Enter your circuit parameters to find the correct wire gauge. Accounts for ampacity (NEC Table 310.16), the NEC 240.4(D) small-conductor limits, and voltage drop.
NEC Table 310.16 lists a 75°C ampacity for every conductor, but for the smallest building-wire sizes NEC 240.4(D) overrides it with a lower hard limit on the overcurrent device. This is the single most common source of wrong answers in wire-size calculators: 12 AWG copper appears in Table 310.16 at 25 A, but you may never put it on a breaker larger than 20 A. This calculator applies the 240.4(D) cap.
| Conductor | Table 310.16 (75°C) | 240.4(D) max breaker |
|---|---|---|
| 18 AWG copper | — | 7 A |
| 16 AWG copper | — | 10 A |
| 14 AWG copper | 20 A | 15 A |
| 12 AWG copper | 25 A | 20 A |
| 10 AWG copper | 35 A | 30 A |
| 12 AWG aluminum | 20 A | 15 A |
| 10 AWG aluminum | 30 A | 25 A |
Above 10 AWG the rule no longer applies and Table 310.16 governs directly. NEC 240.4(E) and 240.4(G) list the specific exceptions — tap conductors, motor circuits, welders, and similar loads sized under their own articles.
Disclaimer: This calculator is for reference only. Always consult a licensed electrician and local building codes for actual electrical work. Wire sizing depends on many factors including insulation type, ambient temperature, conduit fill, and derating factors not covered by this calculator.