & S. gauge, wire must be used for the circuit carrying the
110 volt lamps, while No. 12, B. & S. wire, would be sufficient
for the 220 volt circuit.
In the case shown in fig. 829, the branch circuits A and B are
identical, each supplying four 16 candle power lamps requiring
3.5 watts per candle power at 110 volts or carrying a load of 4
× 16 × 3.5 = 224 watts, = 224 ÷ 110 = 2 _amperes_.
[Illustration: FIG. 837.—Distribution with sub-feeders (multi-center
distribution). The feeder connects at a central point, A, with several
sub-feeders which run to distributing centers, as at B, C, D, and E.
With this arrangement, compound wound dynamos may be so designed that
the pressure at A will remain nearly uniform for all loads. If, for
instance, the wiring be proportioned for 2% drop, the dynamos may
be over compounded to that extent, and the even illumination will
compensate for the extra cost in the installation.]
The distance from the feeder junction or cut out to the
electrical center of each branch circuit is 12.5 _feet_. The
compact area of distribution permits the reduction of the
loss of volts to 1 per cent, or 110 × .01 = 1.1 volts "drop."
Then substituting in formula (5) on page 748 the values for
_amperes_, _feet_ and _drop_ as obtained above
2 × 25 × 21.6
————————————- = 981 circular mils,
1.1
or a value far below that of even No. 18 wire, B. & S. gauge
(see table on page 731), but the smallest wire allowed by the
underwriters for the mains A and B is No. 14, B. & S. gauge.
In calculating the size of wire for the feeders the total load
must be considered. This is equal to eight 16 candle power
lamps, requiring 3.5 watts per candle power at 110 volts =
8×16×3.5 = 448 watts = 4 amperes.
The distance from the entrance cut out to the feeder cut out is
200 feet. The drop should not be greater than 1.5 per cent. or
110×1.5 = 1.6 volts. Then,
4×200×21.6
—————————— = 10,800 circular mils
1.6
a value which indicates that No. 8 wire, B. & S. gauge, must be
used for the feeders in order to keep the drop within the limit
of predetermined value.
TABLE FOR TAPS, BRIDGES OR OTHER WIRES AT NEGLIGIBLE DROP
—————————————————————————————————————————————————————————————————————————
Wire Nos. |0 |1 |2 |3 |4 |5 |6 | 7 |8 |10 |12 |14|16|18
—————————————————————————————————————————————————————————————————————————
Lamp Feet|52 v.| 300| 260|200|160|130|100| 80| 65| 50| 38| 24|15| 9| 6
|110v.|1,280|1,085|860|680|560|435|345|280|220|160|100|60|40|25
—————————————————————————————————————————————————————————————————————————
NOTE.—In using this table, it is only necessary to
calculate the lamp feet of the tap and take the size of wire
corresponding to the nearest greater number of lamp feet in
the table. The lamp feet specified by this table should not be
exceeded by more than 10 per cent. Thus, if a tap measure 108
lamp feet, in 110 volt lamps, No. 12 wire would be used. But if
it measure 115 lamp feet, it would be advisable to use No, 10
wire.
=Constant Voltage Arc Lamp Circuits.=—The branch conductor should
have a carrying capacity about fifty per cent. greater than the normal
current required by the lamp, so as to provide for the heavy current
required when the lamp is started. The underwriters prohibit the use of
any size wire under No. 12 for parallel connected arc light circuits.
=Constant Current Series Arc Lamp Circuits.=—The wiring for series
connected arc lamps should never be concealed nor encased unless
requested by the electrical inspector.
For all interior wiring of this class, approved rubber covered wire
should be used, and the wire should always be rigidly supported on
porcelain or glass insulators which will hold the wires at a distance
of at least one inch from the surface wired over. The wires on all
circuits up to 750 volts, should be kept at least 4 inches from each
other, and 8 inches apart on circuits of over 750 volts. No wires
carrying a current having a pressure exceeding 3,500 volts should
be carried into or over any building except central stations and
sub-stations.
[Illustration: FIG. 838.—Diagram showing "bridge wiring." This method
is used in the case of two parallel mains where one feeder is ample for
both. The feeder is run to a central point as shown and connected to
the two mains by a so called "bridge." The arrangement clearly gives
good distribution and effects a saving in copper and labor, for if the
bridge were omitted, two feeders would be necessary.]
=Wire Calculations for Motors.=—The proper size of wire for a motor
may be readily determined by means of the following formula:
H.P. × 746 × D × 21.6
circular mils = ————————————————————- (6)
E × L × K
in which
H.P. = horse power of motor;
746 = watts per H.P.;
D = length of motor circuit from fuse block to motor;
21.6 = ohms per foot run in circuit where wires are one mil in diameter;
E = voltage at the motor;
L = drop in percentage of the voltage at the motor;
K = efficiency of the motor expressed as a decimal.
The average values for K are about as follows: 1 H.P., .75; 3
H.P., .80; 5 H.P., .80; 10 H.P. and over, 90 per cent.
[Illustration: FIGS. 839 and 840.—Wrong and right methods of loop
wiring. In general, when a large percentage of loss is allowed with
lamps at short distances, the size of wire, calculated simply in
accordance with the resistance rules, will be found too small to carry
the current safely. This fact is often overlooked, and even though
wires may have been correctly calculated for a uniform percentage
of loss, they will become painfully hot simply because the table of
carrying capacity was not consulted. The cross connection of mains
wherever possible, for the purpose of equalizing the pressure, will
also often reduce the heating effects of the current. An example of
this is shown in the above figures. A circle of lights was wired as
in fig. 839, and after the current had been turned on, the wires of
the circle became hot, and there was quite a perceptible difference of
candle power between the lights near A and those near B. Investigation
disclosed the fact that the loop, contrary to instructions, had been
left open. A few inches of wire as shown in dotted lines remedied the
fault. A better arrangement, however, is shown in fig. 840.]
EXAMPLE.—What is the proper size of wire for a 10 H.P. motor,
run at 220 volts, allowable drop 2 per cent. on 200 foot
circuit.
Substituting the given values in the formula on page 758:
10 × 746 × 200 × 21.6
Circular mils = ————————————————————- = 36,991.
220 × 4.4 × .9
The nearest larger value to this result, in the table of
carrying capacities of copper wire (page 731), is 41,740,
corresponding to No. 4 wire, B. & S. gauge.
_In all cases the size of the wire thus formed should be
compared with that allowed by the underwriters for full load
current of motor, plus 25 per cent. of that current_, and if
the size calculated happen to be smaller than the allowable
size, it should be increased to the latter, otherwise it will
not pass inspection.
TABLE OF AMPERES PER MOTOR
——————————————————————————————————————————————————————————————————————————————-
H.P. Per Cent. Eff. Watts Input 50 Volts 100 Volts 220 Volts 500 Volts
¾ 70 800 16 7 4 2
1½ 70 1600 32 15 7 3
3 75 2980 60 27 14 6
5 80 4660 93 42 21 9
7½ 85 6580 132 60 30 13
10 85 8780 176 80 40 18
15 85 13200 264 120 60 26
20 85 17600 352 160 80 35
25 85 21900 438 199 100 44
30 90 24900 498 226 113 50
40 90 33200 664 301 151 66
50 90 41400 828 376 188 83
60 90 49700 994 452 226 99
70 90 58000 1160 527 264 116
80 90 66300 1330 608 302 133
90 90 74600 1490 678 339 149
100 90 82900 1660 755 377 166
120 90 99500 1990 905 453 199
150 90 124000 2480 1130 564 248
——————————————————————————————————————————————————————————————————————————————-
TABLE OF AMPERES PER DYNAMO
——————————————————————————————————————————————————————————————————————————————-
Appx. Appx.
K.W. 125 Vs. 250 Vs. 500 Vs. H.P. K.W. 125 Vs. 250 Vs. 500 Vs. H.P.
1. 8 4 2 1.3 30. 240 120 60 40.
2. 16 8 4 2.7 37.5 300 150 75 50.
