10 vs 12 gauge wire for a 20 amp circuit
Short answer: 12 AWG copper is correct for a 20 A circuit and is all the code requires. Use 10 AWG when the run is long: past about 50–60 ft at heavy load, 12 AWG drops more than the 3% the NEC recommends, while 10 AWG keeps it under 3% to roughly 90 ft.
What the code says
NEC Table 310.16 gives 12 AWG copper an ampacity of 20 A in the 60 °C column, and NEC 240.4(D) caps 12 AWG copper at a 20 A breaker. 14 AWG is limited to 15 A, so it can never go on a 20 A breaker. 10 AWG on a 20 A breaker is always allowed: bigger wire is never a code problem, only a cost and workmanship one.
Voltage drop by distance
Percent drop at 120 V, one-way distance, copper. 16 A is a heavy continuous load (80% of the breaker); 20 A is the breaker's full rating.
| Run | 12 AWG @ 16 A | 10 AWG @ 16 A | 12 AWG @ 20 A | 10 AWG @ 20 A |
|---|---|---|---|---|
| 25 ft | 1.3% | 0.8% | 1.6% | 1.0% |
| 50 ft | 2.6% | 1.6% | 3.2% | 2.0% |
| 75 ft | 3.9% | 2.4% | 4.8% | 3.0% |
| 100 ft | 5.1% | 3.2% | 6.4% | 4.0% |
| 150 ft | 7.7% | 4.8% | 9.7% | 6.1% |
Most household 20 A circuits never see 16 A for long, so 12 AWG is fine for a typical kitchen or bathroom run. It's the long, heavily loaded runs where 10 AWG pays off: a detached garage or shed, a workshop with a table saw or compressor, or an outdoor circuit feeding a heater.
The downsides of 10 AWG
- Cost: 10/2 NM-B costs roughly 1.5–1.7× as much as 12/2 per foot.
- Fit: 10 AWG is stiff, fills boxes faster (box-fill counts 2.5 in³ per conductor vs. 2.25 for 12 AWG) and is awkward on receptacle screw terminals. Backstab holes don't accept it at all.
- A common trick: run 10 AWG for the long home run, then pigtail to 12 AWG at the first box. The breaker still protects 12 AWG properly.