If a copper wire has twice the diameter of another, its current carrying capacity is how many times greater?

• A. One half
• B. Twice
• C. Four times
• D. Sixteen times

Answer: (C)

Doubling the diameter increases the cross-sectional area of the wire by a factor of four, since area goes with diameter squared. The current a conductor can carry without overheating is tied to how much heat it can safely dissipate, which depends on the conductor’s ability to shed heat relative to its resistance. With a larger cross-sectional area, resistance per length drops, so more current can flow before reaching the same temperature rise. Under typical simplifying assumptions used in practice problems, the ampacity scales with cross-sectional area, so four times as much current is possible when the diameter doubles. Real-world ratings can vary with insulation, ambient temperature, and cooling, but the fourfold result is the standard takeaway here.