3. 24 12 6 4.0 40. 320 160 80 53.
5. 40 20 10 6.7 50. 400 200 100 67.
7.5 60 30 15 10. 60. 480 240 120 80.
10. 80 40 20 13. 75. 600 300 150 100.
12.5 100 50 25 17. 100. 800 400 200 134.
15. 120 60 30 20. 125. 1000 500 250 167.
20. 160 80 40 27. 150. 1200 600 300 201.
25. 200 100 50 34. 200. 1600 800 400 268.
——————————————————————————————————————————————————————————————————————————————-
To determine the current required for a motor, as for instance,
the 10 H. P. motor under consideration, _multiply the
horsepower by 746, and divide the product by the voltage of the
motor multiplied by its efficiency as follows_: (10 × 746) ÷
(220 × .90) = 37.6 amperes.
This value increased by 25 per cent. of itself (37.6 × .25 =
9.4 amp.) is equal to 37.6 plus 9.4 = 47 amperes. In the table
of carrying capacities of copper wire (page 731), 46 amperes
is given as the allowable carrying capacity of No. 6, B. & S.
gauge, rubber covered wire; therefore No. 5 wire must be used.
=Calculations for Three Wire Circuit.=—In all cases of interior
conduit work, and in most cases of inside open work, the main feeders
from a three wire source of supply are installed on the three wire
plan, and the sub-feeders and distributing mains, on the two wire plan,
except where the application of the method necessitates the use of
unwieldy sizes.
In laying out sub-feeders and mains, the total load, under normal
operating conditions, should be divided as nearly as possible into two
equal parts, and one part connected on each side of the neutral part
of the entrance cut out, or the neutral bus bar of the switch board or
panel board in an isolated plant, thus making the load on each side of
the neutral wire of the feeder as near equal as possible.
Fig. 841 shows a three wire panel board with connection for
12 mains; those shown in solid lines as A, B, C, etc., being
connected between the neutral wire and the negative wire of the
feeder, and those shown by dotted lines as G, H, I, etc., being
connected between the neutral wire and the positive wire of the
feeder. The total load consists of ninety-one 16 candle power
lamps, which are so distributed that the positive wire of the
feeder carries the current for 46 lamps, and the negative wire,
45 lamps, the neutral wire carrying the difference or current
for 1 lamp.
The proper size of wire for the mains may be calculated as already
explained, but in calculating the outer wires of the three wire feeder,
the neutral wire should be disregarded and the outer wires connected as
a _two wire circuit carrying the total load of 91 lamps at the over all
pressure of 220 volts_.
EXAMPLE.—Ninety-one 16 candle power lamps consuming 3.1 watts
per candle power at a pressure of 110 volts, will require a
current of
16 × 3.1 × 91
————————————- = 41 amperes.
110
The distance from the entrance cut out to the main or feeder
switch is 200 feet, then for a 2 per cent. drop, or a loss of
110×.02=2.2 volts, the cross sectional area of the wire will be,
41 amperes × 200 feet × 21.6
———————————————————————————— = 80,509 circular mils.
2.2 volts
[Illustration: FIG. 841.—Three wire circuit panel board with
connections for 12 mains. The wires shown in solid lines as A, B, C,
etc., are connected between the neutral wire and the negative wire of
the feeder, and those shown by dotted lines, as G, H, I, etc., are
connected between the neutral wire and the positive wire of the feeder.]
The joint resistance of the lamps on a three wire system,
however, would be four times greater than on a two wire system;
consequently the resistance of the outer wires of the feeder in
this case will be four times greater for the same percentage
of loss, and the cross sectional area of each of the outer
wires will be, 80,509÷4=20,127 circular mils. According to the
underwriters' rules, this value compels the use of No. 6 B. &
S. gauge wire.
If the _lamp_ voltage, 110 volts, be used, the two wire formula (5)
given on page 748 must be modified to,
amperes × feet × 21.6
circular mils = ————————————————————-
drop × 4
but if an _over all_ voltage, (that is, the voltage between the outer
wires), of 220 volts be used, the two wire formula does not require any
modification.
The proper size of wire may also be calculated by means of the formula
drop
———————————————————————- = resistance per foot . . . . (1)
2 × distance × amperes
Example.—If in calculating a three wire feeder, the over all
voltage be 220 volts, the drop = 4.4 volts, twice the distance
= 400 feet, and the current = 20.5 amperes, then,
4.4 volts
————————————————————————- = .0005365 ohms per foot.
400 feet × 20.5 amperes
In the table of the properties of copper wire which gives the
resistance of various sizes of wire, it will be noted that
at all of the given temperatures No. 8 wire has a resistance
greater than the value just calculated, therefore, No. 6 B. &
S. gauge wire should be used for the outer wires of the feeder.
In the table the resistance is given per 1,000 feet, hence
it is only necessary to move the decimal point to obtain the
resistance per foot.
If the calculation be based on the lamp voltage, 110 volts, the formula
(1) must be modified to
drop × 4
———————————————————————— = resistance . . . .(2)
2 × distance × amperes
In this case, drop = 2.2 volts, 2 × distance = 400 feet, and current =
41 amperes, then,
2.2 volts × 4 8.8
———————————————————— = ———————— = .005364 ohms.
400 feet × 41 amp. 16,400
=Size of the Neutral Wire.=—In three wire circuits, the size of the
neutral wire will depend to a great extent upon operating conditions.
In the case of installations which occasionally have to be worked as
two wire systems, the cross section of the neutral wire should be equal
to the combined cross section of the two outer wires.
For interior wiring which must pass inspection, the neutral wire must
always be twice the size of one of the outside wires. However, for
general distribution, if it be reasonably sure that the system will
always be worked three wire and that the drop in the two outer wires
does not exceed 1½ per cent., the cross section of the neutral
wire may be made smaller than that of one of the outer wires. In such
a case the size of the neutral wire may be calculated for a maximum
unbalancing of 25 per cent., when the current in one of the outer wires
is 75 per cent. of the current in the other outer wire.
For instance, suppose that in a balanced system, the total load
on each of the outer wires of a feeder be 211 amperes, and that
on account of certain operating conditions, this load has to be
divided unequally so as to put 242 amperes on one of the outer
wires, and 181 amperes on the other outer wire. In this case
the neutral wire will carry 60 amperes, or 25 per cent. of the
current carried by the heavier outer wire.
If the drop in the outer wires exceed 1½ per cent., the
cross section of the neutral wire will have to be equal to or
larger than that of each of the outer wires, otherwise the drop
in the neutral wire will exceed ½ volt with an unbalancing of
25 per cent.
CHAPTER XXXVIII
INSIDE WIRING
The term _wiring_ is commonly understood to mean the methods employed
in laying the conductors used for the transmission and distribution of
electrical energy for lighting, power, and other purposes. Interior
wiring, comprises the various methods of installing the conductors from
the entrance devices in the walls or other parts of the buildings to
the lamps, motors, and other electrical apparatus within the buildings.
The different methods of interior wiring may be conveniently grouped
into the following general classes:
1. Open or exposed wiring;
2. Wires run in mouldings;
3. Concealed knob and tube wiring;
4. Rigid conduit wiring;
5. Flexible conduit wiring;
6. Armoured cable wiring.
=Open or Exposed Wiring.=—This method of wiring possesses the
advantages of being cheap, durable and accessible. It is used a great
deal in factories, mills and buildings where the unsightly appearance
of the wires exposed on the walls or ceilings is of no consequence.
=Ques. What kinds of wires are suitable for this method of wiring?=
Ans. Either rubber covered or slow burning weather proof wire.
Rubber insulation should always be used where the wire is in
a damp place, such as a cellar, and either weather proof or
rubber insulation may be used to protect it against corrosive
vapors.
[Illustration: FIGS. 842 to 844.—Open or exposed wiring. Fig. 842,
wires passing through beams. The holes are bored at an angle and wire
run through in zig-zag course. Porcelain tubes are used where the wire
passes through beams; fig. 843, cleat work across beams, the cleats are
carried by boards attached to the beams; fig. 844, method of carrying
wires on cleats around beams.]
