AC Voltage Drop Calculator

For single-phase AC, the formula is Vdrop = 2 × I × Z × L, where I is current in amps, Z is conductor impedance per foot from NEC Chapter 9 Table 9, and L is one-way distance in feet. The factor of 2 accounts for the round-trip path through the hot and neutral. Power factor adjustments matter for inductive loads (motors, fluorescent ballasts) — multiply the drop by the power factor (typically 0.85 for motors, 1.0 for resistive loads like incandescent or strip heaters). This calculator runs that math for you with NEC-published values.

Three-phase uses Vdrop = √3 × I × Z × L instead of the factor of 2. Why? Because in a balanced three-phase system, current returns through the other two phases rather than a dedicated neutral, and the geometry of the phase angles produces the √3 multiplier (about 1.732). At the same current and distance, three-phase drop is about 13% lower than single-phase, which is one reason large feeders and motor circuits run three-phase whenever practical. This calculator handles single and three-phase automatically when you toggle the phase selector.

Per NEC 210.19(A) Informational Note, branch circuits should not exceed 3% drop. Per NEC 215.2(A), feeders should not exceed 3%, AND the combined feeder + branch drop at the farthest outlet should not exceed 5%. So if your feeder runs at 2.5%, your branch can only run 2.5% to keep the combined under 5%. The 3%/5% rule is a recommendation, not enforceable code, but most AHJs treat it as required for permit approval. Sensitive loads (audio gear, sensitive electronics, hospital instrumentation) target tighter — 1% to 2%.

It depends entirely on amperage and gauge. Roughly: 12 AWG copper at 20A drops about 1.0V per 100 feet on a 240V single-phase circuit (0.4%). 10 AWG at 30A drops about 0.95V per 100 feet. Larger conductors at higher current can run 4-8V per 100 feet. The 3% rule says you can lose up to 7.2V on a 240V circuit before it becomes a problem — but use the calculator on your specific job rather than rules of thumb.

7.2V maximum on the branch (3% of 240V), or 12V combined feeder + branch (5%). For dwelling unit service feeders running at full load, 5% is the hard ceiling. For sensitive loads on a 240V circuit — for example, a TIG welder, a CNC machine, or audio gear — designers often target 2% (4.8V max) to avoid performance issues from sag during heavy loads. Note: the percentages are calculated against nominal voltage at the source, not the actual measured voltage at the panel.

Copper has roughly 60% more current-carrying capacity per area than aluminum, so a 4/0 aluminum feeder is roughly equivalent to a 2/0 copper. Aluminum is cheaper per pound and lighter, which is why most service feeders to commercial buildings are aluminum. Copper is required for branch circuits in most jurisdictions and for any installation where mechanical fatigue or thermal cycling is a concern (panel-to-disconnect short feeders, motor circuits with frequent starts). Aluminum requires antioxidant compound on terminations and torque-spec connections; aluminum-rated lugs are mandatory.

Yes — for inductive loads, the impedance affects voltage drop differently than pure resistance. Motors, transformers, fluorescent and HID lighting, and electronic ballasts have power factors below 1.0 (typically 0.7 to 0.9). NEC Chapter 9 Table 9 publishes effective Z values at 0.85 power factor as the default. If your load has a different PF, multiply the drop by your actual PF for a more accurate result. For purely resistive loads (resistance heaters, incandescent lighting, baseboard heat), use PF = 1.0.

NEC 2026 keeps the 3% branch / 5% combined recommendations as informational notes (210.19 and 215.2). What did change: 210.19(A)(2) added explicit language about EV charging branch circuits, and 215.12 expanded feeder marking requirements. The voltage drop math is unchanged — same Chapter 9 Table 9 impedance values. Most state code adoptions are 2-3 years behind, so check your AHJ for which NEC cycle they enforce.

Running voltage drop and starting voltage drop are different problems. NEMA MG-1 specifies motors must operate within ±10% of nameplate voltage at full load. Starting in-rush can hit 6-8× running current for 0.5-1 second. A motor circuit that calculates pass at 3% running may see 18-24% drop during start-up, which can trigger overload trips, stall the motor, or burn out a contactor. For motor circuits, calculate both running drop (use the calculator) and locked-rotor in-rush drop separately. Increase wire size or shorten the run if start-up drop exceeds 10%.