VOLTAGE DROP

Enter your system voltage (120 for standard outlets/lights, 240 for dryers/AC/EV chargers), the circuit's continuous current draw in amps, and the one-way distance from panel to load in feet (the calculator doubles it for round-trip). Pick the wire size you're planning to use in AWG (American Wire Gauge — lower numbers are thicker wire).

The result is voltage dropped over the run plus drop as a percentage of system voltage. NEC recommends ≤3% drop on branch circuits and ≤5% combined for feeder + branch. The status indicator flags whether your spec passes, is marginal, or needs upsizing. K = 12.9 for copper at 75°C; for aluminum, multiply the result by 1.6.

Say you're running a 20A 120V circuit 75 feet to a detached garage on 12 AWG copper:

VD = (2 × 12.9 × 20 × 75) ÷ 6,530 = 38,700 ÷ 6,530 = 5.93 V. Drop %: 5.93 ÷ 120 = 4.94%.

Status: marginal — exceeds NEC's 3% recommendation. Voltage at the load: 120 − 5.93 = 114 V (lights flicker, motors run hot).

Upsize to 10 AWG: VD = (2 × 12.9 × 20 × 75) ÷ 10,380 = 3.73 V = 3.1%. Status: still slightly marginal but acceptable. Upsize to 8 AWG: 2.34 V = 1.95% — clean install.

For a 50A EV charger at 240V over 100 ft on 6 AWG: VD = (2 × 12.9 × 50 × 100) ÷ 26,240 = 4.92 V = 2.05%. Passes comfortably.

Using one-way distance without doubling. The calculator doubles it for you, but if you're doing the math by hand, remember the current flows panel → load → back to panel. A 75-foot run is 150 feet of conductor.

Assuming voltage drop only matters for long runs. A 14 AWG circuit at 15A drops 4.7% over just 50 feet at 120V — already over NEC recommendation. Long runs make it worse, but short undersized runs also fail.

Forgetting that aluminum needs a bigger conductor. Aluminum wire has 1.6× the resistance of copper at the same AWG. A 10 AWG aluminum run drops the same as 12 AWG copper.

Sizing for ampacity but not drop. NEC requires the wire handle the current safely (ampacity), but drop is a separate calculation. A wire that's safe might still cause equipment to malfunction from low voltage. Always check both.

NEC: ≤3% drop on branch circuits, ≤5% feeder + branch combined.

Quick mental math: at 120V, a 20A circuit on 12 AWG drops about 1% per 15 ft of one-way run. So 75 ft = ~5% drop. Useful for ballpark sizing.

Copper K = 12.9 (75°C terminations). Aluminum K = 21.2 (multiply copper drop by 1.6 for aluminum at same AWG).

Long runs (100+ ft) almost always need the next-larger wire size beyond ampacity minimum.

For 240V circuits carrying the same load, drop percentage is a QUARTER of the 120V case, not half — the current halves so the volts dropped halve, and the voltage you divide by doubles.