What Size Wire for 100 Amps?
A 100 amp feeder is the classic subpanel run: to a garage, a shed, or an addition. The short answer is 3 AWG copper or 1 AWG aluminum at 75 degree terminations, but distance can push you a size or two larger. Here is the table, the rules, and the worked drop math.
A 100 amp circuit needs 3 AWG copper (100 amps at 75C) or 1 AWG aluminum (120 amps at 75C) per NEC 310.16. Long runs add voltage drop: a 200 ft run at 100 amps on 3 AWG copper drops 9.81 volts (4.09 percent), so 1 AWG brings it to 6.17 volts (2.57 percent).
The short answer: 3 AWG copper or 1 AWG aluminum
Per NEC Table 310.16 at the 75 degree column, 3 AWG copper is rated 100 amps and 1 AWG aluminum is rated 120 amps, which comfortably covers a 100 amp feeder. This assumes your breaker terminals and equipment are rated 75 degrees, which is true for virtually all modern panelboards. If you are landing on older 60 degree rated equipment, the 60 degree column governs instead: 3 AWG copper is only 85 amps there, so step up to 1 AWG copper (110 amps at 60 degrees).
Distance changes the answer
Voltage drop grows with distance, and a 100 amp feeder moves enough current that drop shows up fast. Work it with VD = 2 x K x I x L / CM on a 240 volt feeder:
100 foot one-way run, copper: 3 AWG copper gives 2 x 12.9 x 100 x 100 / 52620 = 4.90 volts, or 2.04 percent. Inside the 3 percent guidance, so 3 AWG stands.
100 foot one-way run, aluminum: 1 AWG aluminum gives 2 x 21.2 x 100 x 100 / 83690 = 4.02 volts, or 1.67 percent. Also fine.
200 foot one-way run, copper: 3 AWG copper gives 2 x 12.9 x 100 x 200 / 52620 = 9.81 volts, or 4.09 percent, over the 3 percent branch guidance. Upsize to 1 AWG copper: 2 x 12.9 x 100 x 200 / 83690 = 6.17 volts, or 2.57 percent. That is the correct size for the long run.
The pattern is consistent: at 100 amps, every extra 100 feet of run adds roughly 2 percent of drop on the minimum-size wire, so plan the upsizing before you trench.
100 amp subpanel wiring checklist
A 100 amp subpanel feeder needs four conductors: two hots, a neutral, and a separate equipment ground. Size the ground per NEC 250.122: a 100 amp overcurrent device calls for 8 AWG copper (or 6 AWG aluminum) as the minimum equipment grounding conductor. Keep neutral and ground isolated at the subpanel; they bond only at the main service. Most jurisdictions want a permit and inspection for a new subpanel, and the feeder usually goes in conduit (PVC underground, EMT or SER cable above ground) sized for the conductors.
The 80 percent continuous rule
If the 100 amps is a continuous load (running 3 hours or more, like electric heat), NEC 210.19 requires the conductor to be sized at 125 percent of the load: 125 amps. That means 1 AWG copper (130 amps at 75 degrees) or 2/0 aluminum (135 amps at 75 degrees) for a continuous 100 amp load. Most residential subpanels are noncontinuous mixed loads, so the straight 100 amp sizing applies, but check if one big continuous load dominates.
A note on safety: wire sizing involves the National Electrical Code, breaker rules, and local amendments. The examples below are planning estimates. Have a licensed electrician review any real installation before you buy wire or pull a permit.
Skip the arithmetic
Sizing a subpanel feeder? Enter your run length and load to check the voltage drop on 3 AWG, 1 AWG, or any size, in copper or aluminum.
100 amp wire size questions
Can I use aluminum wire for a 100 amp subpanel?
Yes. 1 AWG aluminum is rated 120 amps at 75 degrees, which covers a 100 amp feeder with margin. Use terminals listed for aluminum, apply anti-oxidant compound, and torque to spec. Aluminum 1 AWG is often much cheaper than copper 3 AWG for long feeder runs.
What size breaker protects 3 AWG wire?
3 AWG copper is rated 100 amps at 75 degrees, so a 100 amp breaker is the standard match. The breaker protects the wire, so the breaker size must not exceed the wire's ampacity at its temperature rating.
How far can you run 100 amp wire on 3 AWG copper?
At 240 volts and 100 amps, 3 AWG copper stays under the 3 percent guidance out to roughly 146 feet one-way (9.81 volts at 200 feet is 4.09 percent, over the line). Past that, upsize to 1 AWG, which holds 2.57 percent at 200 feet.
What size ground wire for 100 amp service?
NEC 250.122 sizes the equipment grounding conductor from the overcurrent device: a 100 amp breaker calls for 8 AWG copper or 6 AWG aluminum minimum. That is separate from the neutral, which is sized from the load calculation.
Does a 100 amp subpanel need 4 wires?
Yes. A subpanel feeder needs two hot conductors, a neutral, and a separate equipment grounding conductor. Neutral and ground stay isolated at the subpanel and bond together only at the main service disconnect.