Current-carrying capacity of wire. A copper wire of 0.040 in. diameter is insulated uniformly with plastic to an outer diameter of 0.12 in. and is exposed to surroundings at 100F. The heat transfer coefficient from the outer surface of the plastic to the surroundings is 1.5 btu/hr x ft^2 x F. What is the maximum steady current, in amperes, that this wire can carry without heating any part of the plastic above its operating limit of 200F? The thermal and electrical conductivities may be assumed constant at the values given here:
Copper 220 btu/hr x ft x F ; 5.1x10^5 ohm^-1 cm^-1
Plastic .20 btu/hr x ft x F ; 0.0 ohm^-1 cm^-1