400 Amp Wire Size
What gauge wire do you need for a 400 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 400 amp circuit needs 600 kcmil copper, or two parallel sets of 250 kcmil aluminium, sized from the 75°C column of NEC 310.16.
At 240 V that copper conductor stays inside a 3% voltage drop for roughly 393 ft one-way. Past that, go up a size.
A 400 A breaker may serve a continuous load of no more than 320 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 400 A load can call for different sizes.
| Termination rating | Copper | Aluminium |
|---|---|---|
| 60°C | 750 kcmil (400 A) | Parallel runs required |
| 75°C (applies here) | 600 kcmil (420 A) | Parallel runs required |
| 90°C | 500 kcmil (430 A) | 750 kcmil (435 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 400 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% |
|---|---|---|
| 600 kcmil | 393 ft | 656 ft |
| 750 kcmil | 476 ft | 793 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 400 amp circuit
Typical loads at this rating: large dwelling services, commercial and industrial feeders, multi-unit buildings.
At 400 A a single conductor per phase becomes large and stiff, so installers commonly run two parallel sets of smaller conductors instead. Parallel runs are permitted by NEC 310.10(G) at 1/0 and larger.
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 400 amp breaker?
600 kcmil copper, or two parallel sets of 250 kcmil aluminium, sized from the 75°C column of NEC 310.16. Where every termination in the circuit is listed for 75°C, 600 kcmil copper is permitted.
Can I use aluminium wire for a 400 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 two parallel sets of 250 kcmil aluminium in place of 600 kcmil copper. It is cheaper and lighter on long feeder runs, which is why services are often aluminium.
How far can I run 600 kcmil at 400 amps?
About 393 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 400 amp circuit?
320 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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