=Ques. How are the wires installed?=
Ans. They are laid on some cornice, wainscoting, beam, or other
architectural feature suitable for the purpose, by means of porcelain
knobs and cleats, as shown in figs. 842 to 844.
Porcelain knobs should preferably be of the two piece type
(fig. 863) in which the wire is carried between the upper and
lower portions rather than being tied to a one piece knob with
a tie wire as in fig. 860. Various porcelain knobs and cleats
are shown in figs. 860 to 866.
=Ques. What are the disadvantages of open wiring?=
Ans. The wiring is not sufficiently protected from moisture, and the
effects of fire which will destroy the insulation of the wires; it is
also liable to mechanical injury.
[Illustration: FIGS. 845 to 847.—Splicing. Figs. 845 and 846, making
a wire splice, and the twist completed; fig. 847, a wrapped joint on
large wire. Splicing of wires or joining a branch to a main wire should
always be made by twisting the wires together or twisting one wire
around the other, so that the joint will be mechanically strong enough
to carry all strain which may come upon it, without any soldering.
The joint should then be carefully tinned and soldered in order to
give good electrical contact and to avoid corrosion along the contact
surface. Where wires are too large to be twisted together, the ends
are given a short bend and the two wires wrapped firmly together with
a smaller bare copper wire, after which the joint is thoroughly tinned
and soldered, preferably by pouring hot solder over the joint. The
joint is then insulated by wrapping it with two layers of pure rubber,
and three layers of tape, sufficient to make the insulation thickness
equal to that of the wire, after which the whole joint should be
painted with water proof paint.]
=Ques. How far apart should the wires be placed?=
Ans. When installed in dry places and for pressures below 300 volts,
the insulators should separate the wires 2½ inches from each other
and ½ inch from the surface along which they pass. For voltages from
300 to 500, the wires should be separated four inches from each other
and one inch from the surface along which they pass.
If the wiring be in a damp place, the wires should be at least
one inch from the surface.
[Illustration: FIGS. 848 to 850.—Crossing of wires. Where wires cross
each other, tubes should be used except in case of large stiff wires
as in fig. 848; here one wire may be bent down and carried under the
other; fig. 849, short bushing strung on the wire—this method is
usually unsatisfactory, especially where a large number of wires cross
each other; fig. 850, wires crossing each other through tubes. Flexible
tubing, such as circular loom may be used in crossing wires in dry
locations. Insulators should always be provided where wires cross to
support the wires, thus preventing the upper wires sagging and touching
those below.]
[Illustration: FIGS. 851 to 853.—Methods of wiring across pipes. The
wires should preferably run over rather than under the pipes. Fig. 851
shows crossing with circular loom, and fig. 852, one in which a tube is
used. Both of these methods are satisfactory in the case of gas pipes,
but for steam pipes or water pipes which are liable to leak or sweat
and drip moisture, the crossing should be above as shown in fig. 853.
On side walls where vertical wires run across horizontal water pipes,
the latter should be enclosed and the moisture deflected to one side.]
=Ques. How should wires be protected when run vertically on walls?=
Ans. They should be boxed in or run in a pipe as shown in fig. 854, the
covering extending 6 feet above the floor.
When placed inside a box there should be a clearance of at
least one inch around the wires; the box should be closed at
the end as shown, and the wires protected where they enter the
top with bushings. When the wires are placed in a pipe they
should be first encased in a piece of flexible tubing that will
extend from the insulator below the end of the pipe to the
first one above it.
[Illustration: Fig. 854.—Methods employed in open wiring when run
vertically on walls. Either a box casing or iron pipe should be used
to protect the wires. The covering need only extend six feet above the
floor.]
=Ques. What kind of incandescent lamp receptacle or wall socket is best
adapted to exposed wiring?=
Ans. One which does not have exposed contact ears, an approved form
being shown in fig. 859.
=Practical Points Relating to Exposed Wiring.=—Some of the principal
points which should be remembered in this connection, together with the
methods which may be applied to special cases, may be briefly stated as
follows:
1. In interior wiring no wires smaller than No. 14 B. & S.
gauge should be used, except as allowed by the underwriters,
and no more than 660 watts should be allowed to a circuit.
2. Tie wires should have an insulation equal to that of the
conductors which they secure.
3. In all cases, whether the wires be run on knobs, split
insulators, or cleats, the wires should be supported at
intervals of at least 4½ feet, and if exposed to mechanical
injury, the supporters should be placed at closer intervals.
4. Wires run on bare ceilings of low basements, especially
where they are liable to injury, should be protected by two
wooden guard strips as shown in fig. 858. The protective strips
should be at least ⅞ inch in thickness and slightly higher
than the knobs, insulators, or cleats. The two circuit wires
should not be run closer than 6 inches apart, and wires run
near water tanks must be rubber covered so as to render them
moisture proof.
5. Cleats should be used for the wiring of stores, offices,
or buildings having flat ceilings, provided the wiring is
installed in dry locations.
6. When the installation is exposed to dampness or acid fumes
such as those developed in stables, bakeries, etc., the wires
should run on knobs or split insulators, and should be rubber
covered.
[Illustration: Figs. 855 to 857.—Methods of carrying wires through
floors. In passing through floors (or walls) the wires often come in
contact with concealed pipes or other grounded material, hence the only
way they can be properly protected is by making the bushing continuous.
This may consist of continuous porcelain tubes as shown in fig. 855,
or short bushings may be arranged in iron pipes as in fig. 856. The
method followed in case of an offset in the wall is shown in fig. 857.
Sometimes the floor can be taken up and an iron conduit, properly
bent, put in place, the wires being reinforced with flexible tubing.
Another method is to attach the wires to insulators; in this case the
floor must not be put down until the wiring has been examined by the
inspector.]
7. When wires are run at right angles to beams which are more
than 4½ feet apart, a running board should be used and the
wires cleated to it as shown in fig. 843. It is desirable,
however, to avoid the use of running boards, whenever possible
by running the wires parallel with the beams, thus reducing the
cost of insulation.
8. In factories or other buildings of open mill construction,
mains of No. 8 B. & S. gauge or larger wire, where they are
not exposed to injury, may be placed about 6 inches apart and
run from timber to timber, not breaking around, and may be
supported at each timber only.
[Illustration: Fig. 858.—Method of protecting exposed wiring on low
ceilings by two guard strips.]
[Illustration: Fig. 859.—Receptacle suitable for use with open wiring,
the requirement being that the contact ears should not be exposed.]
9. The best location for feeders is on the walls. In dry
buildings the fire and weather proof wire can be used with
safety; but covered wire must be used on buildings subject to
any form of dampness. In all cases where feeders are run on
the walls, they should be protected from mechanical injury by
boxings at least 6 feet high on each floor. If floor switches
be used, they may be mounted on the front of the boxing. In
such cases, the holes in the boxing through which the wires
pass to the switches should be provided with porcelain bushings.
10. The rosettes, receptacles, sockets, snap switches, etc.,
used in connection with exposed wiring should conform in all
respects to the standards specified by the underwriters.
[Illustration: FIG. 860 to 866.—Various porcelain knobs and cleats. In
open work various forms of these devices are used.]
[Illustration: FIG. 867.—Porcelain tube for entrance of wire into
a building. There must be a drip loop outside to drain off water,
and the hole through which the conductor passes must be bushed with
a non-combustible, non-absorptive insulating tube slanting downward
toward the outside. The object of the inclination is to allow any water
that might enter the tube to gravitate to the drip loop.]
[Illustration: FIG. 868.—Interior bushing. Wires must be separated
from contact with walls, floors, timbers or partitions through which
they may pass by non-combustible, non-absorptive, insulating tubes,
such as glass or porcelain, except at outlets where approved flexible
tubing is required. Bushings must be long enough to bush the entire
length of the hole in one continuous piece, or else the hole must
first be bushed by a continuous water proof tube. This tube may be a
conductor, such as iron pipe, but in that case an insulating bushing
must be pushed into each end of it, extending far enough to keep the
wire out of contact with the pipe.]
