AmpSizerWhat size wire for a 100 amp service?
A 100 amp dwelling service or main feeder can use 4 AWG copper or 2 AWG aluminium under NEC 310.12. A 100 amp sub panel that is not the entire dwelling load needs 3 AWG copper or 1 AWG aluminium.
The difference is the 83% rule. 310.12 allows service and main feeder conductors for a one-family dwelling to be rated at 83% of the service rating, so a 100 amp service needs conductors good for 83 amps — and 4 AWG copper carries 85.
It applies only where those conductors carry the entire load of the dwelling. A 100 amp sub panel fed from a 200 amp service inside the same house is an ordinary feeder, gets ordinary sizing, and that is 3 AWG copper.
The two answers side by side
| The run | Rule that applies | Copper | Aluminium | Ground |
|---|---|---|---|---|
| Service entrance, one-family dwelling | 310.12, 83% | 4 AWG | 2 AWG | see 250.66 |
| Main feeder carrying the whole dwelling | 310.12, 83% | 4 AWG | 2 AWG | 8 AWG |
| Feeder to a separate structure, whole load | 310.12, 83% | 4 AWG | 2 AWG | 8 AWG |
| Sub panel inside the house | Ordinary feeder | 3 AWG | 1 AWG | 8 AWG |
| Sub panel in a detached garage, partial load | Ordinary feeder | 3 AWG | 1 AWG | 8 AWG |
| 100 A branch circuit to one appliance | Ordinary branch circuit | 3 AWG | 1 AWG | 8 AWG |
The 83% allowance exists because a dwelling’s demand is heavily diversified — nothing in a house runs everything at once, and the load calculation in Article 220 already accounts for that. The code recognises it a second time for the conductors that carry the lot.
The distance question
At 240 volts, 100 amps, copper, 3% target:
| Run | Conductor | Drop |
|---|---|---|
| 50 ft | 3 AWG | 1.0% |
| 100 ft | 3 AWG | 2.0% |
| 147 ft | 3 AWG | 3.0% — the crossover |
| 175 ft | 2 AWG | 2.8% |
| 250 ft | 1/0 AWG | 2.5% |
| 400 ft | 3/0 AWG | 2.6% |
Long feeders to outbuildings are where this matters, and it compounds: at 400 feet the conductor is three sizes larger than ampacity alone would ask for, which is a serious difference in the cost of a trench.
What else a 100 amp panel needs
A disconnect at a separate structure. 225.31 and 225.32 want a means of disconnect at the building it supplies, at the nearest point of entry.
A grounding electrode at that structure. 250.32, and the neutral and ground stay separate in the panel — they are bonded only at the service.
One feeder per structure. 225.30, with limited exceptions. Running a second cable out to the same garage later is not permitted just because there is room in the panel.
Work out your own feeder, including the conduit for four conductors, on the sub panel feeder calculator.
Frequently asked questions
Is 4 AWG enough for a 100 amp service?
For a one-family dwelling service or the main feeder carrying the whole dwelling load, yes — 310.12 permits 83% and 4 AWG copper at 85 A covers the 83 A that requires. For anything else at 100 amps, no: 4 AWG is 85 A and you need 100, which is 3 AWG copper or 1 AWG aluminium.
What size wire for a 100 amp sub panel 100 feet away?
3 AWG copper or 1 AWG aluminium, and voltage drop is 2.0% at that distance so ampacity still decides. The crossover is at about 147 feet, where drop passes 3% and 2 AWG copper becomes the answer. Run four conductors — two hots, a neutral and a ground — in 1 inch conduit.
Why do the tables and the electricians disagree about 100 amps?
Because both are right about different cases. The table gives 3 AWG copper for 100 amps. The electrician who says 4 AWG is quoting 310.12 for a dwelling service, which is where they spend most of their time. Ask which one applies to your run and the disagreement disappears.
Can I use aluminium for a 100 amp service?
Yes, and most utility-supplied service laterals already are. 2 AWG aluminium under the 83% rule, or 1 AWG as an ordinary 100 amp feeder. Aluminium service entrance cable in 4-conductor SER is the standard product for this job, and the terminations need the correct compound and torque.
Is 100 amps still enough for a house?
For a house without electric heat, an EV charger or a heat pump, yes, and millions of them run on it. Add any one of those three and it needs a load calculation rather than an opinion. Where the number comes out over 100, load management devices are often cheaper than a service upgrade and are explicitly recognised by the code.
How we calculate this
2 × amps × R × one-way feet ÷ 1,000 on the Chapter 9 Table 8 resistance figures against a 3% target. The
equipment grounding conductor for a 100 A device is 8 AWG copper from Table 250.122; the grounding electrode
conductor at a service is a separate question sized from Table 250.66, and is generally 4 AWG copper for a 100
to 200 A service.Sources: