Ampacity by temperature rating
| Temperature rating | Ampacity | Typical insulation |
|---|---|---|
| 60°C | 70 A | — |
| 75°C | 85 A | THW |
| 90°C | 95 A | THHN / THWN-2, XHHW-2 |
Reference tables · Copper ampacity
2023 NEC · Table 310.16
4 AWG copper wire ampacity
At a 75°C termination rating, 4 AWG copper carries 85 A under the published table's basis conditions — 30°C ambient, no more than three current-carrying conductors.
What 4 AWG copper is used for
4 AWG copper carries 85 A at 75°C, putting it in feeder and subpanel territory — detached garages, workshops, and service upgrades. It pairs with a 80 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 85 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 | 94.4 A | 75.5 A | 66 A | 47.2 A | 42.5 A |
| 30°C | 85 A | 68 A | 59.5 A | 42.5 A | 38.3 A |
| 40°C | 74.8 A | 59.8 A | 52.4 A | 37.4 A | 33.7 A |
| 50°C | 63.8 A | 51 A | 44.6 A | 31.9 A | 28.7 A |
Properties
| Circular mils | 41,740 |
|---|---|
| DC resistance | 0.308 Ω per 1000 ft |
| Ampacity at 75°C | 85 A |
| Overcurrent ceiling | Set by ampacity |
| Largest standard breaker | 80 A |
| Area, THHN / THWN-2 | 0.0824 in² |
| Area, XHHW-2 | 0.0814 in² |
| Area, THW | 0.0973 in² |
Voltage drop over distance
Single-phase 240 V, carrying 85 A on 4 AWG copper.
| One-way distance | Drop | Within 3%? |
|---|---|---|
| 25 ft | 0.55% | Yes |
| 50 ft | 1.09% | Yes |
| 100 ft | 2.19% | Yes |
| 150 ft | 3.28% | No |
| 200 ft | 4.38% | No |
| 300 ft | 6.57% | 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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