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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 example, 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
B & S Gauge
No. of Wire
Number of wires
Rubber Insulation Amperes
No. 18 No. 1 8 No. 18
7 19 61
25 50 1 2O
No. 1 6 No. 1 6 No. 16
7 19 61
35 70 170
No. 14 No. 14
61 91
235 320
ESTIMATED CARRYING CAPACITY
No. 32 No. 32
40 80
5
10
No. 30
No. 30
15 30
3 6
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 and 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 capacity 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.