Electric-wiring, diagrams and switchboards

Front matter

In

'HI

uin waft

Preface

The contents of this book cover the fundamental facts of wiring, as, well as such of the practise as its modest proportions could be well expected to embrace. It is not offered to -the reader as a scientific treatise — though its statements will be found able to bear the light of scientific investigation — but as a technical work, in which the author has made an effort to present the underlying principles of wiring in language suited to the comprehension of the general reader. Though framed in accordance with the technical requirements of the art of wiring, the subject matter has been presented with the idea and intention of making the reader independent of it as soon as possible. Though a mastery of the principles of rational wiring go hand in hand with its practise, it is frequently found easier to gain the practise than the theory. But it is also true that the best equipped in this particular field of work are those whose power lies within the head and hand, to an extent which makes them independent of text-books or other references. To gain this much-to-be-desired equipment, a knowledge of what is best and most useful must be obtained. What the author considers to be just such knowledge, is presented here in a logical form, as far as its various successive steps are concerned.

6 PREFACE

The elementary relationship of volts, amperes, and ohms is given first consideration; then the pivotal point of drop of potential is emphasized and expanded, and the first applications of this idea brought, as is believed, clearly to the reader's attention. Means of calculating drop, finding the circular mils of the wire, and arriving at its numbered gauge size without a table are given. This may be regarded as the primary object of the book, and will be considered by wiremen who master this method as well worth the slight labor involved. The further expansion of the simple circuit into others of a more complex type represents the next stage of progress. From this step on, the subject matter leads into a consideration of the principles of switchboard design, with reference to shunt and compound wound generators. The apparatus employed on switchboards is of great importance in electric lighting. Though, as is commonly supposed, the switchboard represents the means by which all important circuits are concentrated and controlled ; it is also the measuring and protective, as well as the distributing center of the electric light or power system. Wiring embraces this, as well as the moulding and pipe work, as will be readily understood by the intelligent reader. It is incompletely treated, however, unless the meaning of alternating current phenomena which relate to wiring; as well as simple arithmetical methods of getting the sizes of wire for such circuits, also receive careful attention.

In this respect, the pages of this volume will prove of the utmost value to the student, wireman, or con-

PREFACE 7

tractor. It is not to be inferred from this, that such knowledge is at a high premium; but it may be inferred that such knowledge is often inaccurate, incoherently arranged, and frequently useless. For such reasons as these the latter part of the book was written, and it is hoped that it will fulfil the purpose held in view. All that the author cares to claim is the manner of presentation ; as it is well known that the greater responsibility for those formulas and their derivations, which are the veritable foundations of the science of electricity, must fall upon such masters as Ohm and Helmholtz.

Newton Harrison.

January, 1906.

Chapter I Page

Introduction — Purpose of Wiring — Ohm's Law — Drop of Potential — Meaning of a Milfoot — Meaning of a Circular Mil — Calculation of Drop — Calculation of the Sizes of Wire for a Given Drop — Wiring Rules — Calculation of Resistance for Branch Circuits — What is Power and How Estimated 15

Chapter II

Carrying Capacity of Wires — Effects of Heat upon Resistance^ — Allowance for Heat — A Simple Electric Light Circuit — Wire of a 10 Lamp Circuit with Drop Analyzed — The Wiedemann System of Lighting and Its Defects — The Wiring Table — How to Prepare a Table of Sizes and Circular Mils — Examples of the Application of the Wiring Table and Formula — Calculation of the Drop per 1,000 Feet per Ampere 40

Chapter III

Elements of a Wiring System — Meaning of Mains, Feeders, Branches — Proportioning the Drop in the Various Parts of the System — The Center of Distribution — Examples of the Effects of Drop in Parts of the Circuit — Equalizio CONTENTS

PAGE

ing the Pressure — Dynamos for Incandescent Lighting — Effects of Changes in the Field of Dynamos upon the Lighting — A Wiring System with Four Centers of Distribution — The Life of a Lamp 63

Chapter IV

Measurement of Resistance — Principle of the Wheatstone Bridge — Balancing the Resistance of Four Lamps — Measuring a Lamp Hot and Cold — Measuring Insulation Resistance — The Insulation. Resistance of Buildings Wired for Electric Lighting — The Three Wire System Compared with the Two Wire — Calculating the Three Wire System of Wiring — Circuits of a Three Wire System with Two Centers of Distribution — Calculation of Pounds of Copper for Mains or Feeders . . . . 87

