Voltage drop calculator
The NEC recommends keeping voltage drop under 3% on the branch circuit and 5% from service to load (NEC informational notes to 210.19 + 215.2). This calculator checks your run against that rule using K-factor approximations for single-phase or three-phase circuits.
We compute voltage drop with the standard VD = 2·K·I·L / cmils formula for single-phase or VD = √3·K·I·L / cmils for three-phase, where K = 12.9 (copper) or 21.2 (aluminum) at 75°C and L is the one-way run length. Circular mils are pulled from a standard conductor table. Result is the percent of source voltage lost over the run.
For real designs, voltage drop must be checked alongside ampacity (per NEC Table 310.16 — use our wire size calculator ). The smaller conductor that meets ampacity often fails voltage drop on long runs, requiring an upsize.
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