NEC voltage drop: the 3% rule, the formula, and when to upsize

Reference guidance for field use. NEC is a registered trademark of the NFPA; always follow the code edition and amendments your jurisdiction enforces.

Voltage drop is the quiet spec: inspectors rarely fail you on it, but motors run hot, LEDs flicker, chargers derate — and callbacks arrive — when it's ignored. Here is the short version of what the NEC actually says, the formula worth memorising, and the situations where the wire size on the plan needs a second look.

What the code actually says

The famous numbers — 3% on a branch circuit, 5% total for feeder plus branch — live in NEC Informational Notes, which are advisory. Two nuances make them matter anyway: several specific applications (fire pumps among them) carry mandatory drop limits in their own articles, and project specs or local amendments routinely adopt 3%/5% as binding. In practice: design to 3%/5%, and check whether your job's spec makes it law.

The formula

Single-phase: VD = 2 × K × I × L ÷ CM. Three-phase: replace 2 with 1.732.

• K — conductor constant: ≈ 12.9 for copper, ≈ 21.2 for aluminum (ohms·CM per foot at operating temperature).
• I — load current, amps.
• L — one-way circuit length in feet (the ×2 handles the return).
• CM — circular mils of the conductor (#12 = 6,530; #10 = 10,380; #8 = 16,510…).

Worked example: 16 A load, 120 V circuit, 150 ft one-way on #12 copper: VD = 2 × 12.9 × 16 × 150 ÷ 6,530 ≈ 9.5 V → 7.9%. Far past 3%. Same run on #8 copper: ≈ 3.7 V → 3.1%. That's the whole discipline: long run + real load = check before pulling wire.

Where drop actually bites

• Detached buildings — garages, shops, barns: the run is always longer than it looks.
• Well pumps and gate motors — motors under-volted draw more current and cook.
• EV chargers — continuous high-amp loads magnify every ohm.
• Parking-lot and landscape lighting — daisy-chained fixtures at distance.
• Low-voltage and PoE — 12 V cameras and long Cat-runs have the least headroom of all.

Or skip the arithmetic in the field: AmpCalc runs this exact calculation — copper or aluminum, single- or three-phase, mains or low-voltage cable presets — with a red/amber/green verdict against the 3%/5% targets, offline, on your phone. Every formula is verified against published worked examples. See how AmpCalc works →