Sized for both ampacity and a 3% voltage drop limit, this run needs 6 AWG copper. Voltage drop governs here — ampacity alone would allow a smaller conductor, but not over this distance.
Answer
Copper and aluminum
Material
By ampacity
By voltage drop
Use
Drop
Breaker
Copper
8 AWG
6 AWG
6 AWG
2.05%
50 A
Aluminum
6 AWG
4 AWG
4 AWG
2.12%
50 A
50 A load, 100 ft one-way, 240 V single phase, 75°C termination rating, 30°C ambient, three current-carrying conductors, and a 3% voltage-drop target. Change any of those and the answer changes — use the calculator for your conditions.
Which requirement governs this run
Ampacity alone would permit 8 AWG copper — that conductor carries 50 A safely. But over 100 ft it would drop more than 3% of the supply voltage before the current reached the load, so the conductor is sized up to 6 AWG on voltage-drop grounds.
Worth being precise about what that means: the smaller conductor would be safe, it just would not perform well. Voltage-drop targets appear in the 2023 NEC as recommendations for reasonable efficiency of operation, not as general requirements. Your specification or jurisdiction may make them mandatory, and some equipment has its own limits.
With a 50 A overcurrent device ahead of it, the equipment grounding conductor for this circuit is 10 AWG copper.
Before you buy wire
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