400 kcmil Wire Ampacity & Specifications
400 kcmil copper wire carries 335A at 75°C (NEC Table 310.16), or 270A for aluminum; copper is typically protected by a 300A breaker.
Calculations are built on the 2023 National Electrical Code (NEC) and cross-checked against the published code tables. How we verify. Last updated June 2026.
400 kcmil wire has an ampacity of 335A at 75°C for copper and 270A for aluminum (NEC Table 310.16). Its 60°C and 90°C copper ratings are 280A and 380A. Copper 400 kcmil is commonly protected by a 300A breaker.
Copper 75°C
335A
Copper 90°C
380A
Aluminum 75°C
270A
Copper breaker (75°C)
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. Copper 14, 12 and 10 AWG are limited to 15A, 20A and 30A overcurrent protection, aluminum 12 and 10 AWG to 15A and 25A (240.4(D)). Terminations rated 60°C or 75°C usually cap the usable ampacity below the 90°C value.
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