Chapter V

Types of Motors — Connections of Motors — Meaning and Reason for Back Electromotive Force — Use of a Starting Box — Method of Connecting Up a Shunt Wound Motor — Horse Power of Motors and Efficiencies — Efficiency of Motors and Size of Wires — Advantage of High Pressures — The Alternating Current for Lighting — Meaning of Frequency or Cycles 112

Chapter VI

Reasons for Employing Conduit — The Use of Cleats — The Use of* Moulding — Iron Armored Conduit — Enameled Iron Conduit — Brass Armored Conduit — Asphaltic

CONTENTS ii

PAGE

Paper Tube or Plain Conduit — Flexible Woven Conduit —Flexible Metallic Conduit— The Use of Bends, Elbows, Outlet and Junction Boxes — Directions for Installing Conduit — Conduit Jobs and Their Accessories — Schedule for Wiring Systems . . . . . .124

Chapter VII

Requirements for Iron and Steel Armored Conduit — Laying Out a Conduit System — The Insulation of Conductors — Mechanical Work — Insulating Materials — Concealed and Exposed Work — Insulation Resistance — Grounded Wires — Soldering Solution — A Distribution Sheet for Laying Out Wiring 143

Chapter VIII

The Light of Incandescent Lamps — The Power Consumed by Lamps per Candle Power — Candle Power and Coal — Effect of Low Pressure on Light — Effects of Globes on Light — Size of Room and Number of Lights — Use of Side Lights and Chandeliers — Color of Room Decoration and the Lighting 158

Chapter IX

Switchboards and Their Purpose — The Parts of a Switchboard — Connections of a Shunt Wound Generator — The Circuit Breaker — The Rheostat — Connections of a Compound Wound Generator — Fuses — Connections of Two Shunt Wound Generators and Switchboard — Equal Pressures for Both Dynamos — The Bus Bars — i2 CONTENTS

PAGE

Back View of Switchboard Showing Wiring Connections for Two Shunt Machines — Connections of Compound Wound Machines Showing Bus Bars and Equalizer Bar in Service — Over Compounding — The Series Winding and Its Purpose 168

Chapter X

Generators for Alternating and Direct Current Lighting — Character of Lighting Done — The Switchboard for Two Compound Wound Generators — Connections to Instruments — Switchboard for Control of Six Floors and Two Elevators — Connecting Two Shunt Dynamos According to the Three Wire System at the Switchboard — Switchboards for Electrolytic Work 188

Chapter XI

Panel Switchboards — Street Railway Switchboards — Connections of Compound Wound Generators in a Power House to Switchboard and Instruments — Lightning Arresters — The Panels and Their Functions — Station Fires through Lightning — The Generating, Feeding and Metering Sections . . - . . . . 198

Chapter XII

Testing — The Ground Detector — Testing with a Voltmeter — Use of the Magneto for Testing Insulation — Locating Grounded Circuits — Damp Basements — Use of Insulators — Weatherproof Wire — Cables — Rotary Converters — The Applications of Rotary Converters — Efficiency of Converters 209

CONTENTS 13

Chapter XIII Page

Important Features of Alternating Currents to Consider — Losses in a Line — Inductance Explained — Inductance with Alternating Currents — Resistance Compared with Inductance — Effect of Resistance and Inductance on an Alternating Current — Effect of Capacity on an Alternating Current 223

Chapter XIV

Calculation of Reactance — Value of Induction Reactance — Value of Capacity Reactance — Calculation of Impedance in a Circuit — The Unit of Self Induction — The Power Factor 233

Chapter XV

Circular Mils for Alternating Current Mains — Value of C for Single Phase Currents — Circular Mils Calculated for Single Phase Circuits — Circular Mils Calculated for Two Phase Circuits — Circular Mils Calculated for Three Phase Circuits — Average Power Factors — Weight of Copper — The Induction Motor— Synchronous Motors — Rotaries in Power Transmission — Rotaries in Electric Light Stations — Two and Three Phase Alternator Connections 242

Chapter I

INTRODUCTION. — PURPOSE OF WIRING. — OHM'S LAW. — DROP OF POTENTIAL. — MEANING OF A MILFOOT. — MEANING OF A CIRCULAR MIL. — CALCULATION OF DROP. — CALCU- LATION OF THE SIZES OF WIRE FOR A GIVEN DROP. — WIRING RULES. — CALCULATION OF RESISTANCE FOR BRANCH CIRCUITS. — WHAT IS POWER AND HOW ESTI- MATED.

