Sized for both ampacity and a 3% voltage drop limit, this run needs 4 AWG copper. Voltage drop governs here - ampacity alone would allow a smaller conductor, but not over this distance.
Answer
Copper and aluminum
NEC 2023This answer is transcribed from the 2023 NEC (NFPA 70, 2023). The code is republished on a three-year cycle and adoption varies by jurisdiction - confirm against your locally adopted edition and your authority having jurisdiction (AHJ) before you build.
40 A load, 250 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.
Material
By ampacity
By voltage drop
Use
Drop
Breaker
Copper
8 AWG
4 AWG
4 AWG
2.58%
40 A
Aluminum
8 AWG
2 AWG
2 AWG
2.66%
40 A
Which requirement governs this run
Ampacity alone would permit 8 AWG copper - that conductor carries 40 A safely. But over 250 ft it would drop more than 3% of the supply voltage before the current reached the load, so the conductor is sized up to 4 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 40 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.
FAQ
Frequently asked questions
What size wire do I need for 40 amps at 250 feet on a 240 V circuit?
4 AWG copper. Under the 2023 NEC tables, that is the smallest copper conductor that both carries 40 A at a 75°C termination rating and stays inside a 3% voltage drop over a 250 ft one-way run at 240 V single phase, assuming a 30°C ambient and no more than three current-carrying conductors.
Does voltage drop change the wire size for 40 amps at 250 feet?
Yes. On ampacity alone the 2023 NEC table would allow 8 AWG copper for 40 A, but over 250 ft that conductor would exceed the 3% voltage drop target used on this page, so the run is sized up to 4 AWG copper. The 2023 NEC presents voltage drop targets as recommendations for efficient operation, not general requirements, though a specification or jurisdiction can make them mandatory.
Can I use aluminum wire for 40 amps at 250 feet?
Yes, at 2 AWG. Aluminum conducts less per unit area than copper, so under the same 2023 NEC assumptions this circuit takes 2 AWG aluminum against 4 AWG copper, with an estimated drop of 2.66%. Terminations and lugs must be listed for aluminum conductors.
What size breaker goes with a 40 amp circuit?
A 40 A breaker, the smallest standard overcurrent rating at or above the 40 A load under the 2023 NEC standard ratings. With a 40 A device ahead of it, the equipment grounding conductor for this circuit is 10 AWG copper. A continuous load, one running three hours or more, is sized at 125% and may need the next rating up.
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 2023Transcribed from the 2023 NEC (NFPA 70, 2023). Confirm against your locally adopted edition and your authority having jurisdiction (AHJ).