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brick walls as they are likely to sweat and thus introduce moisture into the moulding.

Approved rubber insulated wire should be used.

CONCEALED KNOB AND TUBE WORK. This is allowed by the underwriters rules but prohibited by ordinances in a great many cities.

Approved rubber covered wire should be used. The wire must be separated at least 1 inch from the surface wired over and must be kept at least 10 inches apart, and, when possible, should be run singly on separate timbers or studdings. They must be separated from contact with walls, floors, timbers, etc., through which they pass by insulating tubes, such as glass or porcelain.

Rigid supports are required under ordinary conditions at least every 42 feet, but a generous use of tubes, cleats, or knobs is advisable in places where the circuits are entirely concealed and where any derangement can not be observed.

All outlets must be protected by insulating tubing or by conduit. In cases of combination fixtures the tubes must extend at least flush with the outer end of the gas cap.

CONDUIT WIRING. Conduit wiring is the best and safest, and the only kind allowed in fireproof buildings. Iron pipes with galvanized or enameled interiors are most exclusively used. The smallest size conduit has an interior diameter of 5% inch. Care should be taken that the insides of the pipes are free from rough spots or projections. The entire system of conduit must be continuous, and permanently and effectively grounded. The inside edge of bends should never have a radius of less than 31⁄2 inches, nor more than the equivalent of four quarter bends should be placed between two outlets.

Rubber covered wire should be used.

In A. C. circuits it is necessary and in D. C. it is advisable that the two sides of the circuit should be contained in the same conduit.

Calculation of Size of Wire

The size of an electrical conductor is usually given in circular mils. A mil is one thousandth of an inch, and a circular mil is the area of a circle, the diameter of which is one mil.

To obtain the area of a conductor in circular mils, knowing the diameter, all that is necessary is to multiply the diameter expressed in mils by itself, i. e., square the diameter. For example a 14-inch cable has a diameter of 250 mils and an area 250 × 250 = 62,500 circular mils. Every conductor offers some resistance to the flow of current. This resistance increases as the length increases and decreases as the cross section increases. For metal conductors this resistance increases as the temperature rises. The resistance of one foot of copper wire with a cross section area of one circular mil has been found to be 10.8 ohms at 75° F. For wiring calculations this is commonly taken as 11. ohms. The resistance of any conductor is, therefore

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L=Length of the conductor in feet. A = cross section area in circular mils.

For example 1,000 feet of 4-inch cable with an area of 62,500 circular mils has a resistance of

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The voltage (E) consumed when current flows through a conductor is equal to the product of the current in amperes (I) and the resistance in ohms (R).

E=IX R

If the cable referred to above were carrying 100 amperes, the volts drop in the cable would be

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From the two equations already given a third can be obtained by substituting the value of (R) from the first into the second

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This equation is used for finding the size of wire (A) in circular mils necessary to carry a known current over a given distance with a certain allowable drop in voltage. For example: What size of wire will be necessary to carry 100 amperes over a distance of 300 feet with voltage drop of 5? To carry the circuit over a distance of 300 feet will require 600 feet of wire.

A =

11 X600 X 100
5.

=132,000 circular mils.

Looking up in the table the nearest size is No. 00. The current values given in the table must not be exceeded and in cases where the allowable drop is large the values given in the table and not the drop will determine the size of the wire to be used.

To calculate the drop it is necessary to know the current flowing in the circuit. In incandescent lamp circuits this depends on the efficiency of the lamps and on the voltage and may be calculated approximately from the following formula:

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I Current in amperes. in parallel in the circuit. Z = Watts consumed per c. p. in the lamps. E Voltage of the circuit.

N = Number lamps connected
Р Candle power of lamps.

=

The following values of Z will give fairly good results. 3.5 for 110 volt carbon filament incandescent lamps. 4.0 for 220 volt carbon filament incandescent lamps.

Tungsten and Tantalum lamps are rated in watts, in which case this value is substituted for P Z, the formula becoming

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Wwatts consumed in one lamp.

In most cases the distance to be used in calculating the size of wire should be an average, for example with a circuit 200 feet long with lamps distributed over the last 100 feet, the distance taken should be 100+ 1⁄2 (100) 150 feet of circuit.

=

The allowable drop is usually given in per cent. of the voltage. To obtain the actual volts drop, multiply the voltage by the per cent. drop, pointing off two places, i. e., a 2% drop on a 100 volt line would be 100 x .02 = 2 volts.

What size of wire is necessary for a 110 volt incandescent lighting circuit 400 feet long carrying 40-16 c. p. carbon filament lamps. The lamps are distributed over the last 50 feet and a 2% voltage drop is allowed.

The distance to the center of distribution is:

350+12 X 50 = 375 feet. Therefore 375 × 2 = 750 feet of wire should be used in the calculations.

40 X 16 X 3.5

The current I=

=20.4 amp.

110

The allowable drop-D = 110 x .02 = 2.2 volts.

11 X 750 X 20.4

A=

=76500 circular mils.

2.2

The nearest size to this is No. 1 B. & S. gauge. The carrying capacity is sufficient for the current.

TABLE 3

Carrying Capacity of Copper Wires

The following table, showing the allowable carrying capacity of copper wires and cables of 98% conductivity, according to the standard adopted by the American Institute of Electrical Engineers, must be followed in placing interior conductors.

For insulated aluminum wire the safe carrying capacity is 84% of that given in the following tables for copper wire with the same kind of insulation.

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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 above tables.

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