Introduction. — Wiring is now one of the most important departments of electrical engineering. In the last 15 years it has developed from a comparatively haphazard attempt to conduct current to the various lamps in a building into a systematized code of principles and practices based upon the rulings of the Board of Fire Underwriters, in conjunction with the recommendations of the most prominent society of electrical experts in the United States.

It has in fact assumed an importance in the building arts second to none. No large structure is erected at present without provision being made in the plans for electric wiring. In many cases it has entirely superseded gas and is the only means of lighting considered.

The development of the art of wiring has meant the development of industries dependent upon it for

*5 their existence. Immense amounts of capital are in use for the manufacture of insulated wire of many descriptions ; for the manufacture of sockets, switches, cut-outs, lamps, lampcord, iron pipe conduit, and a host of smaller appliances essential to the installation of a wiring system. The experimental stage has been passed and electric wiring and electric lighting have entered into the administration of the affairs of our large cities as an economic measure required for public safety and convenience. So we enter, as it were, upon a new era in the application of electricity for electric light and power, and this has had enormous influence upon the development and progress of every large city and town.

Regarding the practice of wiring from such standpoints, it is easy to understand the importance to be attached to those principles which lie at the very root of the subject. When it is remembered, that the greater part of the wiring is still to be done in thousands of homes and buildings of the future; that the universal application of electricity for electric lighting will become a fact as soon as the price of electricity is within the means of the humbler classes, and finally that its hygienic benefits are so pronounced both in summer and winter that it must in the course of time be regarded as an indispensable adjunct to home comfort; it will be seen that the art which will reach its greatest development and application in that direction for that purpose is electric wiring.

Statistics may be found in a variety of magazines showing the enormous growth of electric lighting in the United States, but one of the most unique records is that of the fan motor load which is experienced at certain hours of the day in large cities during the summer months.

Recently the New York Stock Exchange expended thousands of dollars in the construction of an equipment, largely electrical, for keeping the air of the exchange 10 degrees cooler than the air of the street during the heated period. Fans 12 feet in diameter are employed for this purpose attached to powerful motors. The air is filtered as it passes through into the exchange thus relieved of every particle of dust.

Purpose of Wiring. — It is not only the distribution of current which is kept in view by laying out a wiring system, but the proportioning of the sizes of wire employed so as to limit the loss of pressure from point to point as required. In the lighting of incandescent lamps it is necessary to supply a definite pressure to the terminals in order to produce the requisite light. The incandescent lamp is peculiarly sensitive to changes of pressure, losing a large percentage of its illuminating power with a slight drop in pressure and gaining rapidly in candle power as the pressure increases. The life of the lamp is seriously affected by more than the necessary pressure ; it rapidly blackens and soon becomes valueless unless such irregularities are checked in the power supply.

Ohm's Law. — Few discoveries of modern times rank in importance with the discovery of Ohm's Law. A study of the principles of electric wiring cannot be carried on without the reader possessing a thoroughly intelligent conception of the meaning and application of this law.

The law in itself is exceedingly simple, and expresses the relationship between amperes, volts and ohms. In order to understand the law the first thing to be done is to gain a knowledge of what is meant by a volt, an ohm and an ampere. In order to do this satisfactorily, illustrations must be employed which though not presenting an ideal simile, yet will serve to convey the idea in view.

Volts. — When a current of electricity passes through a circuit it is set into motion by what is termed electromotive force. If it is impossible to imagine water or steam or any other fluid passing through a pipe without pressure, it is likewise impossible to imagine a current flowing through a circuit without electromotive force. In other words, the force which moves or tends to move electricity is electromotive force.