=Wires Run in Mouldings.=—Wooden mouldings are extensively used in
connection with the wiring of stores, factories and buildings. The
advantages of this type of construction are: simplicity, cheapness, and
accessibility, and when the moulding is run straight and accurately
mitred it makes a neat job. Any class of wooden moulding wiring,
however, is not sufficiently impervious to moisture to render it
suitable for use in damp places, and it is liable to be crushed or
punctured. Furthermore, it is naturally very combustible. These
difficulties are overcome to a certain extent by impregnating the
moulding with some kind of moisture repellant, or by coating it both
inside and out with water proof paint. Hardwood moulding should be used
wherever possible, but soft wood moulding usually conforms much better
to the wall line.
[Illustration: FIG. 869.—Standard wooden moulding for encasing wires.
Wooden moulding must not be used in concealed or damp places, nor be
placed directly against a brick wall where sweating may introduce
moisture that may ultimately cause a short circuit. Wooden moulding for
concealing electrical conductors is prohibited by ordinances in some
cities.]
=Ques. For what conditions is wiring in mouldings suitable?=
Ans. It is adapted to installations in which the wires have to be laid
after the completion of the buildings.
=Ques. Describe the moulding usually employed.=
Ans. It is made of hardwood in two pieces, a backing and cap, so
constructed as to thoroughly encase the wire.
It should provide a one-half inch tongue between the conductors
and a solid backing which should not be less than three-eighths
of an inch in thickness under the grooves; it should be able to
give suitable protection from abrasion.
The inside of the moulding and the cap must have at least two
coats of waterproof material, or else the whole moulding must
be impregnated with moisture repellant.
Only one conductor is placed in a groove.
The backing is secured to the walls or ceilings by means of
wire nails. The wires are then laid in the grooves and the
capping put in place and fastened by small brads.
The wires should be continuous, and only rubber covered wire
should be employed.
Wooden moulding is made in a great variety of size and design.
The general appearance of this type of moulding being shown in
fig. 869.
=Ques. What other kind of moulding is used?=
[Illustration: FIGS. 870 to 872.—Metal moulding. An approved form
consists, as shown, of two pieces: base (fig. 870), and cap (fig. 871),
so formed as to snap together, the cap snapping over the base as in
fig. 872. The entire moulding should be galvanized or coated with a
rust preventive. When the base is held in place by screws or bolts from
the inside surface, depressions must be provided so that the heads of
the screws will be flush with the surface of the moulding.]
Ans. Metal moulding, as shown in figs. 870 to 872.
Metal moulding is permitted on circuits requiring not more than
660 watts and where the pressure is not over 300 volts. Special
fittings must be used with this class of moulding so that it
is continuous both mechanically and electrically. The moulding
should be grounded. The installation rules are practically the
same as those governing conduit work.
=Ques. What is a kick box?=
[Illustration: FIG. 873.—"Kick box;" a device used to protect wires
encased in porcelain tubes where they pass through floors.]
Ans. A fitting, as shown in fig. 873, for protecting wires at the
points where they enter or emerge from the floor.
=Ques. How is moulding work installed on brick or plaster walls which
are liable to dampness?=
Ans. A backing board must be put on before the moulding is used.
=Ques. How should moulding be placed on a ceiling with respect to
appearance?=
Ans. The appearance is improved if the moulding be carried through to
the side of the room, even if part of it be not used. This will give a
neat and finished appearance to the ceiling as shown in fig. 874.
Moulding should always be run in as inconspicuous position as
possible, and if it be necessary to run it on the open ceiling,
it should be arranged so as to form regular panels. Often it
can be run so as to take the place of a picture moulding or as
a part of the baseboard so that it becomes merely a part of the
wooden trim of the building; and in certain cases it should be
made of material to match the rest of the trim.
[Illustration: FIG. 874.—Treatment of moulding work on ceilings. All
installations should be planned out so as to conform to symmetrical
designs, as far as practicable with the proper distribution of the
lights, etc., and all runs finished off, whenever necessary, by "dead"
mouldings continued to the walls to improve the appearance. In the
figure the sectioned portions show the location of the dead moulding.
Sometimes, especially in the wiring of private houses the use of
special moulding is necessary. In such cases the shape and kind of
wood should match that of the finish or trim of the room, and the
receptacles should be stained to match the moulding. When the moulding
is run along the walls, the capping may be made to match the trim
or the picture moulding already in place, thus giving an apparently
concealed job. In this kind of work the feeders can be run through
the spaces between the walls, and if flexible tubes such as circular
loom or flexiduct be used, no splice box will be necessary where the
system of wiring changes and single braided rubber wire may be used
throughout.]
=Ques. What is the usual character of moulding work?=
Ans. Usually, a certain part of the work will be run as concealed, that
is, inside the partitions, the wires being "fished" from the moulding
to the outlet.
=Practical Points Relating to Wiring in Mouldings.=—The following
practical points will be found useful in the satisfactory execution of
any class of wiring with wooden moulding:
1. Wooden moulding should never be concealed, and should not be
used in damp places or in buildings subject to acid fumes, such
as ice houses, breweries, or stables, etc.
2. Wooden moulding selected for use should be formed of good
straight stock and free from knots, knot holes and other
imperfections. The saving effected in the lower cost of second
hand moulding does not compensate for the additional cost
increase in its working.
3. When wooden moulding is used in connection with solid pipe
or flexible tube conduit, an iron box or conduitlet must be
installed where the system of wiring changes, as shown in fig.
875. The pipe conduit is secured to the box by means of lock
nuts, with porcelain bushings or flexible tubes protecting the
wires. In all cases the loom should run up to the moulding.
[Illustration: FIG. 875.—Method of tapping outlets for feeder circuits
when wiring with wooden moulding.]
=Arc Light Wiring.=—All wiring for high voltage arc lighting
circuits should be done with rubber covered wire. The wires
should be arranged to enter and leave the building through an
approved double contact service switch which should close the
main circuit and disconnect the wires in the building when
turned "off" and be so constructed that they will be automatic
in their action, not stopping between points when started and
to prevent arcing between points under any circumstances, and
should indicate plainly whether the current is "on" or "off."
Never use snap switches for arc lighting circuits. All arc
light wiring of this class should be in plain sight and never
enclosed, except when required, and should be supported on
porcelain or glass insulators which separate the wires at least
one inch from the surface wired over. The wires should be kept
rigidly at least eight inches apart, except, of course, within
the lamp, hanger board or cut out box or switch. On side walls,
the wiring should be protected from mechanical injury by a
substantial boxing, retaining an air space of one inch around
the conductors, closed at the top (the wires passing through
bushed holes) and extending not less than seven feet from the
floor. When crossing floor timbers in cellars or in rooms,
where they are liable to be injured, wires should be attached
by their insulating supports to the under side of a wooden
strip not less than one-half an inch in thickness.
=Arc Lamps on Low Pressure Service.=—For this service there
should be a cut out for each lamp or series of lamps. The
branch conductors for such lamps should have a carrying
capacity about 50 per cent. in excess of the normal current
required by the lamp or lamps to provide for the extra current
required when the lamps are started or should a carbon become
stuck without over fusing the wires. If any resistance coils be
necessary for adjustment or regulation, they should be enclosed
in non-combustible material and be treated as sources of heat;
it is preferable that such resistance coils be placed within
the metal framework of the lamp itself. Incandescent lamps
should never be used for resistance devices. These lamps should
be provided with globes and spark arresters, as in the case
of arc lamps on high voltage series circuits, except when the
closed arc lamps are used.
4. Wooden moulding should never be run in elevator shafts, or
shafts of any kind, and should never be run on the inner side
of the outside walls of the buildings, as these locations are
usually subject to dampness.
5. In laying out feeders it is usually cheaper to use iron
conduit in a shaft, than to run moulding through the floor
timbers.
