What size wire for 20 amps?
12 AWG copper is the minimum for a 20 A circuit under standard conditions. Aluminum: 10 AWG. Long runs, hot spaces and crowded conduit need bigger wire.
Standard conditions only: 30 °C ambient, no more than three current-carrying conductors together, short run. Check your conditions with the wire size calculator. Local codes vary; a licensed electrician should size and install the conductors.
Why 12 AWG?
NEC 110.14(C) limits circuits of 100 A or less to the 60 °C ampacity column unless every termination is marked for 75 °C, and NM-B cable (Romex) is always limited to 60 °C (334.80). In the 60 °C column of Table 310.16, the smallest copper conductor rated for at least 20 A is 12 AWG (20 A).
NEC 240.4(D) also caps 12 AWG copper at 20 A of overcurrent protection, which is why a 12 AWG circuit gets a 20 A breaker even though its 75 and 90 °C ampacities are higher.
Maximum distance for 3% voltage drop
One-way length in feet, copper, at 80% of 20 A (16 A), from NEC Chapter 9 Table 8 resistance.
| Copper size | 120 V | 240 V |
|---|---|---|
| 12 AWG | 57 ft | 114 ft |
| 10 AWG | 91 ft | 181 ft |
| 8 AWG | 145 ft | 289 ft |
| 6 AWG | 229 ft | 458 ft |
| 4 AWG | 365 ft | 731 ft |
For your exact load and length, use the voltage drop calculator.
Questions
What gauge wire for a 20 amp breaker?
12 AWG copper is the minimum under standard conditions (30 °C, no more than three current-carrying conductors, 60 °C terminals or NM-B cable). Aluminum: 10 AWG.
How far can I run 12 AWG on 20 amps?
At 80% load (16 A), 12 AWG copper stays within 3% voltage drop for about 114 ft one-way at 240 V and 57 ft at 120 V. Longer runs need a larger size.
Can I use 14 AWG for 20 amps?
Not on a 20 A breaker under standard conditions. The breaker protects the wire, so the wire's allowable ampacity (and the 240.4(D) limits for 14–10 AWG) must be at least the breaker rating, except where the NEC specifically permits otherwise, such as motor and HVAC circuits.