Voltage Drop Calculator – Wire Size & NEC Check

Single-phase: Vd = 2 × K × I × L ÷ circular mils, where K is 12.9 for copper and 21.2 for aluminum; use √3 instead of 2 for three-phase. The tool runs this against your run length, load, and wire gauge, then flags drops over the NEC's recommended 3% for branch circuits.

NEC Chapter 9 Table 1 limits fill to 53% for one wire, 31% for two wires, and 40% for three or more wires — overfill is a code violation. Enter your conductors and the tool checks your raceway against these limits live.

It depends on the wire sizes and the conduit's inside diameter — the tool applies NEC Chapter 9 fill percentages to your exact conductor mix and tells you whether the raceway clears or violates. Verify against the NEC edition your AHJ enforces before pulling wire.

Pick voltage-drop mode, enter run length and load amps, and the sizer recommends the smallest stock wire gauge that holds drop at or under 3% on the branch circuit. NEC recommends ≤ 3% on branch circuits (5% total feeder plus branch) as a design fine-print note.

The NEC recommends a maximum 3% drop on branch circuits and 5% combined for feeder plus branch, as a fine-print design note rather than a hard rule. The tool flags anything over 3% on branch runs so you can upsize the wire or shorten the run.