6. When tapping outlets for feeder circuits, an iron outlet box
with cover should be used, as shown in fig. 886. The one splice
box is held up to the outlet box already installed by means of
two long screws, and the loom is run right up to the moulding
so as to leave no exposed wire.
[Illustration: FIG. 876.—Circular fixture block for outlet from
moulding work on ceiling.]
7. Wherever fixture outlets are installed, a circular fixture
block as shown in fig. 876 should be used, to give a good
support for the fixture and to make a neat backing for the
fixture canopy. The wires should be brought through the fixture
without cutting and disfiguring the canopy.
=Concealed Knob and Tube Wiring.=—This method of wiring should be
discouraged as far as possible, as it is subject to mechanical injury,
is liable to interference from rats, mice, etc. As the wires run
according to this method are liable to sag against beams, laths, etc.,
or are likely to be covered by shavings or other inflammable building
material, a fire could easily result if the wires become overheated or
short circuited.
[Illustration: FIG. 877.—Concealed knob and tube wiring. The wires are
carried on porcelain knobs attached to the beams. If run perpendicular
to the beams, holes are bored in the latter and porcelain tubes with
a shoulder at one end, inserted in the holes through which the wires
pass. The knobs should support the wires at least one inch from the
surface over which they run, and should not be spaced further than
4½ feet apart. The wires should be attached with tie wires having an
insulation equal to that of the conductor which it secures to the knob.
The use of split knobs does away with the necessity of using tie wires.
The conductors must be at least 5 inches apart and it is better to
support them on separate beams when possible. Each wire must be encased
in a piece of flexible tube at all switches, outlets, etc., and this
piece of tubing should be sufficiently long to extend from the last
insulator and project at least one inch beyond the outlet.]
Concealed knob and tube wiring is still allowed by the
Underwriters, although many vigorous attempts have been made to
have it abolished. Each of these attempts has met with strong
opposition from electric light companies and contractors,
especially in small villages and towns the argument being that
it is the cheapest method of wiring, and if forbidden, many
places which are wired according to this method would not
be wired at all, and the use of electricity would therefore
be much restricted, if not entirely dispensed with in such
communities. This argument, however, is only a temporary
makeshift obstruction against progress, and in the near future,
no doubt, concealed knob and tube wiring will be forbidden by
the underwriters.
[Illustration: FIGS. 878 to 880.—Methods of making fixture outlets in
concealed knob and tube wiring. A cleat consisting of a piece of board
at least ⅞ in. thick, should be nailed between the joists or studs
into which the wood screws supporting the electrolier can be secured.
Holes are then bored through the cleat, through which the flexible
tubing can pass. With a combination gas and electric fixture as shown
in fig. 879, no cleat is necessary, because the gas pipe supports
the fixture. The flexible tubing should be wired to the gas pipe, to
prevent displacement by artisans who have occasion to work around the
outlet.]
=Ques. Describe the method of concealed knob and tube wiring.=
Ans. It consists in running the wires concealed between the floor beams
and studs of a building, knobs being used to support the wires when run
parallel to the beams or studs, and porcelain tubes, when run at right
angles through the beams, or studs as shown in fig. 877.
In this method of wiring, usually nothing need be disturbed on
the first floor as the various outlets can be reached from the
basement and from the second floor.
[Illustration: FIG. 881 and 882.—Arrangement of switch and receptacle
outlets in knob and tube wiring. In wiring for switches, flexible
tubing must be used on the conductor ends from the last porcelain
support, as shown, the same as on conductor ends for other outlets. A
pressed steel switch box should be used to encase each flush switch
mechanism, even though it already be encased in porcelain. A ⅞ in.
wood cleat or cleats are arranged to support the switch box. These
wooden cleats should not be set out flush with the outer edges of the
studs, but should be set about ⅜ in. back, as illustrated, to allow a
space in which the plaster can take a "grip."]
For instance, if it be necessary to make an outlet for the
center fixture in the parlor, a strip of flooring can be
removed from the floor above so as to expose the beams. Then
the wireman can bore two holes through each of the beams,
insert porcelain tubes therein, slip the wires through the
outlet and replace the strip of flooring.
Various simple methods may be employed for carrying the wires
to the outlets on the side walls. For example: a small hole
can be made in the wall, and the wire may be dropped through
the spaces between the walls, or they may be pulled up from
the basement by means of a cord lowered with a weight attached
to its end. Outlets for switches and base receptacles may be
provided for, in a similar manner.
[Illustration: FIGS. 883 and 884.—Elevation and sectional view showing
arrangement of switch outlet in concealed knob and tube wiring.]
[Illustration: FIG. 885.—Arrangement of surface switch in concealed
knob and tube wiring. For a surface snap switch outlet, an iron box is
not necessary, but a ⅞ in. cleat must be installed to hold the tubing
in place and to provide a proper support for the screws that hold
the switch. In wiring old buildings where supporting cleats were not
provided back of the plaster, a ¾ in. wooden block or plate should be
installed on the surface, to which the switch can be attached.]
=Ques. What are the advantages of concealed knob and tube wiring?=
Ans. Its cheapness, especially in wiring completed buildings, and the
absence of any wires or casings on the walls or ceilings.
=Ques. What kind of wire must be used?=
Ans. Wire having an approved rubber insulating covering.
[Illustration: FIGS. 886 to 888.—Switch boxes for concealed knob and
tube wiring. These are for flush switches and are formed from sheet
steel. A single switch box can be expanded for any number of switches,
by using the proper number of spacers. Single and double switch boxes
can be supplied already assembled and are used where feasible, because
it is cheaper to buy them this way than to assemble them. Holes
partially punched, which can be knocked out with a hammer blow, are
provided in the sides and back through which the flexible conduit wire
protection can be extended.]
=Rigid Conduit Wiring.=—The installation of wires in conduits not
only affords protection from mechanical injury, but also reduces the
liability of a short circuit or ground on the wires producing an arc
which would set fire to the surrounding material; the conduit being of
sufficient thickness to blow a fuse before the arc can burn through the
conduit.
=Ques. Describe the unlined type of conduit.=
Ans. It consists of an iron or steel pipe, similar in size, thickness,
and in every other way to gas pipe, except that special precautions
are taken to free it inside from scale or any irregularities; it is
then coated inside with enamel, outside it is sometimes enameled and
sometimes galvanized.
=Ques. Describe the lined type of conduit.=
Ans. It usually consists of a plain iron pipe lined with a tube of
paper which has been treated with an asphaltic or similar compound;
this paper tube is cemented or fastened to the inside of the iron pipe
so that it forms practically an integral part of the same.
=Ques. What are the advantages of unlined conduit?=
Ans. It is cheaper, because having no lining a smaller size of conduit
can be used for any given size of conductor; it is also cheaper to
install, as it can be bent, threaded, and cut more readily than the
lined conduit. Wires may be more easily inserted and withdrawn as the
inside is smoother than that of the lined conduit.
NOTE.—Conduits for inside wiring which are subject to
inspection, must have an inside diameter of not less than
⅝ inch. They must be continuous from outlet to outlet or
to junction bores, and must properly enter and be secured to
all fittings, and the entire system be mechanically secured
in position. In case of service connections and main wires,
this involves running each conduit continuously into a main
cut out cabinet or gutter surrounding the panel board as the
case may be. Conduits must first be installed without the
conductors, and be equipped at every outlet with an approved
outlet box or plate. Outlet plates must not be used where it
is practicable to install outlet bores. The outlet box or
plate must be so installed that it will be flush with the
finished surface, and if this surface be broken, it shall be
repaired so that it will not show any gaps or open spaces
around the edge of the outlet box or plate. In buildings
already constructed where the conditions are such that neither
outlet box nor plate can be installed, these appliances may
be omitted by special permission, providing the conduit ends
are bushed and secured. It is suggested that outlet boxes and
fittings having conductive coatings be used in order to secure
better electrical contact at all points throughout the conduit
system. Metal conduits where they enter junction boxes, and
at all other outlets, etc., must be provided with _approved_
bushings or fastening plates, fitted so as to protect wire from
abrasion, except when such protection is obtained by the use
of _approved_ nipples, properly fitted in boxes or devices.
