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.

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.

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?

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.

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.

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 impedance1 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.

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.

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.

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.

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.)

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.

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.

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.

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.

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."

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

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.

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.

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.)

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.

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 ¹/₃ in., the circular mil area may be found as follows: Reduce the fraction ¹/₃ 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: ¹/₃ = 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.

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:

resistance in ohms = length of wire in ft. × 10.79 circular mils . . . . (1)

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)

resistance= 1,500 × 10.79 10,381 =1.559 ohms.

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:

circular mils = length of wire in ft. × 10.79 resistance in ohms . . . . (2)

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)

circular mils = 1,500 × 10.79 1.559 = 10,381