Ampacity by temperature rating
| Temperature rating | Ampacity | Typical insulation |
|---|---|---|
| 60°C | 35 A | — |
| 75°C | 40 A | THW |
| 90°C | 45 A | THHN / THWN-2, XHHW-2 |
Reference tables · Aluminum ampacity
2023 NEC · Table 310.16
8 AWG aluminum wire ampacity
At a 75°C termination rating, 8 AWG aluminum carries 40 A under the published table's basis conditions — 30°C ambient, no more than three current-carrying conductors.
What 8 AWG aluminum is used for
8 AWG aluminum carries 40 A at 75°C, making it a common choice for 40 A and 50 A circuits — ranges, larger water heaters, EV charging equipment, and hot tubs. It typically pairs with a 40 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 40 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 | 44.4 A | 35.5 A | 31.1 A | 22.2 A | 20 A |
| 30°C | 40 A | 32 A | 28 A | 20 A | 18 A |
| 40°C | 35.2 A | 28.2 A | 24.6 A | 17.6 A | 15.8 A |
| 50°C | 30 A | 24 A | 21 A | 15 A | 13.5 A |
Properties
| Circular mils | 16,510 |
|---|---|
| DC resistance | 1.28 Ω per 1000 ft |
| Ampacity at 75°C | 40 A |
| Overcurrent ceiling | Set by ampacity |
| Largest standard breaker | 40 A |
| Area, THHN / THWN-2 | 0.0366 in² |
| Area, XHHW-2 | 0.0437 in² |
| Area, THW | 0.0556 in² |
Voltage drop over distance
Single-phase 240 V, carrying 40 A on 8 AWG aluminum.
| One-way distance | Drop | Within 3%? |
|---|---|---|
| 25 ft | 1.07% | Yes |
| 50 ft | 2.14% | Yes |
| 100 ft | 4.28% | No |
| 150 ft | 6.42% | No |
| 200 ft | 8.56% | No |
| 300 ft | 12.84% | 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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