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Reference tables · Copper ampacity

2023 NEC · Table 310.16

500 kcmil copper wire ampacity

At a 75°C termination rating, 500 kcmil copper carries 380 A under the published table's basis conditions — 30°C ambient, no more than three current-carrying conductors.

500 kcmil500,000 cmil

Ampacity by temperature rating

Temperature ratingAmpacityTypical insulation
60°C320 A—
75°C380 ATHW
90°C430 ATHHN / THWN-2, XHHW-2
500 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 500 kcmil copper is used for

500 kcmil copper carries 380 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 350 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 380 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°C421.8 A337.4 A295.3 A210.9 A189.8 A
30°C380 A304 A266 A190 A171 A
40°C334.4 A267.5 A234.1 A167.2 A150.5 A
50°C285 A228 A199.5 A142.5 A128.3 A
500 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 mils500,000
DC resistance0.0258 Ω per 1000 ft
Ampacity at 75°C380 A
Overcurrent ceilingSet by ampacity
Largest standard breaker350 A
Area, THHN / THWN-20.7073 in²
Area, XHHW-20.6984 in²
Area, THW0.7901 in²

Voltage drop over distance

Single-phase 240 V, carrying 380 A on 500 kcmil copper.

One-way distanceDropWithin 3%?
25 ft0.20%Yes
50 ft0.41%Yes
100 ft0.82%Yes
150 ft1.23%Yes
200 ft1.63%Yes
300 ft2.45%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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