Ampacity by temperature rating
| Temperature rating | Ampacity | Typical insulation |
|---|---|---|
| 60°C | 170 A | — |
| 75°C | 205 A | THW |
| 90°C | 230 A | THHN / THWN-2, XHHW-2 |
Reference tables · Aluminum ampacity
2023 NEC · Table 310.16
250 kcmil aluminum wire ampacity
At a 75°C termination rating, 250 kcmil aluminum carries 205 A under the published table's basis conditions — 30°C ambient, no more than three current-carrying conductors.
What 250 kcmil aluminum is used for
250 kcmil aluminum carries 205 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 200 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 205 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 | 227.6 A | 182 A | 159.3 A | 113.8 A | 102.4 A |
| 30°C | 205 A | 164 A | 143.5 A | 102.5 A | 92.3 A |
| 40°C | 180.4 A | 144.3 A | 126.3 A | 90.2 A | 81.2 A |
| 50°C | 153.8 A | 123 A | 107.6 A | 76.9 A | 69.2 A |
Properties
| Circular mils | 250,000 |
|---|---|
| DC resistance | 0.0847 Ω per 1000 ft |
| Ampacity at 75°C | 205 A |
| Overcurrent ceiling | Set by ampacity |
| Largest standard breaker | 200 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 205 A on 250 kcmil aluminum.
| One-way distance | Drop | Within 3%? |
|---|---|---|
| 25 ft | 0.36% | Yes |
| 50 ft | 0.72% | Yes |
| 100 ft | 1.45% | Yes |
| 150 ft | 2.17% | Yes |
| 200 ft | 2.90% | Yes |
| 300 ft | 4.35% | 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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