A 400 amp circuit needs 600 kcmil copper wire with 75 °C terminations. Aluminum for 400 amps is larger than 600 kcmil, outside the sizes this page covers. As the main service wire of a house, a 400 amp service may use 400 kcmil copper or 600 kcmil aluminum under the 83% rule: see 400 amp service wire size.
400 amp wire size by termination rating
Wire size for a 400 amp circuit by termination temperature rating (60 °C: NM-B cable; 75 °C: THHN and most breakers), with the ground wire
Termination rating
Copper
Aluminum
75 °C, THHN
600 kcmil
larger than 600 kcmil
Ground wire
3 AWG
1 AWG
400 amp wire size by distance at 240 V
At 400 A on 240 V the wire sized for the current already keeps the voltage drop within 3% up to 300 ft one way: 600 kcmil copper drops 2.2% over 300 ft, so distance does not change the size.
Check your own 400 amp run
What are you sizing for?
A
ft
From the panel to the load, not there and back
Voltage
Conductor
Termination rating
Result
600 kcmil copper
Ampacity sets the size; the voltage drop is within 3%.
Breaker
400 A
Voltage drop
0.9 V (0.4%)
Volts at the load
239.1 V
Ground wire
3 AWG copper
How it is calculated
Breaker: 400 A rounded up to a standard size = 400 A.
Smallest copper conductor rated for 400 A at 75 °C: 600 kcmil.
Drop = 2 × K × 400 A × 50 ft ÷ 600,000 cmil = 0.86 V, target 3% of 240 V.