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 | 1.89 V | 1.6% | Within the 3% branch-circuit target |
| 50 ft | 3.79 V | 3.2% | Over 3% — upsize one gauge |
| 75 ft | 5.68 V | 4.7% | Over 3% — upsize one gauge |
| 100 ft | 7.58 V | 6.3% | Over 5% — upsize two or more gauges |
| 150 ft | 11.36 V | 9.5% | Over 5% — upsize two or more gauges |
| 200 ft | 15.15 V | 12.6% | Over 5% — upsize two or more gauges |