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have a capacity for 25 lamps: Let the loss again be § per cent. The current will be 25 x.6=15.0 amperes. 110.8 (from above) -994=111.74 volts. 111.74—110.8=.94 volts. R=S=-^f=.o6266 ohms. For 1000 feet it would be .6266 and the size corresponding is No. 8 (128 inches).
The size of the 150 foot lengths from the house to the center of distribution will of course be still larger. Allow 1 per cent, then the 99 per cent available will enter the calculation, 100 x .6=60 amperes to be carried in all (2x150) 300 feet. 111.74, (from the previous paragraph) + .99= 112.88. 112.88—111.74 = 1.14 volts lost. R=*=Vs*=.oi9 for 300 feet. For 1000 feet it would be .0633; the size nearest is No. 000 (.409 inches).
From the center of distribution to the station where the dynamos are located was called one-half mile, the length of the circuit will be one mile, or 5280 feet. It is usual to allow more loss in this length than has entered the preceding calculations. Let it be 8 per cent, leaving 92 per cent as effective for lighting. 112.88-92=122.70. 122.70— 112.88=9.82 volts lost in this part of the system, for conveying current for 1000 lamps as stated, iooo x .6=600 amperes. R=J=^f§= .01636ohms resistance for 5280 feet; for 1000 feet it would be .0031 which is outside the reach of the table but could be found to be a wire or rod 11 inches diameter. In practice, this size would be secured by running several smaller wires in multiple.
It is inconvenient for the wire man to figure out these dimensions for every installation, so electric manufacturing companies usually publish tabulated lists from which it is possible to determine the proper sizes of wires to be used. Different sizes and makes of lamps and different working voltages make variations of more or less importance between the Edison, Thomson-Houston, and Westinghouse systems. Besides, all companies do not use the same standard for wire gauges.
The economy of this arrangement of wires cannot be definitely stated. It depends on the "balance" or lack of balance between the number of lamps on both sides of the "neutral" wire. At the best there is a saving of one wire, and the use of lamps of practically twice the ordinary potential. The figuring will be the same as for two ordinary two-wire systems, but with one wire in common.
The principle of this system has been extended to four-wire and five-wire arrangements, but the practical adoption of such was rendered unnecessary by the advent of the more flexible
The advantage of this system lies in the small line wires it is possible to employ, the proper pressure for the lamps being attained by transformers or converters. Lamps of comparatively low voltage can be successfully used.
Two calculations, alike in nature to the previous cases for incandescent lamps are necessary for this system, one for the primary mains, one for the house mains. The tables for the direct current, two-wire system, cannot be made to apply to the secondary, or house mains, with the alternating current as the former are figured for 110 volt-lamps, the latter for 50 or 52 volts,
This is a modification of the ordinary arrangement of lamps, and can be worked by either a continuous or alternating current. The lamps are put in series of 40 on one circuit. The current in each circuit is kept constant, usually at 3^ amperes, so the figuring is the same as for an arc system.
Series motors on arc circuits are to be wired with the same size as the rest of the line wire. As the current is constant, being controlled by the regulator on the generating dynamo, no rheostat is necessary. In figuring the resistance of the line, it is well to assume each horse-power of the motor equivalent to an arc lamp.
Series motors for elevators and some other intermittent machinery are usually supplied from constant potential mains, and in such cases need a rheostat in the circuit to be used in starting only.
In constant potential mains when shunt motors are used a rheostat must always be used in starting. The wiring can be figured on the same basis as if a bank of incandescent lamps were in place of the motor. Leaving out of consideration the size of wires in the street mains, which can be figured in the same way, let it be required to figure for a motor of five horse-power to maintain 110 volts at the brushes; the motor to be located one-fourth mile from the mains and a loss of 5 per cent permitted. The current to be accommodated will be about 40 amperes.
.W=1J6 volts. 116— 110=6 volts loss.
R=J=3%=.I5 ohms for I mile of wire or 2640 feet. ?:<Hb= 0567 ohms per 1000 feet, which is about 000 wire (.409).
GENERAL FORMULAE FOR ALTERNATING CIRCUIT WIRING.
A = Area required in circular mills.
D = Single distance.
N = Number lamps of any voltage.
C — Current in amperes per lamp.
E= Number volts per lamp lost in line.
R= Ratio of conversion.
. _ 21 x D x N x C A E R"~
For example: Let it be required to determine