Voltage Drop Calculator
Enter a wire size, current, one-way distance and source voltage to get the voltage drop in volts and percent, the voltage actually reaching the load, and whether that wire size's ampacity is adequate for the load. Works for single-phase, DC and three-phase circuits, copper or aluminum.
What this calculator does
This tool converts a wire size, load current, one-way run length and source voltage into the voltage drop across that run — in volts and as a percentage of source voltage — and checks the wire's ampacity against your load at the same time, so a wire that's fine on voltage drop but undersized on ampacity still gets flagged.
How the math works
- Volts dropped: 2 × current (A) × one-way distance (ft) × resistance (Ω/1,000 ft) ÷ 1,000 — use √3 in place of 2 for three-phase.
- Percent dropped: volts dropped ÷ source voltage × 100.
- Voltage at the load: source voltage − volts dropped.
Worked example
A 15 A, 120 V single-phase load 100 ft away on 12 AWG copper (1.93 Ω/1,000 ft): 2 × 15 × 100 × 1.93 ÷ 1,000 = 5.79 V dropped, or 4.8% — above the 3% branch-circuit guideline, so 10 AWG (1.21 Ω/1,000 ft) would bring it down to about 3.6 V, or 3.0%.
Frequently asked questions
How do you calculate voltage drop?
Multiply 2 (or 1.732 for three-phase) by the current in amps, by the one-way distance in feet, by the wire's resistance per 1,000 ft, then divide by 1,000 for volts dropped. Divide by source voltage and multiply by 100 for the percentage.
What is an acceptable voltage drop percentage?
The commonly cited guideline is 3% max on a branch circuit and 5% total for a combined feeder and branch circuit. These aren't hard limits for every installation, but going over them usually means dimmer lights, hotter motors and wasted energy.
Does wire size or distance matter more for voltage drop?
Both matter — distance scales the drop linearly, while a larger wire size lowers resistance and therefore drop, often substantially per gauge step.
Is voltage drop calculated differently for three-phase circuits?
Yes — three-phase uses a √3 (≈1.732) multiplier instead of 2, since the phase currents share the return path more efficiently than a single-phase or DC circuit's two conductors.
Limitations
This tool estimates voltage drop and ampacity from standard published wire tables. It doesn't account for ambient temperature, conduit fill, bundled-conductor derating or power factor for inductive AC loads. It's for planning purposes only — confirm final sizing with a licensed electrician or your local code.
Put this calculator on your site
Run a blog or business site for DIYers, homeowners or contractors? You can embed the Voltage Drop Calculator as a free widget — responsive, no registration, and no ads or tracking added to your page.
Cite this calculator
Using this tool in a class, a paper or an article? Copy a ready-made reference: