What size wire for a 20 amp circuit?
Most 20 A receptacle circuits use 12 AWG copper. Length is what changes the answer.
How to run this check
- Set voltage, 20 A, copper, and one-way feet.
- Leave the 3% target unless the job has a tighter number.
- Read the recommended AWG and what limited it.
The short-run starting point
12 AWG copper is the usual 20 A starting size because the small-conductor planning cap is 20 A. This page’s 75 °C effective ampacity for 12 AWG copper is 20 A.
That is not a license to ignore the 60 °C column on NM-B, termination temperature, or the inspector. It is why 14 AWG is the wrong first guess for a 20 A circuit.
When 12 AWG is no longer enough
On 120 V, 20 A, 100 ft one-way, 12 AWG copper drops about 7.9 V (6.6%). That is over a 3% planning target.
The same load at 240 V is about 3.3% — still a long-run check, but half the percent. Voltage, not just amps, decides whether you upsize.
Planning walk-through for 20 A copper, single-phase, 3% target
| Setup | Recommended size | What limited it | Voltage drop |
|---|---|---|---|
| 120 V, 40 ft | 12 AWG | Ampacity | 3.2 V / 2.6% |
| 120 V, 100 ft | 8 AWG | Voltage drop | 3.1 V / 2.6% |
Related
FAQ
Is 12 AWG copper always legal for 20 A?
No. It is the common starting size. Termination temperature, cable type, bundling, and the AHJ can still change the answer.
Can I use 14 AWG on a 20 A breaker?
Do not plan that way. 14 AWG copper is treated as a 15 A small-conductor size on this site.
Does 100 ft always require an upsize?
No. Voltage and actual load current matter. Run the numbers instead of using a one-line rule.
Planning reference only. Verify against the current NEC, manufacturer data, and the authority having jurisdiction. This is not engineering advice or a license substitute.