Ampacity by temperature rating
| Temperature rating | Ampacity | Typical insulation |
|---|---|---|
| 60°C | 215 A | — |
| 75°C | 255 A | THW |
| 90°C | 290 A | THHN / THWN-2, XHHW-2 |
Reference tables · Copper ampacity
2023 NEC · Table 310.16
250 kcmil copper wire ampacity
At a 75°C termination rating, 250 kcmil copper carries 255 A under the published table's basis conditions — 30°C ambient, no more than three current-carrying conductors.
What 250 kcmil copper is used for
250 kcmil copper carries 255 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 250 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 255 A column at the terminals. Check the equipment's listing rather than assuming.
Derating
Ampacity in real conditions
| Ambient | 3 conductors | 4 conductors | 7 conductors | 10 conductors | 21 conductors |
|---|---|---|---|---|---|
| 20°C | 283.1 A | 226.4 A | 198.1 A | 141.5 A | 127.4 A |
| 30°C | 255 A | 204 A | 178.5 A | 127.5 A | 114.8 A |
| 40°C | 224.4 A | 179.5 A | 157.1 A | 112.2 A | 101 A |
| 50°C | 191.3 A | 153 A | 133.9 A | 95.6 A | 86.1 A |
Properties
| Circular mils | 250,000 |
|---|---|
| DC resistance | 0.0515 Ω per 1000 ft |
| Ampacity at 75°C | 255 A |
| Overcurrent ceiling | Set by ampacity |
| Largest standard breaker | 250 A |
| Area, THHN / THWN-2 | 0.397 in² |
| Area, XHHW-2 | 0.3904 in² |
| Area, THW | 0.4596 in² |
Voltage drop over distance
Single-phase 240 V, carrying 255 A on 250 kcmil copper.
| One-way distance | Drop | Within 3%? |
|---|---|---|
| 25 ft | 0.27% | Yes |
| 50 ft | 0.55% | Yes |
| 100 ft | 1.10% | Yes |
| 150 ft | 1.64% | Yes |
| 200 ft | 2.19% | Yes |
| 300 ft | 3.29% | No |
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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