Electromotive force is measured in volts just as steam pressure comparatively is measured in pounds. The general expression, electromotive force and its measurement in volts, may be understood by reference to the general expression, pressure and its measure in pounds.

Amperes. — If a certain quantity of electricity can be delivered by a current in one second it is because the current has a certain strength. If the current is capable of delivering twice as much in one case as another in one second, it obviously possesses twice the strength. A unit quantity of electricity is called a coulomb. The question now is, what is a coulomb? It can be answered in a practical manner by stating that every particle of copper, silver, gold, nickel or any other metal in an electroplating bath is carried over and deposited on the articles to be plated, thickly or thinly, according to the number of coulombs that have been employed. For instance, one coulomb per second will carry over one-twenty-ninth of an ounce of copper in an hour. Each coidomb always carries over a definite quantity. Each second the same amount is carried over, so that in the course of one hour (or 3,600 seconds) a weight of copper equal to one-twenty-ninth of an ounce has been deposited.

A current of electricity which will give one coulomb per second has a strength of one ampere. This means that a current of one ampere will plate over a weight of copper equal to one-twenty-ninth of an ounce per hour. If the current has a strength of two amperes it will plate twice as much per hour, and so on. A current of the strength of five amperes will give five coulombs per second, ten amperes ten coulombs per second, etc., as indicated in the table:

Table Showing Relation Between Coulombs and Amperes.

Strength of current.

Amperes.

Coulombs per second.

I

I

3 4

IO

Table Showing Relationship Between Coulombs, Copper Deposited and Strength of Current.

Amperes.

Coulombs.

Hours.

Pounds.

Ounces.

Grains.

I

36,000

180,000

IO

360,000

IO

720,000

IO

1,080,000

IO

1,440,000

IO

I

1,800,000

IO

I

It is of great importance to grasp the meaning of Ohm's Law, not only as an abstract relationship between current, electromotive force and resistance, but as a physical relationship, which may be proved by illustration in many ways. The following tables are illustrative of the application of Ohm's Law in three successive cases in which the current remains constant, the volts constant and the resistance constant. The influence of this condition is interesting in each table and shows that either amperes, volts or ohms can be calculated by knowing the other two, as follows :

Table I — Amperes = volts -=- ohms. Table II — Volts = amperes X ohms. Table III — Ohms = volts -f- amperes.

Table I. C = E -r- R.

Current Remains Constant.

Amperes.

Volts.

Ohms.

IO

IO

I

IO

IO

IO

IO

Table II. E = C X R.

Volts Remain Constant.

Amperes.

Volts.

Ohms.

IO

IO

3-333

IOO

2-5

2.0

Table III. R = E-r-C.

Ohms Remain Constant.

Amperes.

Volts.

Ohms.

IO

IOO

IO

IO

IO

IO

In the cases cited E is divided twice, once by R and once by C, and R and C are multiplied together. So it is easy to remember that the two factors multiplied together are the two which are respectively

R ^C

Fig. i. — Diagram of Ohm's Law.

divided into E to get either C or R. For instance E -r- by either C or R = either R or C, and C X R = E, which might be represented by the following sketch :

The two lower ones multiplied give E (Fig. 1) ; the upper one, divided by either of the lower, gives the remaining character. It is very convenient to those unaccustomed to algebraic forms to carry an image in the mind as indicated above with the method of handling it.

Drop of Potential. — A fact with which every one should be familiar is that it is impossible to transmit power from place to place without a loss. If steam is sent through a pipe to run an engine, the longer the pipe the greater the loss of power before the steam is utilized. The smaller the diameter of a pipe the greater the waste of power in transmitting. The same principle applies to wire rope transmission, in which a very large percentage of power disappears between the points sending and receiving it, as in the case of cable car systems, passenger elevators, etc.

A wire conducting electric power is subject to the same law, which manifests itself in two ways ; first, the pressure or voltage diminishes ; secondly, the wire develops heat. The loss of pressure, which may be shown by the voltmeter, can be readily calculated by Ohm's Law:

Drop = amperes X ohms.

For instance, if the problem were given : what is the drop of potential in a line of 10 ohms resistance carrying a current of 10 amperes? the answer would be

Drop = 10 X 10 = 100 volts.

2 4

Rule. — To calculate the drop in a line multiply the amperes by the ohms.

