🏠 Wire Size for a Sub-Panel (Shed or Detached Garage)
The question every forum argues about, answered with the actual math — including the distance tables nobody shows you.
The short answer
For a 100 amp sub-panel at 150 feet: #2 copper or 1/0 aluminum. For a 60 amp sub-panel at 100 feet: #6 copper or #4 aluminum. The full table for every common combination is below — and if your run isn't in it, the Wire Size calculator handles any distance.
Why every forum thread disagrees
Ask this question online and you'll get three different "right" answers. That's because it's really two checks stacked together, and different people are answering different ones. First, the wire must be rated to carry the current at all (ampacity). Second, over a long run, the wire must be fat enough that voltage lost along the way stays reasonable (voltage drop — the usual planning target is 3%). At short distances the ampacity check decides; past roughly 100 feet, voltage drop takes over and forces bigger wire than the ampacity table alone suggests. People quoting only the ampacity table and people quoting drop-adjusted sizes are both "right" — for different distances.
The table: minimum size at 240 V, 3% voltage drop
Sized at the breaker rating (conservative), 75°C terminations, one-way distance:
| Feeder | 50 ft | 100 ft | 150 ft | 200 ft | 250 ft | 300 ft |
|---|---|---|---|---|---|---|
| 50 A copper | 8 | 6 | 4 | 4 | 3 | 2 |
| 50 A aluminum | 6 | 4 | 3 | 2 | 1 | 1/0 |
| 60 A copper | 6 | 6 | 4 | 3 | 2 | 2 |
| 60 A aluminum | 4 | 4 | 2 | 1 | 1/0 | 2/0 |
| 100 A copper | 3 | 3 | 2 | 1 | 1/0 | 2/0 |
| 100 A aluminum | 1 | 1 | 1/0 | 2/0 | 3/0 | 4/0 |
AWG sizes; computed with the same verified engine as our calculators (K = 12.9 copper / 21.2 aluminum, NEC Table 310.16 ampacities at 75°C). Cable assemblies like NM-B are limited to the 60°C column — see the ampacity guide — and aluminum here means modern AA-8000 series feeder cable (SER/URD/MHF), which is safe and code-recognized.
Worked example: 100 A at 150 feet
Voltage drop = 2 × K × amps × one-way feet ÷ circular mils. For #2 copper (66,360 cmil): 2 × 12.9 × 100 × 150 ÷ 66,360 = 5.8 V, which is 2.4% of 240 V — inside the 3% target. One size smaller (#3, 52,620 cmil) gives 7.4 V = 3.1% — just over. That's the whole disagreement, resolved: #3 works on ampacity, #2 wins on voltage drop. Check your own numbers in the Voltage Drop calculator.
Four things people miss
1. It's a 4-wire feeder. Two hots, a neutral, and a separate equipment ground — and at the sub-panel, neutrals and grounds land on separate bars (the bonding screw stays out). 2. The famous "2-2-2-4" aluminum cable is rated 100 A only as a dwelling service. Feeding a detached-garage sub-panel, it's a 90 A feeder — a classic inspection surprise. 3. Voltage drop doesn't "reset" at the sub-panel. Drop on the feeder and drop on the branch circuit beyond it add together; a common practice is ~2% budget for the feeder and ~3% total to the farthest load. 4. Size for what you'll actually draw, someday. If the garage will someday host a welder or EV charger, the copper you bury today is the cheapest it will ever be.
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