What Size Conduit for a 400-Amp Service?

For a 400-amp service entrance, conduit sizing depends on whether you use single 600 kcmil conductors or parallel 4/0 sets:

Single 600 kcmil copper THHN for 400A (NEC 310.16)

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.