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Reference tables · Copper ampacity

2023 NEC · Table 310.16

400 kcmil copper wire ampacity

At a 75°C termination rating, 400 kcmil copper carries 335 A under the published table's basis conditions — 30°C ambient, no more than three current-carrying conductors.

400 kcmil400,000 cmil

Ampacity by temperature rating

Temperature ratingAmpacityTypical insulation
60°C280 A—
75°C335 ATHW
90°C380 ATHHN / THWN-2, XHHW-2
400 kcmil copper, at a 30°C ambient with no more than three current-carrying conductors in a raceway or cable. The 90°C column is normally used only as the starting point for derating.

What 400 kcmil copper is used for

400 kcmil copper carries 335 A at 75°C. Conductors in the kcmil range are service, feeder, and distribution conductors, frequently run as paralleled sets under rules this reference does not model. It pairs with a 300 A overcurrent device.

Terminations usually decide the practical answer. Breakers and equipment are typically listed for 60°C or 75°C, so even 90°C-rated insulation on this conductor is normally limited to the 335 A column at the terminals. Check the equipment's listing rather than assuming.

Derating

Ampacity in real conditions

Ambient3 conductors4 conductors7 conductors10 conductors21 conductors
20°C371.9 A297.5 A260.3 A185.9 A167.3 A
30°C335 A268 A234.5 A167.5 A150.8 A
40°C294.8 A235.8 A206.4 A147.4 A132.7 A
50°C251.3 A201 A175.9 A125.6 A113.1 A
400 kcmil copper at a 75°C termination rating, corrected for ambient temperature and adjusted for the number of current-carrying conductors, then capped by any small-conductor limit. Adjustment factors: 4–6 → 80%, 7–9 → 70%, 10–20 → 50%.

Properties

Circular mils400,000
DC resistance0.0321 Ω per 1000 ft
Ampacity at 75°C335 A
Overcurrent ceilingSet by ampacity
Largest standard breaker300 A
Area, THHN / THWN-20.5863 in²
Area, XHHW-20.5782 in²
Area, THW0.6619 in²

Voltage drop over distance

Single-phase 240 V, carrying 335 A on 400 kcmil copper.

One-way distanceDropWithin 3%?
25 ft0.23%Yes
50 ft0.45%Yes
100 ft0.90%Yes
150 ft1.35%Yes
200 ft1.80%Yes
300 ft2.70%Yes
Circular-mil estimate at full load. Lighter loads drop proportionally less. Use the voltage drop calculator for your actual current.

Source: NFPA 70, National Electrical Code, Table 310.16; NFPA 70, National Electrical Code, Table 310.15(B)(1)(1); NFPA 70, National Electrical Code, Table 310.15(C)(1); NFPA 70, National Electrical Code, Chapter 9, Table 8. Published by NFPA.NEC 2023

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