Voltage Drop Calculator: Sizing Conductors So You Pass the First Time
Long run, small wire, and suddenly the motor hums instead of starting. Voltage drop is the quiet failure mode on a job — everything is connected correctly, the breaker holds, and the equipment still misbehaves because the conductor is too small for the distance.
The 3% rule, in one line
The NEC doesn't mandate a hard voltage-drop limit in the body of the code, but the informational notes to 210.19(A) and 215 recommend sizing so that branch-circuit drop stays at or under 3%, with 5% total to the farthest outlet. Most local codes and good practice enforce it. Excess drop wastes energy and can stop equipment from operating as designed.
The formula every sparky should know
Single phase:
VD = 2 × K × I × L ÷ CM
Three phase swaps the 2 for √3 (≈1.732). Where:
- K — resistivity constant: ~12.9 for copper, ~21.2 for aluminum
- I — load current in amps
- L — one-way run length in feet
- CM — conductor circular mils (NEC Chapter 9, Table 8)
Divide the drop by the source voltage for the percentage.
Worked example
A 20 A load, 100 ft away, on #10 copper at 240 V, single phase:
VD = 2 × 12.9 × 20 × 100 ÷ 10,380 = 4.97 V → 2.07%
Under 3% — you're fine. Push that run to 160 ft and you cross the line; upsize to #8 and you're back under. That's the entire decision, and it should take seconds, not a spreadsheet.
Do it on the job, offline
Voltly runs this calculation live as you change phase, material, wire size, length and load — and tells you the moment you're over 3%, then suggests the next size up that gets you back under. Every result shows the formula and the NEC article it came from, so you can show your work to the inspector. No signal, no account, no subscription for the core calculators.
Voltly is a field aid. Always verify against the currently adopted NEC and your local amendments.