Code reference · NEC Table 310.16; 110.14(C); 240.4(D); 210.20(A); 210.19(A) IN
What size wire for a 20 amp breaker?
For a 20 A breaker, the minimum copper conductor is 12 AWG — set by NEC 240.4(D), which pairs 12 AWG copper with a maximum 20 A device. In aluminum, the minimum is 10 AWG (30 A at 75°C, 240.4(D) cap 25 A).
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: 16 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 16 A load and 100 ft one-way on 12 AWG copper at 240 V, the computed voltage drop is 6.18 V (2.57%) — inside the NEC's recommended 3% (210.19(A) Informational Note).
Run the numbers yourself
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