| Rule | Copper | Aluminum |
|---|---|---|
| 60°C column — NM-B cable, or any termination not marked 75°C | 10 AWG | 8 AWG |
| 75°C column — THHN/THWN-2 with 75°C-rated breaker and lugs | 10 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.
Dryers, water heaters, small AC units
| Diameter | 2.5882 mm (0.101897") |
| Area | 5.2612 mm² |
| Resistance at 20°C | 3.2768 Ω/km |
| Weight | 47.14 kg/km |
| Ampacity (75°C) | 35 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.
NEC Table 310.16 rates 10 AWG copper at 35 A at 75°C, which looks like more headroom than a 30A circuit needs. It is not usable headroom. NEC 240.4(D), the small-conductor rule, caps the overcurrent device on 10 AWG copper at 30 A regardless of what the ampacity table says.
This is why 10 AWG is the answer for a 30A circuit and why the next size down is not: the smaller conductor’s own 240.4(D) cap falls below 30 A. Any calculator that reads Table 310.16 without applying 240.4(D) will hand you an undersized, code-violating conductor.
The rule covers 18 through 10 AWG copper and 12 through 10 AWG aluminum. NEC 240.4(E) and 240.4(G) carve out narrow exceptions — tap conductors, motor circuits, air-conditioning equipment, and welders are sized under their own articles instead. See the full table on the wire size calculator.
| One-way run | Drop | % of 240V | Verdict |
|---|---|---|---|
| 25 ft | 1.86 V | 0.8% | Within the 3% branch-circuit target |
| 50 ft | 3.72 V | 1.6% | Within the 3% branch-circuit target |
| 75 ft | 5.58 V | 2.3% | Within the 3% branch-circuit target |
| 100 ft | 7.44 V | 3.1% | Over 3% — upsize one gauge |
| 150 ft | 11.16 V | 4.7% | Over 3% — upsize one gauge |
| 200 ft | 14.88 V | 6.2% | 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 30-amp circuit needs 10 AWG copper or 8 AWG aluminum. NEC 240.4(D) sets the minimum here: 10 AWG copper may not be protected above 30A.
A 30-amp circuit uses a 30A double-pole circuit breaker. The breaker protects the 10 AWG copper (or 8 AWG aluminum) wire from overload. Double-pole breakers are used for 240V circuits and occupy two slots in the panel.
Common 30-amp, 240V appliances include electric dryers, electric water heaters, window or wall air conditioners (up to 240V), and some smaller electric ranges. All require 10 AWG copper or 8 AWG aluminum wire.
No — 12 AWG is only rated for 20A and would be a code violation and fire hazard on a 30A breaker. Electric dryers require 10 AWG copper (or 8 AWG aluminum) wire and a 30-amp, 240V double-pole breaker.