Table Showing Drop in a Line.

Size of wire No. 10 B & S.

Amperes.

Ohms.

Drop in volts.

i,ooofeet

IO

IO

IO

IO

IO

IO

It is evident from an inspection of the table that the drop increases as the resistance or current increases. The loss of power in a line can be diminished by reducing the current in the line or reducing the resistance of the line.

Resistance of Wires. — The resistance of a wire depends upon the length of the wire, its diameter or cross section, and the metal of which it is composed. Resistance is a native property, such as elasticity, ductility, malleability, and depends upon the quality or purity of the metal, or the mixture composing the alloy, as in the case of german silver wire.

If conductors had no resistance, no power would be wasted in transmitting current. In addition, a very small voltage would be sufficient to send heavy currents through a wire. On account of the resistance of a wire being governed by its geometrical dimensions, certain rules have been adopted by means of which the resistance of copper wires of any length or cross section can be readily calculated. The basis which can be employed is the resistance of one foot of copper wire, one onethousandth of an inch in diameter, commonly called a milfoot, which has a resistance of a little less than 11 ohms. The term mil is employed because it means a thousandth of an inch, or a thousandth part, and refers in this case to a round wire of the diameter above mentioned. If two such wires are placed side by side the resistance is reduced to one-half, three such wires will reduce it to one-third, etc. In other words, a rule may be stated as follows :

Rule. — The resistance of a wire of fixed length is inversely proportional to its cross section.

It is customary to call a wire of one mil diameter a circular mil ; a wire of two mils diameter would therefore have four circular mils ; a wire of three mils diameter, nine circular mils, etc.

Table Showing Relation Between Resistance and Cross Section.

Circular mils.

Ohms.

Feet.

10,000

I.O

1,000

20,000

•5

1,000

30,000

•3333

1,000

40,000

.2500

1,000

50,000

.2000

1,000

It is not necessary to show how the resistance increases or diminishes as the wire increases or diminishes in length, while retaining the same cross section in circular mils, because it is obvious that a current must move through twice as much resistance in 1,000 feet of wire as 500 feet of the same cross section. As a proof of this fact the drop of potential with a given current in a fixed cross section is just twice as great with twice the length of wire, but as drop of potential equals C X R it is evident that if the current remains constant the drop in both can only increase or double if the resistance doubles.

A simple and practical rule can be deduced from these facts which will assume the following form :

Rule. — The resistance of a wire is proportional to its length in feet and inversely proportional to its cross section in circular mils.

Table Showing Relation Between Resistance, Cross Section and Length.

Circular mils.

Ohms.

Feet.

10,400

IO

10,000

5,200

IO

5,000

2,600

IO

2,500

1,300

IO

1,250

IO

Calculating Drop in Volts. — In a single circuit the calculation of the drop of pressure is made by a* using Ohm's Law in the form previously given : E = C X R> or volts drop = amperes X ohms. For instance, what is the loss of volts in a simple circuit whose resistance is 10 ohms carrying a current of 3 amperes? According to the rule drop in volts = 10 ohms X 3 amperes or 30 volts. If the circuit is supplying current to lamps, then the volts are no where the current enters; where it leaves in the above case, it would be 30 volts less, or only 80 volts (Fig. 4) ; 30 volts disappearing through the effect of the resistance and current.

+0

LAMP LAMP 1

2 OHMS 2 OHMS 2 OHMS 2 OHMS 2 OHMS 4 VOLTS DROP 4 VOLTS DROP

■o olSJ

VOLTMETER VOLTMETER

Fig. 2. — Lamps in Series, 4 Volts per Lamp.

In the sketch (Fig. 2) the lamps are shown in series with each other; that is, the same current passing through one lamp after the other. As two amperes pass through each lamp as indicated, and as each lamp has two ohms resistance, the drop between the ends of each lamp would be 2X2 = 4 volts. Voltmeters are shown in position across the terminals, each giving a reading of four volts, which is a reading of the drop taking place in the lamp. An experiment of this kind can be tried with five no volt lamps arranged as shown. Only one voltmeter is necessary for readings from every lamp. When they have all been obtained their sum will equal the total voltage applied.

