Free Voltage Drop Calculator Online
Use this free voltage drop calculator to determine how much voltage is lost across a wire run. Enter your source voltage, load current, wire length, AWG gauge, and conductor material to instantly see the voltage drop in volts and as a percentage of source voltage.
Voltage Drop
Enter your details to calculate
How the Voltage Drop Calculator Works
This voltage drop calculator uses the standard formula VD = (2 x L x R x I) / 1000, where L is the one-way wire length in feet, R is the resistance per 1,000 feet for the selected AWG gauge and material, and I is the current in amperes. The multiplier of 2 accounts for the full circuit round-trip distance.
For example, a 120V circuit drawing 20A through 100 feet of 12 AWG copper wire produces a voltage drop of about 6.4V (5.3%). That exceeds the recommended 3% branch-circuit limit, so the voltage drop calculator would recommend upsizing to 10 AWG, which cuts the drop to roughly 4.1V (3.4%).
The calculator supports both copper and aluminum conductors and covers AWG sizes from 14 gauge through 2/0. It also flags results that exceed NEC recommended limits so you can select a safer wire size before pulling cable.
Common Wire Resistance Values (per 1,000 ft)
Aluminum resistance is about 1.6x higher than copper for the same gauge. Use the voltage drop calculator to compare materials side by side.
3 Key Voltage Drop Factors
Wire Length
Voltage drop increases linearly with distance. A 200-foot run has exactly double the drop of a 100-foot run at the same current and gauge. For long runs, upsizing the wire is often the most cost-effective solution.
Load Current
Higher amperage means more voltage drop. A 30A load on 10 AWG copper drops about 3V per 100 feet, while a 15A load on the same wire drops only 1.5V. Always calculate for the maximum expected load, not average usage.
Wire Gauge
Each step up in wire size (lower AWG number) roughly cuts resistance by 37%. Going from 12 AWG to 10 AWG reduces voltage drop significantly and can bring a non-compliant circuit within NEC limits.
Frequently Asked Questions
Disclaimer: This voltage drop calculator is for educational and estimation purposes only. Always consult a licensed electrician and follow local electrical codes (NEC/CEC) when sizing conductors. Actual voltage drop may vary based on temperature, conduit fill, and connection quality.