Safe Carrying Capacity of Wire.—All wires will heat when a current of electricity passes through them. The greater the current or the smaller the wire, the greater will be the heating effect. Large wires are heated comparatively more than small wires because the latter have a relatively greater radiating surface.
Fig. 815.—Pothead wires. The standard wire for pothead work is either No. 19, 20 or 22 B. & S. gauge, either single conductor or twisted pair, insulated to a diameter of 3/32 inch over rubber, without any outer braid or protection. In the case of twisted pairs one conductor is usually made of a differently colored rubber than the other, so as to distinguish between them.
The temperature of a wire increases approximately as the square of the current, and inversely as the cube of the diameter of the wire.
The elevation in temperature of a wire carrying a current represents so much lost energy.
From these considerations it must be clear that it is important not to overload conductors in order to secure efficient working, and to avoid risk of fire on inside installations.
The Board of Underwriters specifies that the carrying capacity of a conductor is safe when the wire will conduct a certain current without becoming painfully hot.
In the following table of carrying capacity, prepared by the underwriters, a wire is assumed to have a safe carrying capacity when its temperature is not increased by the given current over 30° Fahr. above that of the surrounding air.
SAFE CARRYING CAPACITIES OF WIRES
(Maximum amperes allowed by the Underwriters.)
| Brown and Sharpe Gauge | Circular mils | Rubber insulation ——— Amperes | Other insulations ——— Amperes |
|---|---|---|---|
| 18 | 1,624 | 3 | 5 |
| 16 | 2,583 | 6 | 8 |
| 14 | 4,107 | 12 | 16 |
| 12 | 6,530 | 17 | 23 |
| 10 | 10,380 | 24 | 32 |
| 8 | 16,510 | 33 | 46 |
| 6 | 26,250 | 46 | 65 |
| 5 | 33,100 | 54 | 77 |
| 4 | 41,740 | 65 | 92 |
| 3 | 52,630 | 76 | 110 |
| 2 | 66,370 | 90 | 131 |
| 1 | 83,690 | 107 | 156 |
| 0 | 105,500 | 127 | 185 |
| 00 | 133,100 | 150 | 220 |
| 000 | 167,800 | 177 | 262 |
| 0000 | 211,600 | 210 | 312 |
| 200,000 | 200 | 300 | |
| 300,000 | 270 | 400 | |
| 400,000 | 330 | 500 | |
| 500,000 | 390 | 590 | |
| 600,000 | 450 | 680 | |
| 700,000 | 500 | 760 | |
| 800,000 | 550 | 840 | |
| 900,000 | 600 | 920 | |
| 1,000,000 | 650 | 1,000 | |
| 1,100,000 | 690 | 1,080 | |
| 1,200,000 | 730 | 1,150 | |
| 1,300,000 | 770 | 1,220 | |
| 1,400,000 | 810 | 1,290 | |
| 1,500,000 | 850 | 1,360 | |
| 1,600,000 | 890 | 1,430 | |
| 1,700,000 | 930 | 1,490 | |
| 1,800,000 | 970 | 1,550 | |
| 1,900,000 | 1,010 | 1,610 | |
| 2,000,000 | 1,050 | 1,670 |