Ampacity by temperature rating
| Temperature rating | Ampacity | Typical insulation |
|---|---|---|
| 60°C | 240 A | — |
| 75°C | 285 A | THW |
| 90°C | 320 A | THHN / THWN-2, XHHW-2 |
Reference tables · Copper ampacity
2023 NEC · Table 310.16
300 kcmil copper wire ampacity
At a 75°C termination rating, 300 kcmil copper carries 285 A under the published table's basis conditions — 30°C ambient, no more than three current-carrying conductors.
What 300 kcmil copper is used for
300 kcmil copper carries 285 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 285 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 | 316.4 A | 253.1 A | 221.4 A | 158.2 A | 142.4 A |
| 30°C | 285 A | 228 A | 199.5 A | 142.5 A | 128.3 A |
| 40°C | 250.8 A | 200.6 A | 175.6 A | 125.4 A | 112.9 A |
| 50°C | 213.8 A | 171 A | 149.6 A | 106.9 A | 96.2 A |
Properties
| Circular mils | 300,000 |
|---|---|
| DC resistance | 0.0429 Ω per 1000 ft |
| Ampacity at 75°C | 285 A |
| Overcurrent ceiling | Set by ampacity |
| Largest standard breaker | 250 A |
| Area, THHN / THWN-2 | 0.4608 in² |
| Area, XHHW-2 | 0.4536 in² |
| Area, THW | 0.5281 in² |
Voltage drop over distance
Single-phase 240 V, carrying 285 A on 300 kcmil copper.
| One-way distance | Drop | Within 3%? |
|---|---|---|
| 25 ft | 0.26% | Yes |
| 50 ft | 0.51% | Yes |
| 100 ft | 1.02% | Yes |
| 150 ft | 1.53% | Yes |
| 200 ft | 2.04% | Yes |
| 300 ft | 3.06% | 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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