| Rule | Copper | Aluminum |
|---|---|---|
| 60°C column — NM-B cable, or any termination not marked 75°C | 4 AWG | 2 AWG |
| 75°C column — THHN/THWN-2 with 75°C-rated breaker and lugs | 4 AWG | 3 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.
Large sub-panels, commercial equipment
| Diameter | 5.1894 mm (0.204307") |
| Area | 21.1506 mm² |
| Resistance at 20°C | 0.8151 Ω/km |
| Weight | 189.51 kg/km |
| Ampacity (75°C) | 85 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 | 1.08 V | 0.4% | Within the 3% branch-circuit target |
| 50 ft | 2.16 V | 0.9% | Within the 3% branch-circuit target |
| 75 ft | 3.23 V | 1.3% | Within the 3% branch-circuit target |
| 100 ft | 4.31 V | 1.8% | Within the 3% branch-circuit target |
| 150 ft | 6.47 V | 2.7% | Within the 3% branch-circuit target |
| 200 ft | 8.62 V | 3.6% | Over 3% — upsize one gauge |
Aluminum has about 61% of copper’s conductivity, so a 70A feeder that takes 4 AWG copper needs 3 AWG aluminum — typically one to two gauge sizes larger.
| Aluminum conductor | 3 AWG |
| Ampacity (75°C, Table 310.16) | 75 A |
| Copper equivalent | 4 AWG (85 A) |
| Resistance | 0.3029 Ω/1000 ft |
| Voltage drop at 100 ft, 70A | 5.64 V (2.4% of 240V) |
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 70-amp circuit needs 4 AWG copper or 2 AWG aluminum. With THHN/THWN-2 conductors and breakers and lugs rated 75°C, 4 AWG copper or 3 AWG aluminum is permitted.
A 70-amp circuit uses a 70A double-pole circuit breaker. The breaker protects the 4 AWG copper (or 3 AWG aluminum) wire from overload. Double-pole breakers are used for 240V circuits and occupy two slots in the panel.
A 70-amp sub-panel can power a large workshop, an addition to a home, or multiple high-draw appliances simultaneously. It supports 240V circuits for welders, air compressors, EV chargers, and heating units alongside 120V outlets and lighting.
No. 6 AWG copper is rated 65A at 75°C and 55A at 60°C, both below 70A. Use 4 AWG copper (70A at 60°C, 85A at 75°C), or 3 AWG aluminum with 75°C terminations (2 AWG aluminum at 60°C).