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What size wire for a 15 amp 120 volt circuit?
What size wire for a 20 amp 120 volt circuit?
What size wire for a 30 amp 120 volt circuit?
What size wire for a 20 amp 240 volt circuit?
What size wire for a 30 amp 240 volt circuit?
What size wire for a 40 amp 240 volt circuit?
What size wire for a 50 amp 240 volt circuit?
What size wire for a 60 amp 240 volt circuit?
What size wire for a 70 amp 240 volt circuit?
What size wire for a 80 amp 240 volt circuit?
What size wire for a 100 amp 240 volt circuit?
What size wire for a 125 amp 240 volt circuit?
What size wire for a 150 amp 240 volt circuit?
What size wire for a 200 amp 240 volt circuit?

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Check this against the Code These results come from NEC Table 310.16, 240.4(D) and the standard voltage-drop formula, and they assume no more than three current-carrying conductors in a raceway at 30 degC ambient. Bundling, heat, solar gain and local amendments all change the answer. This is a reference tool, not a design, and it is not a substitute for a licensed electrician or the current printed Code.

Where the numbers come from

Ampacity: NFPA 70 (National Electrical Code), Table 310.15(B)(16), formerly 310.16 - copper, no more than three current-carrying conductors in a raceway, 30 degC ambient.
Overcurrent limits on 14, 12 and 10 AWG: NEC 240.4(D).
Continuous loads sized at 125%: NEC 210.19(A) and 210.20(A).
Standard breaker ratings: NEC 240.6(A).
Conductor geometry and resistance: the AWG definition, d(n) = 0.005 in x 92^((36-n)/39).
Voltage drop: V = 2 x L x R x I, the round trip on a single-phase run.

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