40 Amp Wire Size
What gauge wire do you need for a 40 amp circuit? The short answer, the full NEC 310.16 table at all three termination ratings, and how far you can run it before voltage drop forces a bigger conductor.
Short answer
A 40 amp circuit needs 8 AWG copper, or 6 AWG aluminium, sized from the 60°C column of NEC 310.16.
At 240 V that copper conductor stays inside a 3% voltage drop for roughly 115 ft one-way. Past that, go up a size.
A 40 A breaker may serve a continuous load of no more than 32 A (NEC 210.19/215.2, the 125% rule).
Minimum conductor by termination rating
Terminations on breakers, lugs and equipment carry a temperature rating. You size the conductor from the lowest-rated component in the circuit, so the same 40 A load can call for different sizes.
| Termination rating | Copper | Aluminium |
|---|---|---|
| 60°C (applies here) | 8 AWG (40 A) | 6 AWG (40 A) |
| 75°C | 8 AWG (50 A) | 8 AWG (40 A) |
| 90°C | 10 AWG (40 A) | 8 AWG (45 A) |
NEC 110.14(C)(1) requires the 60°C column for circuits rated 100 A or less unless every termination is listed for a higher temperature, and the 75°C column above 100 A. Most modern breakers and panels are listed 75°C, which is why the 75°C size is often what actually gets installed. The 90°C column is normally used only as a starting point for derating, not for the final size.
How far can you run it?
Ampacity sets the minimum size; voltage drop sets the maximum distance. These are one-way run lengths at 40 A on a 240 V single-phase circuit, holding drop to 3%.
| Copper size | Max one-way run at 3% | Max one-way run at 5% |
|---|---|---|
| 8 AWG | 115 ft | 192 ft |
| 6 AWG | 183 ft | 305 ft |
| 4 AWG | 289 ft | 482 ft |
| 3 AWG | 360 ft | 600 ft |
3% is the NEC's recommended limit for a branch circuit and 5% for feeder plus branch combined. Both are recommendations in informational notes, not enforceable rules, but most inspectors and specifiers treat 3% as the working target.
What runs on a 40 amp circuit
Typical loads at this rating: electric ranges and cooktops, smaller EV chargers, some heat-pump air handlers.
At the 75 C column 8 AWG copper carries 50 A, comfortably above 40 A. Where terminations are only rated 60 C, 8 AWG is exactly at its 40 A limit.
Check your exact run
These figures assume a standard installation with no derating. For your actual length, voltage, conductor material and temperature, run the numbers:
Ampacities are NEC 310.16 reference values for not more than three current-carrying conductors at 30°C ambient. They do not include ambient-temperature or conductor-bundling derating, and local wiring rules take precedence. Verify against the applicable code before installing.
Frequently Asked Questions
What size wire do I need for a 40 amp breaker?
8 AWG copper, or 6 AWG aluminium, sized from the 60°C column of NEC 310.16. Where every termination in the circuit is listed for 75°C, 8 AWG copper is permitted.
Can I use aluminium wire for a 40 amp circuit?
Yes, provided the terminations are rated for aluminium (marked AL or CU-AL) and an antioxidant compound is used. Aluminium has lower ampacity for a given size, so you need 6 AWG aluminium in place of 8 AWG copper. It is cheaper and lighter on long feeder runs, which is why services are often aluminium.
How far can I run 8 AWG at 40 amps?
About 115 ft one-way on a 240 V single-phase circuit before voltage drop reaches 3%. Beyond that, step up one conductor size for every further increment of length.
What is the largest continuous load on a 40 amp circuit?
32 A. A continuous load is one expected to run for three hours or more, and NEC 210.19 and 215.2 require the circuit to be rated at 125% of it. That is why, for example, EV chargers are always on a breaker well above their charging current.
Does this include derating?
No. These are base ampacities for not more than three current-carrying conductors at 30°C ambient. If you have more conductors in a raceway, a hot attic, or conduit in direct sun, apply the NEC 310.15 adjustment and correction factors, which will push the required size up.
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