640 ELECTRICAL EQUIPMENT 



and corresponding current, and the following approximate formula 

 may be used. 



Volts drop per wire = IR cos <j>+IX sin $, 



where / = current per wire in amperes; 

 R = resistance in ohms per wire; 

 X= reactance in ohms per wire; 

 Cos = power-factor of load. 



Volts drop of two-phase circuit = 2 X (volts drop per wire). 



Volts drop of three-phase circuit = 1. 73 X( volts drop per 

 wire). 



Resistance as well, as reactance values for single-conductor 

 cables are given in Table LIX. The values ate for 2000 feet of 

 wire, i.e., for each wire of a circuit of that length, and apply equally 

 well to bare or lead-covered cables as the insulation or lead cov- 

 ering has practically no effect on the self-induction. 



Table LX gives reactance and impedance values for one mile 

 three-conductor cables. Unlike the reactance values given in 

 Table LIX, which were single-phase, these values are three-phase, 

 i.e., by multiplying them by the current the drop in the full-line 

 voltage (not voltage to neutral) is obtained directly. In calculat- 

 ing the values a 2 per cent allowance for spiral of strands and a 

 2 per cent allowance for spiral of conductors has been made. All 

 the results are based on a cable one mile long but can, of course, be 

 obtained for any shorter distance by reducing the figures given in 

 direct proportion. Similarly, the values correspond to a fre- 

 quency of 60 cycles. For any other frequency, the values given 

 must be multiplied by that frequency and the result divided by 60. 