Conduits must have the metal of the conduit permanently and
effectually grounded. Conduits and gas pipes must be securely
fastened in metal outlet boxes so as to secure good electrical
connections. If conduit, couplings, outlet boxes or fittings
having protective coating of insulating material, such as
enamel, be used, such coating must be thoroughly removed from
threads of both couplings and conduit and from surfaces of
boxes and fittings where the conduit is secured in order to
obtain requisite good connection. Where boxes used for centers
of distribution do not afford good electrical connection, the
conduits must be joined around them by suitable bond wires.
Where sections of metal conduit are installed without being
fastened to the metal structure of buildings or grounded metal
piping, they must be bonded together and joined to a permanent
and efficient ground connection. Junction boxes must always be
installed in such a manner as to be accessible. All elbows or
bends must be so made that the conduit or lining of same will
not be injured. The radius of the curve of the inner edge of
any elbow must not be less than three and one-half inches. Must
have not more than the equivalent of four quarter bends from
outlet to outlet, the bends at the outlets not being counted.
[Illustration: FIG. 889.—Conduit box showing arrangement for
combination side outlet with open cover. Outlet or junction boxes are
of two general types: 1, those which are made for a particular position
and have a given number of outlets, and 2, those which have a variable
number of outlets which are plugged with metal discs in such a manner
that the latter can be knocked out by a slight blow of a hammer. The
illustration shows a universal plugged steel conduit box, which can be
used as a straight electric, or combination gas and electric, ceiling
or side wall outlet, or for flush rotary or push button switches, or
for flush receptacles. When rigid conduits are used, they are screwed
to the outlets by means of lock nuts and washers. In the case of
flexible conduits, the entering ends of the conduits are provided with
clamp bushings which are secured to the outlet by means of lock nuts.
All outlet boxes are fitted with covers of various designs, which
permit their use for various types of construction such as ceiling and
wall work in lath or plaster, fireproofing ceiling work, etc., while
many designs of outlet plates and receptacle plates may be obtained
from the supply houses to satisfy the requirement of any special case.]
=Ques. What are the disadvantages of the unlined conduit?=
Ans. The Underwriters require the use of double braided conductors
instead of single braided which are allowed for lined conduits.
=Ques. Where may unlined conduits be used?=
Ans. In buildings where the conduit is not liable to corrosive action.
=Flexible Conduit Wiring.=—Flexible conduits are used to advantage
in many cases where rigid conduits would not be desirable. It is
especially adapted to completed buildings where it is desired to
install the wiring by "fishing" without greatly disturbing the walls,
floors, or ceilings.
[Illustration: FIGS. 890 and 891.—Greenfield flexible steel conduit;
fig. 890 single strip type; fig. 891 double strip type. The former
(fig. 890) is formed with a single strip of galvanized steel,
interlocked and gasketed in such a manner as to be suitable for
concrete construction. The double strip type (fig. 891) is constructed
of a concave and convex steel strip, spirally wound upon each other
in such a manner as to interlock their concave surfaces. Thus the
convex surfaces of the two strips form respectively the outer and inner
surfaces of the conduit. This construction insures a smooth interior
surface, thus reducing the possibility of friction in the drawing in
of conductors. A gasket is provided between the inner and outer strips
rendering the conduit moisture proof. This form of flexible conduit
is especially adapted to use where the wiring is installed after
completion of building, because it is very flexible.]
=Ques. How is a flexible conduit installed by "fishing"?=
Ans. It is "fished" under floors, in partitions between the floor and
ceiling, by making pockets in the floors, walls or ceilings, say every
15 or 20 feet, and fishing through first a stiff metal wire called a
"snake," and then attaching the conduit to same and pulling the conduit
in place from pocket to pocket.
[Illustration: FIG. 892.—Insulating joint. This fitting is used
in fixture work. The part A screws on to the gas pipe and B to the
fixture. The parts are separated by insulating material E, and
the outside of the joint is covered with moulded insulation D. In
connecting fixtures to the wiring, all wires should be kept away
from the gas pipe above the joint, but they may be bunched in below
the insulating joint after the wires have been spliced, soldered,
and taped. It is important to protect the gas pipe at this point.
Insulating joints should be tested before being used.]
[Illustration: FIG. 893.—Canopy insulator. This fitting should be
installed wherever there are metal ceilings against which the canopies
of fixtures might come. The canopy is the brass cup shaped piece used
at the top of fixtures to cover the joint, and is simply an insulating
ring placed between the canopy and the ceiling. It is in contact with
the fixture; hence, it is important that it be insulated from metal
ceilings, or else all the benefits derived from an insulating joint
will be lost.]
=Ques. How is the conduit fished on vertical runs?=
Ans. A chain or weighted string is used which is dropped from the
outlet to the floor and its lower end located by sound of the chain end
or weight striking the floor.
[Illustration: FIG. 894.—Section of flooring illustrating use of
fishing hook. In fishing wires, punch a hole through the plastering
at the required position, being careful that there is no studding at
that place. Use a brad awl and cut the hole large enough to permit
running of the wires. With a short length of small brass spring wire,
push through the opening a few inches of number 19 double jack chain
such as is used for general fishing purposes, first having connected
the end of the chain with a piece of heavy linen thread. Run out the
thread between the laths and the outside wall until the chain touches
the floor beneath; move the thread and locate the chain by the sound;
bore a hole through the baseboard or floor, as the case may be, toward
the chain. Use a two or three foot German twist gimlet. With a small
brass spring wire bent at the end in the shape of a hook, fish for the
chain and draw it out. At the other end of the thread attach the wire
and draw it through with the thread. Passing under the floor bore a
second hole through the floor as near the other as possible. Run into
this a piece of snake or fishing wire with a hook at the end, until it
comes to an obstruction. Locate the obstruction by sound. In running
wires under the flooring first carefully examine all parts and find
the direction in which the beams and timbers run, and run the wires
parallel with these. After locating the end of the fishing wire see
if the obstruction be a timber; if so, find the center and bore from
the middle diagonally through it in the direction of the fishing wire.
Drop the jack chain and thread through the hole; fish for it and draw
it through hole number 2; attach the insulated wire and draw it back.
Starting hole number 3, bore hole number 4 diagonally through the
timber in the direction in which the wire is to be run, making holes
3 and 4 form an inverted "V" through the timber. Run the fishing wire
through hole number 4 until it meets an obstruction. If at the end of
the room, bore through the floor, drop the chain, fish it out, attach
wire and draw it home. Putty up holes after having finished the work,
in case of hard finish, plug them up with wood. In lightly built houses
it is often found easier to take off the moulding above the baseboard
and run the wire under it. In such cases care should be taken to break
off the old nails, as any attempt to drive them out would cause a bad
break. In closets and around chimneys it is usually found easy to work.
A "mouse" or lead weight attached to a string may often be dropped from
the attic to the cellar ceiling through the space outside the chimney.]
=Ques. What is the difference between flexible conduit and flexible
tubing?=
Ans. Flexible conduits are made of metal while flexible tubing is
non-metallic.
=Ques. Describe a flexible conduit.=
Ans. It is a continuous flexible steel tube composed of convex and
concave metal strips, wound spirally upon each other in such a way as
to interlock their concave surfaces.
=Ques. What are the advantages of this form of flexible conduit?=
Ans. It possesses considerable strength and can be obtained in long
lengths (50 to 200 feet); elbow fittings are not required as the
conduit may be bent to almost any radius. The fissures of the conduit
provide some ventilation; this is an advantage in some places and a
disadvantage in others.
[Illustration: FIGS. 895 to 897.—Greenfield flexible steel conduit
and fish plug, showing method of insertion. Fish plugs are made for
⅜ inch, ½ inch, and ¾ inch conduit and are useful in drawing in
the conduit in finished buildings where it is desired to fish it under
doors or in partitions. After the conduit has been cut off square in
the special vise, the fish plug may be screwed into the tube and the
fish wire or drawing-in line should then be attached to the eyelet on
the end of the plug.]
