⚡ Voltage Drop, Explained

The formula

For DC and single-phase AC: voltage drop = 2 × K × amps × one-way feet ÷ circular mils. For three-phase, replace the 2 with √3 (1.732). The pieces: K is the wire material's resistance constant — 12.9 for copper, 21.2 for aluminum (ohm·cmil/ft at 75°C). Circular mils is the wire's cross-section (#12 = 6,530; #6 = 26,240; #2/0 = 133,100). The 2 is the round trip — current flows out and back, so measure your distance one way and let the formula double it. (Mixing this up is the #1 reason two calculators "disagree.")

Three worked examples

1. Garage receptacle: #12 copper, 20 A, 80 one-way ft, 120 V → 2 × 12.9 × 20 × 80 ÷ 6,530 = 6.3 V = 5.3%. Too much — go to #10 or shorten the run.

2. Hot tub feeder: #6 copper, 40 A actual load, 150 ft, 240 V → 2 × 12.9 × 40 × 150 ÷ 26,240 = 5.9 V = 2.5%. Inside the 3% target.

3. 12-volt trolling motor (DC): #14 copper, 8 A, 15 ft, 12 V → 2 × 12.9 × 8 × 15 ÷ 4,110 = 0.75 V = 6.3%. Same formula, brutal result — at 12 V, even short thin runs bleed real percentage. This is why automotive, marine, solar, and landscape-lighting wiring runs so much fatter than the amps suggest, and why our calculator has a DC mode.

Is the 3% rule actually code?

Honest answer: it's an official recommendation, not an enforceable requirement — in the NEC it lives in Informational Notes (3% for a branch circuit, 5% total from service to the farthest load). You generally can't fail a standard inspection on voltage drop alone, with real exceptions: some loads (like fire pumps) have hard drop limits, some local jurisdictions and energy codes adopt stricter enforceable versions, and low-voltage systems often need tighter targets for the reason example 3 shows. Treat 3%/5% as the line between "performs well" and "expect complaints," not as law.

Where "K" comes from (and why calculators disagree)

K bundles the wire's resistivity into one field-friendly number. We use the standard conservative values (12.9/21.2); some tools instead use the code's DC resistance table, which is ~20–25% more optimistic. Neither is wrong — conservative leaves margin for temperature and real-world conditions. Every result in our tools opens with "How we calculated this" so you can compare constants against any other source directly.

How to fix excessive voltage drop

In order of usual practicality: go up a wire size (each AWG step cuts drop ~21%... two steps cuts it ~37%), raise the voltage (running the same watts at 240 V instead of 120 V halves the amps AND doubles the tolerable drop — a 4× improvement), shorten the route, or for very long rural runs, step up with a transformer. For motors that stumble on startup, drop during inrush is the culprit — size for the start, not just the run.

Related

Voltage Drop calculator · Max Wire Length · Sub-panel guide · How far can you run each gauge?