Wire size calculator (NEC)
The smallest copper or aluminum conductor for your load and breaker, by NEC Table 310.16, with an optional 3 % voltage drop check.
Assumes 30 °C ambient and no more than 3 current-carrying conductors. Check local code, equipment ratings and derating.
Electrician Calc: Wire & Panel
Full cable sizing with ambient and grouping derating, IEC and NEC, step-by-step working and PDF reports. See what's inside.
Questions
What size wire for a 30 amp breaker?
With copper THHN and 75 °C terminations, 10 AWG (35 A at 75 °C, and 240.4(D) limits 10 AWG copper to 30 A). Long runs may need a larger size for voltage drop.
Why 125 % for continuous loads?
NEC 210.19(A)(1) and 215.2 require conductors for continuous loads (3 hours or more, like EV chargers and water heaters) to carry 125 % of the continuous current.
Why does NM-B use the 60 °C column?
NEC 334.80 limits NM cable ampacity to the 60 °C column, even though the conductors are rated 90 °C.