=Ques. In what places are flexible conduits not desirable?=
Ans. In damp places.
=Ques. Why?=
Ans. Because of the fissures.
=Practical Points Relating to Inside Conduit Wiring.=—The following
instructions apply to the installation of wiring in both rigid and
flexible conduit:
1. All conduits should be made continuous from one junction or
outlet box to another, or to the various fixtures. A conduit
installation is made a complete system by the use of outlets,
outlet boxes, switch or junction boxes, and panel boxes with
doors and locks, which serve to thoroughly protect the circuit
at all points.
[Illustration: FIGS. 898 to 901.—Pull boxes and their use in conduit
work. A pull box is a convenient device used for the purpose of
avoiding the disadvantages of having too many bends in one continuous
line of conduit; too many bends will give trouble when the conductors
are drawn in. Pull boxes are also useful in places where the
arrangement of the conduit is such that trouble would be experienced in
bending it to a fit, and also in the case of conduits which are first
run on a side wall and then have to be carried across the ceiling at
right angles to the wall. Fig. 898 shows an example of objectionable
bends, and fig. 899, the method of overcoming the difficulty by the use
of a pull box. It is evident that it would be impossible to make some
of these bends so as to permit the drawing in of the conductors. This
difficulty is overcome, as shown, by placing a pull box on the wall,
with its top close to the ceiling. A board B, having the proper size
holes for the conduits is fastened to the front of the box and close
to the ceiling. After the conductors have been drawn into the conduits
along the wall as far as the pull box, they can be readily pulled away
from the box through the holes in the board into the corresponding
conduit on the ceiling. Fig. 901, shows the use of a pull box in a case
where it is necessary to run conduit through partitions at right angles
to each other. Pull boxes can be designed to suit any condition liable
to occur in practice, and when properly used will always save much
time and labor. Locknuts should be placed on the ends of all conduits,
both inside and outside the pull box in order to prevent their being
displaced when drawing in the conductors. After all the conductors have
been drawn into the conduit, all the outlets should be plugged up with
wood or fibre plugs made in parts to fit around the wires and cables,
and the outlets given a coating of some compound which will render the
whole system air tight and moisture proof. A final test should then be
made to ascertain that there are no grounds on the different parts of
the wiring, and that the insulation comes up to the requirements of the
underwriters. The metal of all conduits, and the sheathing of steel
armoured cables should be effectually and permanently grounded.]
2. In the installation of interior conduit wiring, the tubes
are usually put in place as soon as the partitions of the
buildings have been constructed. In non-fireproof buildings,
the tubes are usually supported from the underside of the floor
beams, but in fireproof buildings they are placed on top of the
floor beams and under the floor as in fig. 902.
3. When conduit is used in damp places, lead encased wires
should be used, and the wires drawn in very carefully so as to
prevent any injury to the casings.
4. For wiring installations in buildings constructed entirely
of reinforced concrete, the preliminary work should be laid out
during the progress of the building operations so as to avoid,
as much as possible, the necessity of drilling holes in the
finished concrete work.
[Illustration: FIG. 902.—Method of installing conduits in fire proof
buildings. The installation of the conduit includes the placing of
all outlet boxes, and when this has been completed, the lathing or
plastering work is executed, and after that is finished, the wire is
pulled into the tubes, and the receptacles, switches, etc., put in
position. The work of pulling in the wires may be greatly facilitated
by the use of _pull boxes_ as shown in figs. 899 and 901.]
5. For concealed wiring, the location of all the outlets
should be marked by sheet iron tubes large enough to hold the
conduits. These tubes should be properly plugged, and set in
the false work before the concrete is poured in. In a similar
manner, threaded pieces of conduit of the proper size, should
be placed in the false work for risers.
6. For exposed wiring on concrete walls and ceilings, suitable
cast iron supports should be set in the moulds at regular
intervals. When liberally used, these supports will also serve
as good supports for other pipes.
7. Where a conduit line terminates on the outside of a building
some suitable fitting such as a pipe cap should be used, as
shown in fig. 903, to prevent the entrance of moisture into the
conduit system. A variety of devices suitable for this purpose
are available at supply houses; but those having porcelain
covers which spread the wires the proper distance apart are the
most satisfactory.
8. Where it is desirable or necessary to continue open wiring
from conduits, or where the character of the wiring makes it
necessary to bring the wires over from the conduit, as in an
arc lamp, neat and safe work can be done by use of a suitable
form of _condulet_ as shown in fig. 904.
[Illustration: FIG. 903.—Service entrance to interior conduit system;
showing method of preventing moisture reaching the interior of the
conduit system.]
[Illustration: FIG. 904.—Outlet to arc lamp from conduit by use of
condulet. The wires are brought out from the conduit system at a
distance of 2½ inches apart. Conduits are made in a great variety
of design with interchangeable porcelain covers which render them
adaptable to almost all cases requiring the installation of outlet
boxes.]
9. Where a conduit line terminates in a switch or panel box,
the lining or casing of the panels should be of iron, and the
conduit firmly secured to it so as to make good electrical
contact. Vertical lines of conduit should be fastened to the
wall or other supports in such a manner as to prevent the
weight of the conduit coming on the panel box, and each length
of conduit installed should be fastened so as to bear only
its own weight. The best method of fastening conduit to brick
walls is by the use of expansion bolts and screws. In the case
of fire brick ceilings or other plastered walls, toggle bolts
should be used. When conduits are run on wooden or iron beams,
various kinds of pipe hanger may be employed.
10. There are numerous devices on the market for bending
conduit for the making of elbows, offsets, etc., but the
majority possess the disadvantage that the conduit must be
taken to them to be bent. In the case of the smaller sizes,
this difficulty is avoided by the use of some form of conduit
bender such as shown in figs. 910 and 911.
[Illustration: FIGS. 905 to 909.—Sprague multilet covers. Fig. 905,
six wire porcelain cover; 906, P & S. rec. cover; 907, cover for five
ampere snap switch; 908, G. E. and P. & S. rec. cover; 909, cover for
ten ampere snap switch.]
11. In all cases, the interior diameter of the conduit
installed should be amply sufficient to permit of the wires
being drawn in easily, thus providing a substantial raceway for
the conductors. The practice of pulling wires through conduit
by means of a block and tackle is very objectionable. It is
evident that if the wires be pulled in by the application of
much force the insulation is very liable to become damaged;
furthermore, much difficulty will be experienced in pulling
them out again, especially in warm places where the heat tends
to soften the lining of the conduit, and also the rubber
covering of the wire. Powdered soapstone put in the pipe while
the wires are being drawn in will lessen the friction and
permit the wire to go in more readily.
[Illustration: FIG. 910.—Ordinary form of hickey or conduit bender. It
consists of a piece of one inch steam pipe about three feet long with a
one-inch cast iron tee screwed onto one end of the pipe. This device is
used as follows: the conduit to be bent is placed on the floor and the
tee slipped over it. The workman then places one foot on the conduit
close to the tee, and pulls the handle of the bender towards him. As
the bending progresses, the workman should take care to continually
move the bender away from himself, to prevent the buckling of the
conduit.]
[Illustration: FIG. 911.—Commercial form of hickey or conduit bender.]
[Illustration: FIGS. 912 and 913.—Methods of bending large conduits. A
substantial support is necessary which may consist, as in fig. 912, of
two pieces of 2 × 4 studding A and B securely fastened to an upright.
The conduit is placed under the block A and over the block B, and then
bent by a downward pressure exerted at C, the conduit in the meantime
being gradually advanced in the direction D to give a curve of the
required radius. The method shown in fig. 913, may be used wherever
a ring A can be attached to a beam or girder by means of clamps or
otherwise to serve as a support. In this case the conduit is slipped
through the ring and placed on the top of blocking B. The bending is
accomplished by means of a block and tackle rigged to an overhead beam
as shown. Where ring supports cannot be arranged, the application of
frame bending methods give the most satisfactory results.]
