Code reference · NEC Table 310.16; 110.14(C); 240.4(D); 210.20(A); 210.19(A) IN
What size wire for a 30 amp breaker?
For a 30 A breaker, the minimum copper conductor is 10 AWG — set by NEC 240.4(D), which pairs 10 AWG copper with a maximum 30 A device. In aluminum, the minimum is 8 AWG (40 A at 75°C).
That answer assumes ≤3 current-carrying conductors, 30°C ambient, and a load that isn't continuous. A continuous load (3+ hours — EV charging, water heaters, most lighting) must be limited to 80% of the device: 24 A on this circuit (NEC 210.20(A)). Hot locations or bundled raceways derate the conductor further (310.15) and can push you up a size.
Distance is the other size driver. At 24 A load and 100 ft one-way on 10 AWG copper at 240 V, the computed voltage drop is 5.81 V (2.42%) — inside the NEC's recommended 3% (210.19(A) Informational Note).
Run the numbers yourself
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