Current-carrying capacity of wire. a copper wire of 0.040 in. in diameter is insulated uniformly with plastic to an outer diameter of 0.12 in. and is exposed to surroundings at 100∘f. the heat-transfer coefficient from the outer surface of the plastic to the surroundings is 1.5 btu∕hr ⋅ ft2 ⋅ ∘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 200∘f? the thermal and electrical conductivities may be assumed constant at the values given here: