What Size Wire for a Sub-Panel?
Wire and breaker sizing for a Sub-Panel — the typical 240V circuit, plus the code details (NEC 310.16) that change the answer.
Calculations are built on the 2023 National Electrical Code (NEC) and cross-checked against the published code tables. How we verify. Last reviewed June 2026.
On a 100A 240V circuit at 75°C terminations. 60°C terminations: 1.
Protected by a 100A breaker (NEC 240.6).
Sub-Panel current varies by model. The size above is the common case — always size the conductor to the unit’s nameplate rating and protect it at the labeled maximum breaker/fuse.
Sub-Panel Circuit Details
- Feed the sub-panel at its rating: 60A → 6 AWG, 100A → 3 AWG copper (or 1/0 aluminum), 125A → 1 AWG copper.
- A sub-panel feeder is a 4-wire run with isolated neutral and separate ground bars.
- The NEC 310.12 dwelling-service reduction does NOT apply to ordinary sub-panel feeders.
Long run to your sub-panel?
Over distance, voltage drop can require a larger wire than the table above. See the 100A wire size by distance:
Size it for your exact install
Open the calculator pre-filled with 100A at 240V to add distance, material and temperature.