Series Electric Lighting. — A very practical example of the above case of series lighting can be found in high tension arc lighting (Fig. 3), so uni-

O O O

-o — o — o — o

Fig. 3. — High Tension Arc Light System, Series Wiring.

versally employed in large cities. The lamps are placed on street corners as a rule, and extend through the city in this manner for a distance of several miles. A current of about 10 amperes is employed, and each lamp has the equivalent of a resistance of 5 ohms. According to these figures each lamp will have a drop of 50 volts ; therefore, if 10 lamps are lit, 500 volts are required, for 20 lamps 1,000 volts, 40 lamps, 2,000 volts, etc. In a lighting system of this kind all wiring is done in series, in contradistinction to incandescent light wiring, which is done in multiple. The difference between the series system and the multiple system of wiring is readily illustrated by a simple sketch (Fig. 4)-

Fig. 4. — Incandescent Light System, Multiple Wiring.

Multiple Wiring. — In a multiple circuit the current divides up ; each part of the circuit taking current according to its resistance, as shown by Ohm's Law. In the cases mentioned three amperes divide up into three separate currents of one ampere apiece. The current divides as shown because the resistance of each branch will not permit any more to pass through.

Example: The lamps each take 1 ampere at no volts, what is the resistance of each lamp? Referring to the table previously given, it will be seen that this is a case where volts and amperes are given to find ohms. According to the rule, volts divided by amperes gives ohms; therefore no divided by 1 gives no ohms per lamp.

What is Meant by Percentage of Drop. — The drop in either a series circuit or a multiple circuit is calculated from the amperes and ohms of the circuit. A very simple formula is employed for .the purpose of obtaining the size of wire in circular mils, in which a stated loss of volts occurs. For instance, if a building is wired for incandescent lights, so ELECTRIC-WIRING, DIAGRAMS

it is customary to make an allowance beforehand for the drop in volts. This allowance may be 2 per cent., 3 per cent., etc., as the circumstances warrant. If no volts are supplied to the lamps, 2 per cent, or 2.2 volts will be purposely wasted in the circuits before it reaches the lamps. The lamps will therefore receive only 107.8 volts. In using the formula the number of volts to be dissipated in the circuit under consideration must be given.

Formula : Circular mils = feet of wire X amperes in wire X 1 1 -7- volts drop in wire.

Example : Take a circuit 250 feet long carrying 10 amperes, in which 3 volts drop will be allowed, how many circular mils cross section must be supplied for the wire? According to the above, a circuit with a 250 foot run must have 500 feet of wire, giving circular mils equal to 500 X 10 X 11-7-3 = 18,333. The formula is given in symbols in the following form :

where F = feet of wire, A = amperes, n is a constant, V = volts drop and C. M. = circular mils. The constant n is the resistance in ohms of 1 mil foot of copper wire.

Table Showing Effect of Percentage of Drop on Circular Mils of Wire Required.

Formula:

Amperes.

Percentage of drop.

Circular mils.

Feet of wire.

Volts drop.

IO

I

100,000

1,000

I.I

IO

50,000

1,000

2.2

IO

33,333

1,000

3-3

IO

25,000

1,000

4-5

IO

20,000

1,000

5-5

Volts for lighting = no.

The volts supplied are supposed to be fairly constant ; the amperes may vary according to the number of lamps burning. The amount of copper is well represented by the circular mils in each case where the percentage of drop is varied. With 5 per cent, drop only one-fifth of the copper required in the first case is necessary.

Sizes of Wire and Circular Mils. — The sizes of wire are known by reference to the number of circular mils they represent and vice versa. The number of circular mils of a round wire may be obtained by squaring the diameter of the wire in mils.

For instance, a wire one-tenth of an inch in diameter is y 1 -^, or 100 mils in diameter; the square of 100 mils is 100X100=10,000 circular mils.

Resistance and Circular Mils. — For practical purposes it is safe to assume n ohms resistance for a wire I foot in length and one circular mil in cross section. Therefore the resistance of a wire I foot long and having two circular mils cross section will be one-half of n ohms or 5.5 ohms. On this basis fewer circular mils to a wire mean more resistance and more circular mils mean less resistance. The resistance of wires can be calculated by a simple formula which expresses the idea just stated in a concise form.