| Rule | Copper | Aluminum |
|---|---|---|
| 60°C column — NM-B cable, or any termination not marked 75°C | 8 AWG | 6 AWG |
| 75°C column — THHN/THWN-2 with 75°C-rated breaker and lugs | 8 AWG | 8 AWG |
Most breakers and panelboards rated 100A or less are listed 60/75°C, but NM-B cable is always limited to the 60°C column (NEC 334.80). When in doubt, use the 60°C answer.
Electric ranges, large AC units
| Diameter | 3.2636 mm (0.12849") |
| Area | 8.3656 mm² |
| Resistance at 20°C | 2.0608 Ω/km |
| Weight | 74.955 kg/km |
| Ampacity (75°C) | 50 A |
Important: For long wire runs, you may need to upsize the wire to compensate for voltage drop. NEC recommends keeping voltage drop under 3% for branch circuits. Use our voltage drop calculator to check.
| One-way run | Drop | % of 240V | Verdict |
|---|---|---|---|
| 25 ft | 1.56 V | 0.6% | Within the 3% branch-circuit target |
| 50 ft | 3.11 V | 1.3% | Within the 3% branch-circuit target |
| 75 ft | 4.67 V | 1.9% | Within the 3% branch-circuit target |
| 100 ft | 6.22 V | 2.6% | Within the 3% branch-circuit target |
| 150 ft | 9.34 V | 3.9% | Over 3% — upsize one gauge |
| 200 ft | 12.45 V | 5.2% | Over 5% — upsize at least two gauges |
Using the 60°C column of NEC Table 310.16, which NEC 110.14(C) requires by default for circuits of 100A or less and always for NM-B cable, a 40-amp circuit needs 8 AWG copper or 6 AWG aluminum. With THHN/THWN-2 conductors and breakers and lugs rated 75°C, 8 AWG copper or 8 AWG aluminum is permitted.
A 40-amp circuit uses a 40A double-pole circuit breaker. The breaker protects the 8 AWG copper (or 6 AWG aluminum) wire from overload. Double-pole breakers are used for 240V circuits and occupy two slots in the panel.
A 40-amp electric range circuit uses 8 AWG copper or 6 AWG aluminum wire on a 40A double-pole breaker. Note: most modern full-size electric ranges are now rated for 50A circuits instead.
No — 10 AWG copper wire is rated for 30A maximum. For a 40-amp circuit you must use at least 8 AWG copper or 6 AWG aluminum wire. Using undersized wire on an oversized breaker is a serious fire hazard.