Minimum size by temperature column
| Conductor | 60°C | 75°C | 90°C |
|---|---|---|---|
| Copper | 750 kcmil400 A | 600 kcmil420 A | 500 kcmil430 A |
| Aluminum | 1250 kcmil405 A | 900 kcmil425 A | 700 kcmil425 A |
Equipment grounding conductor
A circuit protected at 400 A takes a 3 AWG copper or 1 AWG aluminum equipment grounding conductor, from the 400 A row of NEC Table 250.122. Raise the ground with the ungrounded conductors when you increase them for voltage drop or any reason other than 310.15(B) or (C) derating, because 250.122(B) requires the same proportion. The ground is never required to be larger than the circuit conductors.
Longest run at 3% voltage drop
| Conductor | 240 V single phase | 240 V three phase |
|---|---|---|
| 600 kcmil copper | 420 ft | 485 ft |
| 900 kcmil aluminum | 380 ft | 440 ft |
NEC 210.19(A)(1) Informational Note 4 recommends keeping branch-circuit drop at or below 3%. These lengths come from the Table 8 resistance at 75°C for a stranded conductor at unity power factor, rounded to the nearest 5 ft. Reactance is not included, so a large conductor in a steel raceway falls short of the figure. A balanced three-phase run reaches about 15% farther than the single-phase figure.
Dwelling service allowance under 310.15(B)(7)
A 120/240 V single-phase dwelling service or feeder rated 100 A through 400 A may use conductors with an ampacity of 83% of the rating. For 400 A that is 332 A, which the 75°C column meets with 400 kcmil copper or 600 kcmil aluminum.
The allowance covers service and feeder conductors only. Branch-circuit conductors, the equipment grounding conductor, and any local amendment are unaffected, and the neutral may be reduced separately under 220.61.
What a 400 A circuit feeds
- Large residential and light commercial services
- Heavy equipment and multi-unit feeders
Four hundred amps is a large dwelling service or a light commercial service, and it is where parallel conductors become normal. 600 kcmil copper or 900 kcmil aluminum reaches it at 75°C. Two parallel sets of 3/0 copper, or two sets of 250 kcmil aluminum, also add up to 400 A and are easier to pull and terminate than a single 600 kcmil conductor.
Parallel conductors carry rules of their own: 310.10(H) requires every set to be the same length, material, size, and insulation type, and to be terminated the same way. Continuous loads top out at 320 A, and a service this size is often metered through current transformers rather than a socket.
A parallel service is usually built as two runs in separate conduits rather than one large raceway, which keeps each conductor in a size that can be bent and landed by hand. The equipment grounding conductor is sized from Table 250.122 for the overcurrent device rating rather than for the parallel set, and 250.122(B) raises it if the phase conductors were increased for voltage drop.
400 A wire size questions
- What size wire do I need for a 400 amp circuit?
- 600 kcmil copper or 900 kcmil aluminum at the 75°C column, which 110.14(C)(1)(b) requires for equipment rated over 100 A. On the 60°C column the same circuit would need 750 kcmil copper or 1250 kcmil aluminum.
- What size ground wire for a 400 amp circuit?
- 3 AWG copper or 1 AWG aluminum, taken from the 400 A row of NEC Table 250.122.
- What size aluminum wire for 400 amps?
- 900 kcmil aluminum at the 75°C column carries 425 A. Terminations on aluminum conductors must be identified for the material.
- Can I use 500 kcmil copper for 400 A?
- Table 310.16 lists 500 kcmil copper at 320 A at 60°C, 380 A at 75°C, and 430 A at 90°C. It falls short at 75°C, the column this circuit uses. The 90°C column does reach 400 A, and a higher column applies only where every termination is listed for it.