MasterElecWire size · NEC 2023
What size wire for 20 amps?
12 AWG copperThat's the minimum for a typical 20-amp circuit. 14 AWG copper is not permitted on a 20-amp breaker.
Why 12 AWG
Two rules decide this, and you need both. Table 310.16 gives the wire's ampacity. Section 240.4(D) then limits the breaker size on small conductors.
| Copper size | 60°C | 75°C | 90°C | Max breaker, 240.4(D) |
|---|---|---|---|---|
| 14 AWG | 15 A | 20 A | 25 A | 15 A |
| 12 AWG | 20 A | 25 A | 30 A | 20 A |
| 10 AWG | 30 A | 35 A | 40 A | 30 A |
Why 14 AWG doesn't work
This is the common mistake. The table shows 14 AWG copper at 20 A in the 75°C column, so it looks fine. But 240.4(D) caps its breaker at 15 A, so it can't go on a 20-amp circuit.
What about aluminum?
Step up to 10 AWG aluminum or copper-clad aluminum. 240.4(D) limits 12 AWG aluminum to a 15 A breaker and 10 AWG to 25 A.
Long runs: check voltage drop
12 AWG can pass the rules above and still drop too much voltage. Here's a 20 A load, 100 ft one way, on a 120 V single-phase circuit:
| Copper size | Voltage drop | Percent of 120 V |
|---|---|---|
| 12 AWG | 7.9 V | 6.6% |
| 10 AWG | 5.0 V | 4.1% |
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Get MasterElec on iPhoneGeneral guidance for a typical circuit under NEC 2023. Ambient temperature, bundling, terminations and local amendments can change the result. Check the code adopted in your area and your local inspector. Spot an error? Email contact@twoscripts.com.