What size wire do I need for a 400 amp breaker?

500 kcmil copper is the smallest conductor our model estimates at or above 400 A using a 75 °C insulation rating; 750 kcmil is required if the terminations are limited to 60 °C. In aluminium the equivalent is 750 kcmil. Verify against the code edition adopted locally before installing.

Can I use aluminium wire on a 400 amp circuit?

Yes, where the terminations are rated for aluminium. Because aluminium carries roughly 78% of the current of the same copper size, a 400 A circuit steps up to 750 kcmil and uses K = 21.2 in the voltage-drop formula instead of 12.9.

How far can a 400 amp circuit run?

On 500 kcmil copper the 3% voltage-drop guideline is reached near 174 ft one-way at 120 V and 348 ft at 240 V. Longer runs need the next size or two up — length, not ampacity, decides most feeder sizes.

Is a 400 amp breaker sized at 100% of the load?

Only for non-continuous loads. A continuous load is designed at 125%, so 400 A of continuous current becomes 500 A of design current and lands on a 500 A device with conductors sized to match.

What is typically on a 400 amp circuit?

Commonly large residential and commercial service entrance. The load type also drives the receptacle and disconnect ratings, not just the conductor size.

What Size Wire for a 400 Amp Breaker?

Conductor size, aluminium equivalent, derating and maximum run length for a 400 amp circuit — all derived from published conductor geometry and the voltage-drop formula.

Quick answer

A 400 amp circuit needs 500 kcmil copper with 75 °C terminations, or 750 kcmil aluminium. 750 kcmil copper applies where equipment terminals are limited to 60 °C. Voltage drop reaches 3% at roughly 348 ft one-way on 240 V and 174 ft on 120 V.

Formula estimate

Copper @ 75 °C

500 kcmil

Copper @ 60 °C

750 kcmil

Aluminium @ 75 °C

750 kcmil

3% drop @ 240 V

348ft

Conductor options for 400 A

Estimated ampacity of each candidate size, so you can see the margin you are buying.

Material / rating	Minimum size	Estimated ampacity	Margin over 400 A
Copper, 60 °C	750 kcmil	444 A	+44 A
Copper, 75 °C	500 kcmil	403 A	+3 A
Copper, 90 °C	500 kcmil	454 A	+54 A
Aluminium, 75 °C	750 kcmil	409 A	+9 A
Aluminium, 90 °C	750 kcmil	461 A	+61 A

Ampacity estimated as I = 0.0670 × cmil^0.6505, scaled for insulation rating and material.

Source & method: Conductor areas come from the ASTM B258 relationship d = 5 × 92^((36 − n) / 39). Ampacity is a fitted physical model, not a reproduction of any code table, and runs within roughly ±8% of published values.

Maximum run length on 500 kcmil copper

One-way distance before voltage drop passes the 3% and 5% guidelines.

System	3% limit	5% limit	Drop @ 100 ft
120 V 1Ø	174 ft	290 ft	1.72%
240 V 1Ø	348 ft	581 ft	0.86%
208 V 3Ø	349 ft	581 ft	0.86%
480 V 3Ø	805 ft	1,342 ft	0.37%

Source & method: VD = 2 × K × I × L / cmil single phase, 1.732 × K × I × L / cmil three phase, with K = 12.9 for copper. 500 kcmil is 500,000 circular mils and 0.0207 Ω per 1000 ft at 20 °C.

Wire size by run length for 400 A

Ampacity sets the floor; distance is what actually forces the upsize on long runs. Smallest copper conductor that carries the load and still holds 3% drop.

One-way run	120 V 1Ø	240 V 1Ø	480 V 3Ø
50 ft	500 kcmil	500 kcmil	500 kcmil
75 ft	500 kcmil	500 kcmil	500 kcmil
100 ft	500 kcmil	500 kcmil	500 kcmil
150 ft	500 kcmil	500 kcmil	500 kcmil
200 ft	600 kcmil	500 kcmil	500 kcmil
250 ft	750 kcmil	500 kcmil	500 kcmil
300 ft	—	500 kcmil	500 kcmil

Sized on both criteria at once: estimated 75 C ampacity and 3% voltage drop at the stated one-way distance.

Source & method: A dash means no listed size holds 3% at that distance and voltage — split the load, raise the system voltage, or move the panel closer.

Hot attics, conduit runs and bundling

What a 400 A circuit actually needs once ambient temperature and conductor count are applied.

Ambient	3 conductors	4–6 conductors	7–9 conductors
30 °C	500 kcmil	750 kcmil	—
40 °C	600 kcmil	—	—
50 °C	—	—	—
60 °C	—	—	—

Source & method: Ambient correction F = √((Tc − Ta) / (Tc − 30)) with Tc = 75 °C; bundling adjustment uses the standard stepped schedule (1.00 / 0.80 / 0.70). The conductor shown is the smallest copper size whose corrected ampacity still carries 400 A.

Where 400 A circuits are actually used

400 A is the top of the range covered here: large commercial and industrial services, switchboard mains, multi-metered apartment services, and high-power EV charging or PV interconnection. Residential 400 A services exist, typically split into two 200 A panels.

A 400 A service is a distribution design, not a single conductor selection: typically parallel sets per phase, grouped service disconnects, and a grounding electrode conductor sized to the service-entrance conductors. Every overcurrent device must have an interrupting rating at or above the available fault current at its point of installation, and equipment must be applied within its short-circuit current rating — that applies at every rating, not only at 400 A. What varies is how the available fault current is established: a utility-supplied figure or a simple calculation is often enough, while a formal fault-current study is warranted where the supply is stiff, the service is large or the authority having jurisdiction asks for one. The tables on this page give the conductor floor; equipment listings and the fault-current basis determine what is actually installed.

Continuous loads

A continuous load — one expected to run three hours or more, like commercial lighting or an EV charger — is designed at 125% of the load current. 400 A continuous becomes 500 A of design current, which lands on a 500 A overcurrent device and 750 kcmil copper.

Frequently asked

What size wire do I need for a 400 amp breaker?: 500 kcmil copper is the smallest conductor our model estimates at or above 400 A using a 75 °C insulation rating; 750 kcmil is required if the terminations are limited to 60 °C. In aluminium the equivalent is 750 kcmil. Verify against the code edition adopted locally before installing.
Can I use aluminium wire on a 400 amp circuit?: Yes, where the terminations are rated for aluminium. Because aluminium carries roughly 78% of the current of the same copper size, a 400 A circuit steps up to 750 kcmil and uses K = 21.2 in the voltage-drop formula instead of 12.9.
How far can a 400 amp circuit run?: On 500 kcmil copper the 3% voltage-drop guideline is reached near 174 ft one-way at 120 V and 348 ft at 240 V. Longer runs need the next size or two up — length, not ampacity, decides most feeder sizes.
Is a 400 amp breaker sized at 100% of the load?: Only for non-continuous loads. A continuous load is designed at 125%, so 400 A of continuous current becomes 500 A of design current and lands on a 500 A device with conductors sized to match.
What is typically on a 400 amp circuit?: Commonly large residential and commercial service entrance. The load type also drives the receptacle and disconnect ratings, not just the conductor size.