Wire gauge for amperage and run length.
Pick the minimum AWG that handles a given current and stays within an allowable voltage drop over the run. Uses NEC ampacity tables and standard voltage-drop math.
How this works
Two separate constraints both have to be satisfied:
- Ampacity. The wire's NEC-rated ampacity at the chosen insulation temperature must be at least the load current. This guards against overheating and is governed by NEC §310.16.
- Voltage drop. The voltage lost to the wire's resistance must stay below the allowable percentage of the supply voltage. Long runs and high currents push voltage drop up, often forcing a wire size larger than ampacity alone would require.
The voltage-drop formula
For a single-phase AC circuit or DC:
Vdrop = 2 × I × Rper ft × L
where I is current in amps, L is one-way distance in feet, and Rper ft is the conductor's resistance per foot. The factor of 2 accounts for the return path — every circuit needs a wire going out and coming back.
For 3-phase AC (208 V, 480 V), the formula uses √3 instead of 2 because of how the phases interact. This calculator handles that automatically.
Why "smaller AWG number = bigger wire"
AWG is a logarithmic gauge: each step up in number corresponds to a fixed ratio decrease in diameter (roughly 0.890× per step). So 4 AWG is bigger than 12 AWG, and 4/0 AWG is huge. This trips up almost everyone the first time. See the AWG cheat sheet for the full reference.
When to size up beyond this calculator's result
- Bundled conductors. Wires in conduit with more than 3 current-carrying conductors derate per NEC Table 310.15(B)(3)(a). Multiply ampacity by the derating factor before sizing.
- High ambient temperature. Above 30 °C (86 °F), ampacity derates per NEC Table 310.15(B)(2)(a). In an attic in Phoenix, this matters a lot.
- Continuous loads. NEC requires sizing for 125% of a load that runs more than 3 hours continuously. The result here doesn't include that — multiply your amperage input by 1.25 first if relevant.
- Motor circuits. Motors have inrush currents and different sizing rules per NEC Article 430. This calculator is for general branch and feeder circuits, not dedicated motor circuits.
- Aluminum needs slightly different terminations. Many devices are rated for "CU only" or "CU/AL"; check before substituting.
Sources
- Ampacity: NFPA 70 (NEC), Table 310.16, copper and aluminum at 60/75/90 °C insulation, ≤ 3 current-carrying conductors, 30 °C ambient.
- Resistivity: Annealed copper 1.7241 × 10⁻⁸ Ω·m (IEC 60228). Aluminum (1350 alloy) 2.65 × 10⁻⁸ Ω·m. Both at 20 °C.
- Geometry: ASTM B258 AWG diameter formula.
Disclaimer. This calculator is for reference and educational use. Final wire sizing for any installation must be done by a qualified electrician and must comply with the NEC and local codes.