Voltage drop calculator
Long runs from a generator to the house, out to a detached garage, or down to a well pump lose voltage in the wire. Enter the load and the one-way length (not the round trip) to see the drop and the smallest wire that stays within target.
Basis
Resistance values are 2023 NEC Chapter 9 Table 8 (uncoated stranded conductors, 75 C), ohms per 1000 ft; reactance is ignored. The 3% branch and 5% total targets come from 210.19(A) Informational Note 4 and 215.2(A)(1) Informational Note 2, which are recommendations, not requirements. The motor start check uses 3x running amps unless you enter the locked rotor or start-up amps from the nameplate, and its 10% limit is a rule of thumb, not an NEC figure. Wire must also be rated for the current (ampacity, NEC 310.16) and protected by the right breaker; this tool only checks voltage drop. Direct burial, conduit and underground runs have their own rules. See inlet breaker and wire size and conduit fill.