| Item | Value |
|---|---|
| Breaker type | Double-pole, 240 V |
| Recommended copper conductor | 1 AWG |
| Recommended aluminum conductor | 1/0 AWG |
| Minimum per NEC Table 310.16, 75°C | 3 AWG copper (100 A) |
| Minimum NM-B cable, 60°C column | Not available: NM-B is made only in 14–2 AWG copper (NEC 334.104). Use SER cable or THHN in conduit. |
| Equipment grounding conductor | 8 AWG copper / 6 AWG aluminum (NEC Table 250.122) |
| Maximum continuous load (125% rule) | 80 A |
| Maximum non-continuous load | 100 A |
Main service for a small home, detached garage and ADU feeders, large workshop sub-panels
| Load | Notes |
|---|---|
| Detached garage / workshop sub-panel | Very common 100 A feeder |
| Accessory dwelling unit | Feeder sized from a load calculation |
| Small home main service | The old residential standard |
| Large shop equipment panel | Multiple 240 V machines |
At 100 A the choice of conductor depends on what the circuit is. A general feeder is sized from Table 310.16, where 3 AWG copper reaches exactly 100 A at 75°C. For a dwelling service, or a feeder carrying the entire load of a dwelling, NEC 310.12 permits 83% sizing — which is why 4 AWG copper or 2 AWG aluminum is accepted for a 100 A house service. 1 AWG copper is the conservative choice that covers either case with margin for voltage drop.
Feeders do not require GFCI. A 100 A feeder to a separate structure must include an equipment grounding conductor and requires a grounding electrode system at the structure (NEC 250.32(A)), with neutral and ground kept separate in the sub-panel.
| One-way run | Drop | % of 240V | Verdict |
|---|---|---|---|
| 25 ft | 0.77 V | 0.3% | Within the 3% branch-circuit target |
| 50 ft | 1.54 V | 0.6% | Within the 3% branch-circuit target |
| 75 ft | 2.31 V | 1.0% | Within the 3% branch-circuit target |
| 100 ft | 3.08 V | 1.3% | Within the 3% branch-circuit target |
| 150 ft | 4.62 V | 1.9% | Within the 3% branch-circuit target |
| 200 ft | 6.16 V | 2.6% | Within the 3% branch-circuit target |
The 80% rule: a continuous load is one expected to run for three hours or more — EV chargers, electric heat, and most commercial lighting. NEC 210.20(A) requires the overcurrent device to be rated at not less than 125% of that continuous load, which leaves 80 A usable on a 100 A breaker. A breaker that trips after an hour or two under a steady load is usually a continuous-load sizing mistake, not a faulty breaker.
1 AWG copper or 1/0 AWG aluminum, per NEC Table 310.16 at the 75°C column. The equipment grounding conductor is 8 AWG copper per NEC Table 250.122.
80 amps. NEC 210.19(A)(1) and 210.20(A) require the circuit to be rated at 125% of a continuous load — one that runs for three hours or more — which works out to 80% of the breaker rating available for continuous use. Non-continuous loads may use the full 100 A.
8 AWG copper or 6 AWG aluminum, from NEC Table 250.122. The table lists maximum overcurrent device ratings, so you round up to the next listed row. If the circuit conductors are upsized for voltage drop, NEC 250.122(B) requires the ground to be upsized proportionally too.
No. NEC 334.80 limits NM-B to the 60°C column of Table 310.16, which would require 1 AWG copper for 100 A, but NM-B is only manufactured in 14 through 2 AWG copper (NEC 334.104). Use SER cable or THHN/THWN-2 in conduit, sized from the 75°C column: 3 AWG copper.