A copper wire with twice the diameter of another wire has a current carrying capacity which of the following?

• A. Half as great
• B. Twice as great
• C. One fourth as great
• D. Four times as great

Answer: (D)

Current carrying capacity is determined by how much heat the wire can safely dissipate as current flows through it. For a copper conductor, resistance is inversely related to its cross-sectional area. When the diameter doubles, the cross-sectional area increases by a factor of four (area ∝ diameter^2). With a larger area, the same amount of current produces less heat per unit length, and the conductor can carry more current before reaching the temperature limit imposed by insulation and surroundings. In practice, ampacity is roughly proportional to cross-sectional area, so quadrupling the diameter yields about four times the current-carrying capacity.