| Rule | Copper | Aluminum |
|---|---|---|
| 60°C column — NM-B cable, or any termination not marked 75°C | 12 AWG | 10 AWG |
| 75°C column — THHN/THWN-2 with 75°C-rated breaker and lugs | 12 AWG | 10 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.
Kitchen, bathroom, garage circuits
| Diameter | 2.0525 mm (0.080808") |
| Area | 3.3088 mm² |
| Resistance at 20°C | 5.2104 Ω/km |
| Weight | 29.647 kg/km |
| Ampacity (75°C) | 25 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 12 AWG copper at 25 A at 75°C, which looks like more headroom than a 20A circuit needs. It is not usable headroom. NEC 240.4(D), the small-conductor rule, caps the overcurrent device on 12 AWG copper at 20 A regardless of what the ampacity table says. The same applies to aluminum: 10 AWG aluminum shows 30 A in Table 310.16 but is capped at 25 A by 240.4(D).
This is why 12 AWG is the answer for a 20A circuit and why the next size down is not: the smaller conductor’s own 240.4(D) cap falls below 20 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 120V | Verdict |
|---|---|---|---|
| 25 ft | 1.98 V | 0.8% | Within the 3% branch-circuit target |
| 50 ft | 3.96 V | 1.7% | Within the 3% branch-circuit target |
| 75 ft | 5.94 V | 2.5% | Within the 3% branch-circuit target |
| 100 ft | 7.92 V | 3.3% | Over 3% — upsize one gauge |
| 150 ft | 11.88 V | 5.0% | Over 3% — upsize one gauge |
| 200 ft | 15.84 V | 6.6% | 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 20-amp circuit needs 12 AWG copper or 10 AWG aluminum. NEC 240.4(D) sets the minimum here: 12 AWG copper may not be protected above 20A.
A 20-amp circuit uses a 20A single-pole circuit breaker. The breaker protects the 12 AWG copper (or 10 AWG aluminum) wire from overload. Double-pole breakers are used for 240V circuits and occupy two slots in the panel.
No — NEC prohibits using 14 AWG wire on a 20-amp breaker. 14 AWG is rated for 15A maximum. Always use 12 AWG copper (or 10 AWG aluminum) for any 20-amp circuit.
NEC 210.11(C) requires at least two 20A small-appliance circuits for kitchen and dining countertop receptacles, at least one 20A circuit for bathroom receptacles, one 20A laundry circuit, and (since 2020) at least one 20A circuit for garage receptacles. The NEC does not require dedicated circuits for refrigerators, dishwashers or microwaves, although manufacturer instructions and local codes often do.