=Armoured Cable Wiring.=—Where a conduit system cannot be conveniently
installed, armoured cable is used. Armoured cable is made by winding
steel strips over the insulated conductors, the latter being
permanently retained inside the steel casing. Armoured cable is
manufactured in long lengths, the actual lengths being determined by
convenience in handling.
[Illustration: FIGS. 914 and 915.—Greenfield flexible steel armoured
conductors. The armour is composed of convex and concave galvanized
metal strips, wound spirally upon each other and over the insulated
conductors. A gasket is placed between the inner and outer metal
strips, thus further rendering the conductor moisture proof.
FIG. 916.—Greenfield flexible steel armoured lead covered conductors
for use in wet places, such as breweries, packing houses, cold
storage buildings, coal breakers and the like, and for underground
construction, in which classes of work these materials are being
extensively and satisfactorily used.]
=Ques. What are the features of armoured cable?=
Ans. It is flexible and the conductors are well protected from
mechanical injury. While this form of wiring has not the advantage of
the conduit system—namely, that the wires can be withdrawn and new
wires inserted without disturbing the building in any way whatever—yet
it has many of the advantages of the flexible steel conduit, and it
has some additional advantages of its own. For example, in a building
already erected, this cable can be fished between the floors and in the
partition walls, where it would be impossible to install either rigid
conduit or flexible steel conduit without disturbing the floors or
walls to an extent that would be objectionable.
[Illustration: FIGS. 917 to 920.—Greenfield flexible conduit tools.
Special tools are necessary for installing this type of conduit. Fig.
917, universal reamer; fig. 918, bushing tool; fig. 919, cable armour
cutter; fig. 920, vice for holding conduit. To remove cable armours,
clamp the conductor firmly in the armour cutter and with a pair of
cutting pliers back the armour off, one strip at a time, to the point
of contact with the cutting edge of the tool. The vise for holding
conduit takes all sizes. The conduit can be cut with an ordinary
hacksaw. To protect the insulation against any possible injury while
the wire is being drawn in, a soft metal bushing should be inserted in
the end of the tube and secured permanently thereto by means of the
bushing tool. The bushing provided for this purpose has an outside
thread, which permits its being screwed into the end of the tube and
then expanded by the use of the tool. The tool should always be used
after the bushing has been screwed into the pipe, then the bushing tool
should be inserted.]
=Ques. How should armoured cable be installed?=
Ans. It should be continuous from outlet to outlet, without being
spliced and installed on the loop system. Outlet boxes should be
installed at all outlets, although, where this is impossible, outlet
plates may be used under certain conditions. Clamps should be provided
at all outlets, switch boxes, junction boxes, etc., to hold the cable
in place, and also to serve as a means of grounding the steel sheathing.
=Ques. Is armoured cable wiring expensive?=
Ans. It is less expensive than the rigid conduit or the flexible steel
conduit, but more expensive than cleat wiring or knob and tube wiring,
and is strongly recommended in preference to the latter.
CHAPTER XXXIX
OUTSIDE WIRING
In the equipment of lighting and power plants, the cost of the outside
wiring represents a considerable proportion of the total investment,
sometimes costing more than the engines, boilers and dynamos.
A thorough knowledge of outside wiring is therefore necessary to
properly proportion and install the wires so that the system will prove
economical and safe.
=Materials for Outside Conductors.=—Copper wire is now considered to
be the most suitable material not only for the transmission of current
for electric light and power purposes, but also for telegraph and
telephone lines, in place of the iron wire formerly employed.
Hard drawn copper wire is used in outside construction, because its
tensile strength ranges from 60,000 to 70,000 pounds or about twice
that of soft copper. This is desirable to withstand the stresses to
which the wire is subjected which, in the case of long spans, are
considerable.
The table on the next page gives the tensile strength, in pounds per
square inch of cross section, hard drawn copper wire of various sizes
B. & S. gauge.
The metal _aluminum_ possesses certain advantages as a material
for overhead wires. Its conductivity is about .6 that of
copper. The specific gravity of aluminum is about 2.7, while
that of copper is 8.89, so that a given volume of copper will
weigh 3.3 times more than an equal volume of aluminum, and
copper wire of given length and resistance would be about twice
as heavy as an aluminum wire of equal length and resistance.
There are several disadvantages, such as, low tensile
strength, high electro-positive quality of the metal, higher
electrostatic capacity, etc.
TENSILE STRENGTH OF COPPER WIRE
+——————————————-+————————————————+——————————————-+————————————————+
| Size of wire | Tensile | Size of wire | Tensile |
| B. & S. gauge | strength, lbs. | B. & S. gauge | strength, lbs. |
+——————————————-+————————————————+——————————————-+————————————————+
| 0000 | 9971 | 9 | 617 |
| 000 | 7907 | 10 | 489 |
| 00 | 6271 | 11 | 388 |
| 0 | 4973 | 12 | 307 |
| 1 | 3943 | 13 | 244 |
| 2 | 3127 | 14 | 193 |
| 3 | 2480 | 15 | 153 |
| 4 | 1967 | 16 | 133 |
| 5 | 1559 | 17 | 97 |
| 6 | 1237 | 18 | 77 |
| 7 | 980 | 19 | 61 |
| 8 | 778 | 20 | 48 |
+——————————————-+————————————————+——————————————-+————————————————+
=Pole Lines.=—In the majority of cases overhead conductors are
supported by wooden poles. In tropical countries, however, such as
India, Central America, etc., where wood is rapidly destroyed by
the ravages of white ants and other insects, iron poles are almost
exclusively used for telegraph, telephone, and other electric
transmission lines. The form of iron pole generally adopted consists of
tapering shells of sheet iron of convenient length, riveted together at
their ends and set into cast iron base plates which are buried in the
ground.
[Illustration: FIGS. 921 to 929.—Pole construction tools. Fig. 921,
long handled digging shovel; fig. 922, digging bar; fig. 923, crow and
digging bar; fig. 924, tamping and digging bar; fig. 925, wood handle
tamping bar; fig. 926, slick digging tool; fig. 927, post hole augur;
fig. 928, carrying hook; fig. 929, tamping pick.]
=Wooden Poles.=—On account of their size and straightness, various
species of northern pine, cedar and cypress are especially suitable for
large poles. Chestnut, which can be readily sawed and hewed is a very
good material for smaller poles. Sawed redwood is extensively used in
California.
=Preservation of Wooden Poles.=—The preservation of wooden poles
employed in line work is a matter of importance. Decay of the pole at
or near the soil line is caused primarily by various forms of bacteria
or fungi, and in some cases by insects. Bacteria and fungi attack
either dead or living timber. In the case of dead timber, such as that
of poles, they attack the walls of the cells and cause the familiar rot
or decay which eventually destroys the usefulness of the pole.
It is well known that the rapid multiplication or action of the
bacteria and the growth of the fungi are induced by a certain
per cent. of moisture and the heat of the sun, that is, the
portion of the pole at or near the soil line is alternately
moistened and dried. Therefore, in order to protect it against
this action, it is necessary to sterilize the pole by the
application of an antiseptic which will penetrate the pores of
the wood.
=Preservation Processes.=—There are several processes which may be
successfully employed for the preservation of poles or other exposed
timber. The best known of these are the creosoting, burnettizing,
kyanizing, carbolizing, and vulcanizing processes.
In England, creosoted poles showed no sign of decay at the
end of 35 years of service. In the United States they have an
average life of 22 years. In Europe impregnation with copper
sulphate has been extensively used, but this impregnation must
take place within a few days after cutting down the tree.
Uniformly good results have been obtained by impregnation with
corrosive sublimate, involving simply immersion in the liquid
from ten to fourteen days. German authorities state that the
average life of such poles is about 17 years, compared with 14
years for natural or untreated poles.
The application of pitch and tar oftentimes results in more
harm than good. It is authoritatively stated, howev
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