Wire size for a 100 amp breaker
| Wire | Condition | Minimum size for 100 A |
|---|---|---|
| Copper | NM-B (Romex) cable or 60°C terminals | 1 AWG |
| Copper | THHN/THWN in conduit, 75°C terminals | 3 AWG |
| Aluminum | 75°C terminals | 1 AWG |
| Aluminum | 60°C terminals | 1/0 AWG |
3 AWG copper is rated 85 A in the 60°C column, 100 A at 75°C and 115 A at 90°C. The size above is the smallest whose ampacity is at least 100 A. Most modern breakers and panels have 75°C terminals, but a 100 A circuit is presumed 60°C unless the equipment is marked otherwise (NEC 110.14(C)).
Voltage drop: how far can 3 AWG run at 100 A?
The 3 AWG above carries 100 A safely but loses voltage over distance. At the full 100 A and 240 V, 3 AWG copper stays within the NEC's 3% recommendation for about 147 ft one way. 2 AWG stretches that to 185 ft.
| One-way distance | 3 AWG copper | 2 AWG copper | 1 AWG copper |
|---|---|---|---|
| 25 ft | 0.5% | 0.4% | 0.3% |
| 50 ft | 1.0% | 0.8% | 0.6% |
| 100 ft | 2.0% | 1.6% | 1.3% |
| 150 ft | 3.1% | 2.4% | 1.9% |
| 200 ft | 4.1% | 3.2% | 2.6% |
Percent drop at 100 A and 240 V.
On 120 V circuits, the 100 A percentages are twice these values.
Check your own 100 A run with the voltage drop calculator, or let the wire size calculator choose the size for you.
What a 100 amp circuit is used for
- Sub panels feeding a garage, barn or addition
- Level 2 EV chargers drawing 80 A continuous
- Small residential services (see the note below)
- Large heat pumps and electric heating
Cable type matters
NM-B for 100 A must be 1 AWG, since 3 AWG at 60°C is only 85 A. In practice a 100 A feeder uses 3 AWG THHN in conduit on 75°C terminals, or aluminum SE or feeder cable.
Continuous loads and common mistakes
On a 100 A breaker, a continuous load running three hours or more should not exceed 80% of the rating. That is 80 A, or about 19,200 W at 240 V. A 100 A dwelling service or main feeder is treated differently. NEC 310.12 allows 4 AWG copper (or 2 AWG aluminum) for a 100 A single-phase dwelling service, because homes rarely load every circuit at once. That rule does not apply to a sub panel feeder in the same house, which uses the 3 AWG minimum above.
For a 100 A breaker the ground wire comes from NEC Table 250.122. Use the ground wire size calculator. For other breaker sizes, see the breaker to wire size chart, or jump to 20 amps, 30 amps, 40 amps, 50 amps, 60 amps.
Frequently asked questions
What size wire for a 100 amp breaker?
3 AWG copper at 75°C terminals, or 1 AWG for NM-B cable. In aluminum use 1 AWG.
How far can 3 AWG wire run on 100 amps?
About 147 feet one way at 240 V before the voltage drop passes the NEC's 3% recommendation. Past that distance, step up to 2 AWG.
What size copper wire for a 100 amp sub panel?
3 AWG copper at 75°C, or 1 AWG for 60°C terminals. Aluminum needs 1 AWG at 75°C.
What size wire for a 100 amp service?
4 AWG copper or 2 AWG aluminum for a single-phase dwelling, under NEC 310.12. For anything else use 3 AWG copper.