calcs.homes
calcs.homes
Enter your circuit's amperage, one-way wire length, and voltage — we check both ampacity and voltage drop, since a long run can need a thicker wire than amperage alone suggests.
Estimate only — copper THHN/THWN ampacity assumed. Have an electrician verify before installing.
We start from the smallest AWG size in our table whose ampacity rating meets or exceeds your circuit amps, then check its voltage drop using drop % = (2 × 12.9 × amps × one-way length ÷ circular mils) ÷ voltage × 100 — the 2× accounts for the outgoing and return conductor, and 12.9 is copper's resistivity constant. If that gauge's drop exceeds your target percentage, we step up to the next larger gauge and recheck until both ampacity and voltage drop pass.
Ampacity alone only guarantees the wire won't overheat — it says nothing about voltage lost to resistance over distance, which is why a technically-rated gauge can still cause dim lights or a struggling motor at the end of a long run. 3% is a widely used voltage-drop target for branch circuits (5% for a feeder-plus-branch total), even though the NEC doesn't hard-mandate a specific limit for most circuits — so we check both constraints and let the stricter one determine the final gauge.
Longer wire runs have more resistance, so more voltage is lost getting to the load. A 20A circuit might be fine with 12 AWG at 20 ft but need 10 AWG or larger at 100 ft to stay within an acceptable voltage drop.
3% for branch circuits and 5% for the total feeder-plus-branch-circuit run are common industry recommendations, though not always a hard NEC requirement — check your local code.