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.
A wire's ampacity rating alone tells you the minimum safe size — but on a long run, that minimum size can still lose too much voltage by the time it reaches the load, causing dim lights or underperforming motors. We start from the ampacity-rated gauge, then check its voltage drop and step up a size at a time until it's within your target (3% is a common recommendation, though NEC doesn't mandate a specific limit for most branch circuits).
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.