| Rule | Copper | Aluminum |
|---|---|---|
| 60°C column — NM-B cable, or any termination not marked 75°C | 1 AWG | 1/0 AWG |
| 75°C column — THHN/THWN-2 with 75°C-rated breaker and lugs | 3 AWG | 1 AWG |
| Dwelling service or main feeder — NEC 310.12 (83% = 83A) | 4 AWG | 2 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.
Main panels, large sub-panels, service entrance
| Diameter | 7.3481 mm (0.289297") |
| Area | 42.4077 mm² |
| Resistance at 20°C | 0.4065 Ω/km |
| Weight | 379.973 kg/km |
| Ampacity (75°C) | 130 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 | 0.77 V | 0.3% | Within the 3% branch-circuit target |
| 50 ft | 1.54 V | 0.6% | Within the 3% branch-circuit target |
| 75 ft | 2.31 V | 1.0% | Within the 3% branch-circuit target |
| 100 ft | 3.08 V | 1.3% | Within the 3% branch-circuit target |
| 150 ft | 4.62 V | 1.9% | Within the 3% branch-circuit target |
| 200 ft | 6.16 V | 2.6% | Within the 3% branch-circuit target |
Aluminum has about 61% of copper’s conductivity, so a 100A feeder that takes 1 AWG copper needs 1/0 AWG aluminum — typically one to two gauge sizes larger.
| Aluminum conductor | 1/0 AWG |
| Ampacity (75°C, Table 310.16) | 120 A |
| Copper equivalent | 1 AWG (130 A) |
| Resistance | 0.151 Ω/1000 ft |
| Voltage drop at 100 ft, 100A | 4.02 V (1.7% 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 100-amp circuit needs 1 AWG copper or 1/0 AWG aluminum. With THHN/THWN-2 conductors and breakers and lugs rated 75°C, 3 AWG copper or 1 AWG aluminum is permitted.
A 100-amp circuit uses a 100A double-pole circuit breaker. The breaker protects the 1 AWG copper (or 1/0 AWG aluminum) wire from overload. Double-pole breakers are used for 240V circuits and occupy two slots in the panel.
A 100-amp dwelling service can be sized under NEC 310.12 at 83% of the rating (83A): 4 AWG copper or 2 AWG aluminum, the familiar #4 Cu / #2 Al service. A 100A feeder to a sub-panel does not get that allowance and needs 1 AWG copper or 1/0 aluminum at 60°C, or 3 AWG copper or 1 AWG aluminum with 75°C terminations.
A 100-amp panel (with correctly sized service conductors) can power a small to medium home. It supports a mix of 15A and 20A branch circuits, plus one or two 240V appliances like a range or dryer — but may be limiting for homes with EV chargers or electric heat.