638 



ELECTRICAL EQUIPMENT 



so as to deduct as little as possible from the permissible instru- 

 ment load on the transformer, it is the recommended practice to 

 make runs up to 75 feet of 19/25 multi-conductor cable, corre- 

 sponding in conductivity to a No. 12 B. & S. wire. For runs of 

 from 75 to 150 feet, 19/22 cable, corresponding in conductivity 

 to No. 10 B. & S. wire, should be used for mechanical reasons 

 as well as for increased conductivity. For potential and control 

 wiring, 19/25 cable may be used in practically all instances. The 

 above distances refer to 110-volt circuits and for 220 volts they 

 can, of course, be doubled. In general, the size of control leads 

 must also be determined from the standpoint of voltage drop, the 



TABLE LVI 

 SIZE AND AMPERE CAPACITY OF COPPER TUBING 



permissible drop depending on the minimum voltage required for 

 the apparatus in question. This is generally stipulated by the 

 manufacturers. 



Instrument transformer secondaries should be permanently 

 grounded. Where secondaries cannot be grounded at any point, 

 as for instance in the case of instruments and meters which have 

 secondary current and primary potential coils, the secondary 

 wiring must be insulated and installed to safely withstand primary 

 potential. One common ground bus, not less than No. 4 B. & S., 

 should be run across the back of the switchboard, to which appa- 

 ratus mounted on the switchboard intended for grounding should 

 be connected. The switchboard pipe framework, except when 

 insulated, should be connected to this ground bus, one connec- 

 tion being made for every three pipe joints in series. 



Steel work supporting high-potential switching equipment 

 should be carefully grounded at several points so as to prevent 

 the possibility of high voltage occurring between sections of the 



