Ampacity by temperature rating
| Temperature rating | Ampacity | Typical insulation |
|---|---|---|
| 60°C | 260 A | — |
| 75°C | 310 A | THW |
| 90°C | 350 A | THHN / THWN-2, XHHW-2 |
Reference tables · Copper ampacity
2023 NEC · Table 310.16
350 kcmil copper wire ampacity
At a 75°C termination rating, 350 kcmil copper carries 310 A under the published table's basis conditions — 30°C ambient, no more than three current-carrying conductors.
What 350 kcmil copper is used for
350 kcmil copper carries 310 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 310 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 | 344.1 A | 275.3 A | 240.9 A | 172.1 A | 154.8 A |
| 30°C | 310 A | 248 A | 217 A | 155 A | 139.5 A |
| 40°C | 272.8 A | 218.2 A | 191 A | 136.4 A | 122.8 A |
| 50°C | 232.5 A | 186 A | 162.8 A | 116.3 A | 104.6 A |
Properties
| Circular mils | 350,000 |
|---|---|
| DC resistance | 0.0367 Ω per 1000 ft |
| Ampacity at 75°C | 310 A |
| Overcurrent ceiling | Set by ampacity |
| Largest standard breaker | 300 A |
| Area, THHN / THWN-2 | 0.5242 in² |
| Area, XHHW-2 | 0.5166 in² |
| Area, THW | 0.5958 in² |
Voltage drop over distance
Single-phase 240 V, carrying 310 A on 350 kcmil copper.
| One-way distance | Drop | Within 3%? |
|---|---|---|
| 25 ft | 0.24% | Yes |
| 50 ft | 0.48% | Yes |
| 100 ft | 0.95% | Yes |
| 150 ft | 1.43% | Yes |
| 200 ft | 1.90% | Yes |
| 300 ft | 2.86% | Yes |
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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