| Rule | Copper | Aluminum |
|---|---|---|
| 60°C column — NM-B cable, or any termination not marked 75°C | 14 AWG | 12 AWG |
| 75°C column — THHN/THWN-2 with 75°C-rated breaker and lugs | 14 AWG | 12 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.
Standard lighting and outlet circuits
| Diameter | 1.6277 mm (0.064084") |
| Area | 2.0809 mm² |
| Resistance at 20°C | 8.2848 Ω/km |
| Weight | 18.645 kg/km |
| Ampacity (75°C) | 20 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 14 AWG copper at 20 A at 75°C, which looks like more headroom than a 15A circuit needs. It is not usable headroom. NEC 240.4(D), the small-conductor rule, caps the overcurrent device on 14 AWG copper at 15 A regardless of what the ampacity table says. The same applies to aluminum: 12 AWG aluminum shows 20 A in Table 310.16 but is capped at 15 A by 240.4(D).
This is why 14 AWG is the answer for a 15A circuit and why the next size down is not: the smaller conductor’s own 240.4(D) cap falls below 15 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 | 2.35 V | 1.0% | Within the 3% branch-circuit target |
| 50 ft | 4.71 V | 2.0% | Within the 3% branch-circuit target |
| 75 ft | 7.07 V | 2.9% | Within the 3% branch-circuit target |
| 100 ft | 9.42 V | 3.9% | Over 3% — upsize one gauge |
| 150 ft | 14.13 V | 5.9% | Over 5% — upsize at least two gauges |
| 200 ft | 18.84 V | 7.8% | 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 15-amp circuit needs 14 AWG copper or 12 AWG aluminum. NEC 240.4(D) sets the minimum here: 14 AWG copper may not be protected above 15A.
A 15-amp circuit uses a 15A single-pole circuit breaker. The breaker protects the 14 AWG copper (or 12 AWG aluminum) wire from overload. Double-pole breakers are used for 240V circuits and occupy two slots in the panel.
No. NEC 240.4(D) limits 14 AWG copper to a 15A overcurrent device, even though Table 310.16 lists it at 20A in the 75°C column. A 20A breaker requires 12 AWG copper or 10 AWG aluminum.
15-amp circuits power most bedroom outlets, living room outlets, hallway lighting, and general-purpose circuits. They use 14 AWG wire and a 15A single-pole breaker. Kitchens, bathrooms, and garages require 20A circuits instead.