Wire size calculator · 100 A breaker
100 A · 75 ft · 240 V single phase
Sized for both ampacity and a 3% voltage drop limit, this run needs 3 AWG copper. Ampacity governs here; voltage drop is comfortably inside the limit.
Answer
| Material | By ampacity | By voltage drop | Use | Drop | Breaker |
|---|---|---|---|---|---|
| Copper | 3 AWG | 4 AWG | 3 AWG | 1.53% | 100 A |
| Aluminum | 1 AWG | 3 AWG | 1 AWG | 1.58% | 100 A |
Ampacity governs here. 3 AWG copper is the smallest conductor that carries 100 A at a 75°C termination rating, and over 75 ft its voltage drop of 1.53% is comfortably inside the 3% target.
That comfort disappears as the run gets longer. The same 100 A load at triple this distance would likely need a size or two more purely for voltage drop.
With a 100 A overcurrent device ahead of it, the equipment grounding conductor for this circuit is 8 AWG copper.
This page fixes several assumptions that may not match your job. It assumes a 75°C termination rating, which is typical but not universal — check what your breaker and equipment are actually listed for. It assumes a 30°C ambient, which an attic, a rooftop, or a boiler room will exceed. It assumes three current-carrying conductors in the raceway; more than that triggers an adjustment that reduces ampacity further.
It also assumes a non-continuous load. A load that runs three hours or more is sized at 125% of its rating, which frequently pushes the conductor up a size. And it does not model motor circuits, air conditioning equipment, or the other load types that have their own sizing articles.
Run your real numbers through the wire size calculator, then have the result confirmed by a licensed electrician before anything gets pulled.
Source: NFPA 70, National Electrical Code, Table 310.16; NFPA 70, National Electrical Code, 210.19(A) and 215.2(A) informational notes. Published by NFPA.NEC 2023
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