Minimum wire size
8 AWG
copper at 75°C
50A
ampacity of that size
40A
required (with 125% if cont.)
6 AWG
next size up
NEC 310.16, 30°C ambient, ≤3 conductors. Derate for conduit fill, higher ambient, or long runs — and check the 15/20/30A breaker limit on 14/12/10 AWG (240.4(D)).
Long run? Also check voltage drop. Then price the wire, breakers, and labor into a free estimate.
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Cómo se calcula
The calculator picks the smallest conductor whose allowable ampacity (NEC 310.16, at your temperature column) is at least the load — sized to 125% if you mark it continuous.
Base ampacities assume 30°C ambient and three or fewer current-carrying conductors. Real installs derate for conduit fill, ambient heat, and bundling, must respect the weakest termination’s temperature rating (110.14(C)), and cap 14/12/10 AWG at 15/20/30A breakers (240.4(D)). Use this to get in the ballpark, then verify against the full install conditions.
Es una estimación de planeación: conviene confirmarla siempre con la normativa, las especificaciones y tu proveedor antes de pedir o instalar.
Wire Size Calculator: FAQ
What size wire do I need for my amps?
Match the load to the conductor ampacity in the NEC 310.16 table for your metal and termination temperature. At the common 75°C column, copper handles 20A on 12 AWG, 30A on 10 AWG, 50A on 8 AWG, 65A on 6 AWG, and 100A on 3 AWG. This calculator picks the smallest size that carries your load.
What is the difference between the 60, 75, and 90°C columns?
They are the conductor’s insulation temperature ratings. You must size to the temperature rating of the weakest termination — most breakers and lugs are rated 75°C, so the 75°C column is the usual choice even with 90°C wire.
Why does a continuous load need bigger wire?
For a load running 3 hours or more, the NEC requires the conductor and breaker to be sized at 125% of the load. A 40A continuous load is sized as 50A. Toggle "continuous" and the calculator applies the 125%.
Do I still need to derate?
Often, yes. These are base ampacities at 30°C with three or fewer conductors. More conductors in a conduit, higher ambient temperature, or bundling require derating to a smaller allowable ampacity — and 14, 12, and 10 AWG have breaker limits of 15, 20, and 30A regardless (NEC 240.4(D)).
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