Example.—A certain circuit supplies 20 kw. to motors at 220 volts and .8 power factor. What is the apparent current?

true watts 20,000
Apparent Current = ————————— = ———— = 113.6 amperes
power factor × volts .8 × 220

Ques. What else, besides power factor, should be considered in making wire calculations for motor circuits?

Ans. The efficiency of the motor, and the heavy starting current.

The product of the efficiency of the motor multiplied by the power factor gives the apparent efficiency, which governs the size of the wires, apparatus, etc., necessary to feed the motors.

Allowance should be made for the heavy starting current required for some motors to avoid undue drop.

TABLE OF APPROXIMATE AMPERES PER TERMINAL FOR INDUCTION MOTORS


Single phase		Two phase
four wire		Three phase
three wire
Horse
power 		110
volts 		220
volts 		440
volts 		110
volts 		220
volts 		440
volts 		110
volts 		220
volts 		440
volts 		550
volts
.5  6.6 3.4 1.8 3.3 1.7 .9 3.7 1.8 1
1  14 7 3.5 6.4 3.2 1.6 7.4 3.7 1.9
2  24 12 6 11 5.7 2.9 13 6.6 3.3 2.5
3  34 17 8.5 16 8.1 4.1 19 9.3 4.7 3.5
4  52 26 13 26 13 6.5 30 15 7.5 6
5  74 37 18.5 38 19 9.5 44 22 11 9
10  94 47 23.5 44 22 11 50 25 12.5 11
15  66 33 16.5 76 38 19 16
20  88 44 22 102 51 25.5 22
25  111 55 28 129 64 32 25
30  134 67 33.5 154 77 38.5 32
40  178 89 44.5 204 107 53.5 44
50  204 102 51 236 118 59 52
75  308 154 77 356 178 89 77
100  408 204 102 472 236 118 100

Ques. What are the usual power factors encountered on commercial circuits?

Ans. A mixed load of incandescent lamps and induction motors will have a power factor of from .8 to .85; induction motors above .8 to .85; incandescent and Nernst lamps .98; arc lamps, .85.