What Gauge Wire for a 100-Amp Circuit?
Use 3 AWG copper or 1 AWG aluminum for 100-amp feeders. 3 AWG copper THHN handles 100A at 75°C terminals.
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
| Conductor | NEC Table 310.16 @ 75°C | NEC 310.12 dwelling exception |
|---|---|---|
| 3 AWG copper THHN | 100A | — |
| 4 AWG copper THHN | 85A | 100A (dwelling main only) |
| 1 AWG aluminum | 100A | — |
| 2 AWG aluminum | 90A | 100A (dwelling main only) |
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
| Application | Conductors | Min EMT |
|---|---|---|
| 100A service (3 AWG copper): 2 hots + N + EGC | 4 | 1" EMT (36.2% fill) |
| 100A dwelling (4 AWG copper): 2 hots + N + EGC | 4 | 1" EMT (29.3% fill) |
| 100A sub-panel (3 AWG): 2 hots + N + EGC (8 AWG per 250.122) | 4 | 1" EMT (34.4% fill) |
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.