MasterElecWire size · NEC 2023
What size wire for a 100-amp subpanel?
1 AWG copper, or 3 AWG at 75°CAt exactly 100 A the 60°C column applies by default, which gives 1 AWG copper. 3 AWG copper works only if the terminations qualify for 75°C.
Why the answer depends on terminations
Terminations matter here. Under 110.14(C), circuits of 100 A or less use the 60°C column unless the equipment is listed and identified for higher-temperature conductors. Above 100 A, the 75°C column applies.
| Size | 60°C | 75°C | Note |
|---|---|---|---|
| 3 AWG copper | 85 A | 100 A | Only with 75°C-rated terminations |
| 2 AWG copper | 95 A | 115 A | Too small at 60°C |
| 1 AWG copper | 110 A | 130 A | Works in either column |
What about aluminum?
Use 1/0 AWG aluminum, rated 100 A at 60°C. 1 AWG aluminum reaches 100 A at 75°C only, under the same termination rule.
Ground wire
Table 250.122 gives a minimum 8 AWG copper equipment grounding conductor for a 100-amp breaker. It may need to be larger if you upsize the phase conductors.
Long runs: check voltage drop
A 100 A load, 100 ft one way, on 240 V single-phase, with 1 AWG copper drops about 3.1 V, or 1.3%. Longer runs may need a larger wire.
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Get MasterElec on iPhoneGeneral guidance for a typical circuit under NEC 2023. Ambient temperature, bundling, terminations and local amendments can change the result. Check the code adopted in your area and your local inspector. Spot an error? Email contact@twoscripts.com.