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Will that 100-foot run actually deliver 120 volts?
Every long wire run loses voltage — to a shed, an RV pedestal, a well pump, a shop. Undersized wire means dim lights, hot conductors, and motors that die young. This calculator runs the standard NEC formula (Vd = 2 × K × L × I ÷ CM), checks the result against the 3% branch-circuit guideline, and tells you the smallest copper or aluminum gauge that passes.
Electricians sizing runs to outbuildings, DIYers wiring a shed, and RV owners spec-ing a pedestal all hit the same wall: the wire is legal by ampacity but drops too many volts over distance. This catches that before the trench is backfilled.
The NEC recommends (as a performance guideline, not a mandate) no more than 3% on a branch circuit and 5% total for feeder plus branch. At 120 V, 3% is 3.6 volts. Sensitive electronics and motors care more than light bulbs.
12 AWG copper carries 20 A legally, but at 100 feet it drops 7.9 V (6.6%) — over the guideline. Even 10 AWG fails at 5.0 V (4.1%). You need 8 AWG (3.1 V, 2.6%) to pass. Distance, not just amps, is what upsizes wire — run the numbers before you buy.