What Size Conduit for a 400-Amp Service?
For 400A residential or light commercial service, typical conduit is 3-inch EMT/IMC/RMC with 600 kcmil copper or 4/0 parallel runs.
For a 400-amp service entrance, conduit sizing depends on whether you use single 600 kcmil conductors or parallel 4/0 sets:
| Conductor configuration | Cu vs Al | Conduit per raceway |
|---|---|---|
| Single 600 kcmil per phase + neutral | Copper | 3-inch EMT/IMC/RMC |
| Single 600 kcmil per phase + neutral | Aluminum | 3-1/2 inch |
| 2 × parallel 4/0 sets per phase + neutral | Copper | 3-inch (single raceway with 6 conductors) |
| 2 × parallel 250 kcmil sets per phase + neutral | Aluminum | 3-inch+ |
Single 600 kcmil copper THHN for 400A (NEC 310.16)
| Variable | Value |
|---|---|
| 600 kcmil THHN area | 0.8676 in² |
| 3 conductors (no EGC in service entrance) | 2.603 in² |
| 40% fill required interior | 6.508 in² |
| 3" EMT (8.846 in²) | 29.4% fill ✓ |
Parallel 4/0 copper for 400A (NEC 310.10(H))
Two parallel sets per phase. Each set must be:
- Identical length, gauge, insulation, terminations
- ≥ 1/0 minimum gauge
6 phase conductors + 2 parallel neutrals = 8 conductors of 4/0 in the raceway. 8 × 0.3237 = 2.590 in² → 29.3% fill in 3" EMT. Fits.
Aluminum 400A (NEC 310.16, 600 kcmil Al)
600 kcmil aluminum at 75°C = 380A — insufficient for 400A. Step to 700 kcmil aluminum at 75°C = 400A.
700 kcmil aluminum is unusual stocking; many installs use 2 parallel sets of 250 kcmil aluminum instead: 2 × 215A at 75°C = 430A.
Two sets of 250 kcmil aluminum × 3 phases = 6 conductors. 6 × 0.4596 (250 kcmil aluminum THHN) = 2.758 in² → 31.2% fill in 3" EMT.
Underground service lateral
For 400A laterals, the typical setup:
- 3-1/2 inch PVC Sch 80 above grade for the riser (where exposed to physical damage)
- 3-inch PVC Sch 40 below grade in trench
If using 600 kcmil aluminum in Sch 80 risers, jump to 4-inch Sch 80 to clear the 40% margin (600 kcmil Al × 3 = 1.379 in² × 3 = 4.137 in² > 40% of 6.442 in² = 2.577 in² — over).
Local AHJ considerations
400A is at the boundary of residential/light commercial. Many AHJs require:
- Separate raceway per phase (parallel install) for ampacity adjustment relief
- IMC or RMC for the riser (not EMT) for added physical protection
- Bonding bushings at every metallic fitting
Quick reference
- Standard service raceway: 3-inch EMT or IMC for 400A copper
- Aluminum: 3-1/2 inch or 4-inch
- Parallel 4/0 Cu: 3-inch EMT (single raceway, 6 conductors)
- Underground: PVC Sch 80 above, Sch 40 below
For commercial 400A feeders with multiple sets, use the conduit fill calculator.