Wire size
Wire size for 100 amps
A 100 amp circuit needs 3 AWG copper at the 75 degree C column of NEC Table 310.16, or 1 AWG aluminium. If the terminations are only rated 60 degrees C, which is common on equipment at 100 amps or less, you need 1 AWG copper instead.
Conductor size for 100 amps
NEC 110.14(C) limits you to the lowest-rated termination in the circuit, which on most equipment at 100 amps or less is 60 °C. Check the breaker and the device before using the 75 °C column.
| Material | 60 °C | 75 °C | 90 °C |
|---|---|---|---|
| Copper | 1 | 3 | 3 |
| Aluminium | 1/0 | 1 | 2 |
| One-way run | Drop | Percent | Within 3% |
|---|---|---|---|
| 50 ft | 2.5 V | 1.0% | Yes |
| 100 ft | 4.9 V | 2.0% | Yes |
| 150 ft | 7.4 V | 3.1% | No, size up |
| 200 ft | 9.8 V | 4.1% | No, size up |
What runs on a 100 amp circuit
Subpanels, detached garages and workshops, small dwelling services and larger HVAC equipment.
The thing to watch on this circuit
Most 100 A panels and breakers carry 75 degree C terminations, but check: a 60 degree C rating forces the 60 degree column and pushes you from 3 AWG copper to 1 AWG.
Common questions
What size wire do I need for 100 amps?
- A 100 amp circuit needs 3 AWG copper at the 75 degree C column of NEC Table 310.16, or 1 AWG aluminium. If the terminations are only rated 60 degrees C, which is common on equipment at 100 amps or less, you need 1 AWG copper instead.
What breaker goes with a 100 amp circuit?
- A 100 amp breaker. The conductor is sized to the breaker, not the other way round, and NEC 240.4 requires the overcurrent device to protect the conductor.
How far can I run 100 amps before voltage drop matters?
- On 3 AWG copper at 240 volts, voltage drop reaches 1.0 percent at 50 feet and 4.1 percent at 200 feet. The NEC recommends staying under 3 percent on a branch circuit in the informational note to 210.19(A)(1), which is advice rather than a requirement, so beyond about 150 feet you should go up a size.
Can I use aluminium wire for a 100 amp circuit?
- Yes, at 1 AWG for the 75 degree C column, which is larger than the copper equivalent. Aluminium is common and cost-effective on feeders and services, but the terminations must be listed for aluminium and an antioxidant compound is normally required.
What is a 100 amp circuit usually for?
- Subpanels, detached garages and workshops, small dwelling services and larger HVAC equipment.
Conductor sizes are computed from NEC Table 310.16 with the 240.4(D) small-conductor cap applied. Table 310.16 assumes not more than three current-carrying conductors at 30 °C ambient; correct for ambient (310.15(B)(1)) and conductor count (310.15(C)(1)) before use, and verify against the code edition your jurisdiction has adopted.