CH. XIII] 



INSTALLATION OF ARC LAMPS 



501 



694a. The carrying capacity of the different wires in this table is the 

 amperage which can be safely and continuously carried by the wires without 

 injury to the insulation or to the wire. The rubber covered wire is capable of 

 carrying as great an amperage as the wires with more resistant insulation, but 

 the amperage given, is that which experience has shown can be carried without 

 undue injury to the rubber insulation, and with entire safety in continuous use. 



Furthermore, it should be said that the carrying capacity given in the table 

 is by no means the maximum capacity which the wire could carry. For exam- 

 ple, one might send a current of 20 amperes through a No. 18 wire, but this 

 would soon injure the insulation from the overheating. By following the 

 Electrical Code, one is on the safe side. 



695. Table of allowable carrying capacity of 

 cables and cords composed of several small wires. 



flexible 



ESTIMATED CARRYING CAPACITY 



1913 National Electrical Code, 94, p. 186-187. 



695a. This estimate is based upon the law that "The conductivity of a 

 wire is directly proportional to its sectional area." Thus, No. 30 wire has a 

 diameter of .01003 m - an d an area in circular mils of 010.03 x 010.03 = 100.6. 

 The area in circular mils of No. 1 8 wire is 1624 The allowable carrying capa- 

 city of No. 1 8 wire is three amperes when there is rubber insulation (see table 

 above). Assuming the same proportional carrying capacity for the No. 30 wire 



then its capacity would be l624 = IO0 ' 6 , whence I624X =301.8 andX = .18 



3 X 



amp. If one small wire can carry .18 ampere, 15 should carry .18 x 15 = 2.7 

 amperes or in round numbers, 3 amperes. If both cords are united into one 

 conductor there would be 30 small wires with the capacity of .18 x 30 = 5.4 

 amps, or 6 amperes in round numbers. 



For No. 32 wire in the same way: Thus, No. 32 wire has a diameter of 

 .00795 in. The circular mils = 7.95 x 7.95 = 63.21 for each wire. 



