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 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:

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k (Btu/hr • ft · F) k, (ohm"' cm"') ст 220 5.1 × 105 Copper 0.20 Plastic 0.0