What Gauge Wire for a 20-Amp Circuit?

12 AWG copper is the standard wire gauge for 20-amp branch circuits, per NEC 240.4(D)(3). 12 AWG THHN has a 90°C ampacity of 30A but is limited to 20A by the NEC small-conductor protection rule.

NEC reference

Required 20-amp circuits per NEC

• Kitchen counter receptacles (≥2 branches per NEC 210.11(C)(1))
• Bathroom receptacles (NEC 210.11(C)(3))
• Laundry receptacle (NEC 210.11(C)(2))
• Garage receptacles (NEC 210.52(G))

Optional 20-amp circuits

• General receptacles in living areas (15A is acceptable; some installers use 20A everywhere)
• Outdoor receptacles
• Workshops, basements

Conduit fill for multi-circuit 20A homeruns

12 AWG THHN homerun configurations:

• 2 circuits (6 conductors): 6 × 0.0133 = 0.0798 in² → 26.3% fill in 1/2" EMT ✓
• 3 circuits (9 conductors): 9 × 0.0133 = 0.1197 in² → 39.4% fill in 1/2" EMT ✓ (at limit)
• 4 circuits (12 conductors): 12 × 0.0133 = 0.1596 in² → 52.5% in 1/2" EMT ✗ → 3/4" EMT instead

Ampacity derating

With 4–6 current-carrying conductors in a raceway, NEC 310.15(C)(1) applies 80% derate. 12 AWG THHN: 30A × 0.80 = 24A. Still ≥ 20A breaker rating — compliant.

Push to 7–9 CCCs (70% derate): 30 × 0.70 = 21A. Marginal — verify against terminal temperature rating.

Voltage drop

12 AWG copper resistance: 1.588 Ω/1000 ft. At 20A on a 100 ft one-way run:

V drop = 2 × 100 × 20 × (1.588 / 1000) = 6.35V (5.3% of 120V)

Over the 3% recommendation. For runs >50 ft at 20A on 12 AWG, upsize to 10 AWG.

Quick reference

• Wire: 12 AWG copper THHN/THWN-2 (or 10 AWG aluminum, rare for branches)
• Breaker: 20A
• Receptacle: 20A T-slot (required for 20A circuits per NEC 406)
• Conduit minimum: 1/2" EMT for up to 3 branches

Use the conduit fill calculator for mixed-gauge runs.