What Gauge Wire for a 100-Amp Circuit?

For 100-amp circuits, choose:

• 3 AWG copper THHN — standard table-310.16 sizing, 100A at 75°C terminals
• 1 AWG aluminum — equivalent aluminum size at 100A at 75°C
• 4 AWG copper or 2 AWG aluminum — permitted under NEC 310.12 for residential dwelling main service/feeder only (not for sub-panels in commercial)

NEC reference

NEC 310.12 — the 100A residential exception

For one-family dwellings, two-family dwellings, and multifamily dwellings, NEC 310.12 permits service-entrance conductors and the main feeder to be sized at 83% of the service rating:

• 100A service → 4 AWG Cu or 2 AWG Al
• 125A service → 2 AWG Cu or 1/0 Al
• 150A service → 1 AWG Cu or 2/0 Al
• 200A service → 2/0 Cu or 4/0 Al

This applies only to the main service or main feeder. Sub-panel feeders downstream use Table 310.16 directly.

Conductor count + conduit

Ampacity derating

3 AWG copper THHN: 110A at 90°C. With 4 CCCs derate by 0.80: 110 × 0.80 = 88A — below 100A. Either upsize to 2 AWG (130A × 0.80 = 104A) or limit to 3 CCCs.

For 100A protection with derating, 2 AWG copper is the safer choice in shared-raceway installations.

Voltage drop at 100A

3 AWG copper: 0.197 Ω/1000 ft. At 100A on 100 ft (240V, 200 ft round-trip):

V drop = 200 × 100 × (0.197 / 1000) = 3.94V (1.6% of 240V)

Within 3%. For runs over 150 ft, consider upsizing to 1 AWG.

Quick reference

• Wire (standard): 3 AWG copper THHN/THWN-2 or 1 AWG aluminum
• Wire (dwelling main): 4 AWG copper or 2 AWG aluminum per NEC 310.12
• Breaker: 100A 2-pole main
• EGC: 8 AWG copper per NEC 250.122
• Conduit: 1" EMT typical; 1-1/4" for longer runs with derating margin

For a 200A service, see what gauge wire for 200 amp.