400 kcmil Wire Ampacity & Specifications
NEC ampacity, resistance and typical uses for 400 kcmil copper and aluminum wire.
Calculations are built on the 2023 National Electrical Code (NEC) and cross-checked against the published code tables. How we verify. Last reviewed June 2026.
Copper 75°C
335A
Copper 90°C
380A
Aluminum 75°C
270A
Typical breaker
300A
400 kcmil Wire Specifications
| Ampacity — Copper (60 / 75 / 90°C) | 280 / 335 / 380 A |
| Ampacity — Aluminum (60 / 75 / 90°C) | 225 / 270 / 305 A |
| Cross-section | 400,000 circular mils |
| DC resistance — Copper | 0.0321 Ω / 1000 ft |
| DC resistance — Aluminum | 0.0529 Ω / 1000 ft |
| Typical voltage drop (240V) | 0.4% at 50 ft · 0.8% at 100 ft (at 300A) |
Common Uses for 400 kcmil Wire
- 335A copper feeders
- Industrial distribution
Ampacities are from NEC Table 310.16. 14, 12 and 10 AWG are limited to 15A, 20A and 30A overcurrent protection (240.4(D)). Terminations rated 60°C or 75°C usually cap the usable ampacity below the 90°C value.
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