Voltage Drop Calculator
Calculate voltage drop over a wire run. Enter your voltage, current, one-way length, and gauge, and this voltage drop calculator returns the drop, percent drop, and voltage at the load for copper or aluminum.
Reviewed & updated for 2026 by Rakesh Choudhary, PhDMethodology & sources · How we calculate
How voltage drop is calculated
As current flows through a wire, the wire's resistance turns a little of the voltage into heat, so the voltage at the far end is lower than at the source. The estimate is Vdrop = factor × I × L × R ÷ 1,000, where I is amps, L is the one-way length in feet and R is the wire's resistance in ohms per 1,000 feet. The factor is 2 for single-phase and √3 (about 1.732) for three-phase.
Copper resistance comes from NIST's Copper Wire Tables (NBS Handbook 100, Table 6) at 75 °C, a wire running warm. That is the room-temperature (20 °C) figure times 1 + 0.00393 × 55, about 22% higher. Aluminum uses the annealed (EC-0) figures in NIST's Aluminum Wire Tables (NBS Handbook 109). Both are for solid conductors; stranded building wire reads slightly higher. For 12 AWG that gives 1.93 ohms per 1,000 ft in copper and 3.15 in aluminum.
Percent drop is the voltage drop divided by the source voltage. That percentage matters more than the raw volts, because a 3-volt drop is trivial on 240V but severe on 12V.
How to reduce voltage drop
- Use a larger wire. A lower AWG number means more copper and less resistance. Going up one or two gauges is the most common fix.
- Shorten the run. Drop is proportional to length, halving the distance halves the drop.
- Use copper instead of aluminum. In NIST's tables copper has about 39% less resistance than annealed aluminum of the same gauge.
- Raise the voltage. At the same current, doubling the voltage halves the percent drop. For the same wattage the current halves as well, so the percent drop falls to a quarter.
Estimates for planning only. The 3% flag follows the National Electrical Code's suggestion; the NEC is a model code, so check the version your state or city has adopted, and have a licensed electrician size and install circuits.
FAQs
How do you calculate voltage drop?
Multiply the amps by the one-way run in feet, then by the wire's resistance per foot, then by 2 for a single-phase circuit (current goes out and comes back) or by √3 for three-phase. Resistance comes from NIST's copper and aluminum wire tables at 75 °C. Dividing the result by the supply voltage gives the percentage.
What is an acceptable voltage drop?
Informational notes in the National Electrical Code, a model code that states and cities adopt with their own amendments, suggest about 3% for a branch circuit and 5% for feeder and branch together. They are advice rather than a rule. The flag here uses 3%; your local code and the equipment maker's tolerance are what count.
How do I reduce voltage drop?
Pick a thicker conductor (a smaller AWG number), make the run shorter, choose copper over aluminum, or supply the load at a higher voltage. For long runs, going up a gauge or two is the usual answer.
Is copper or aluminum better for voltage drop?
Gauge for gauge, copper loses less: in NIST's tables a 12 AWG copper wire has about 39% less resistance than the same size in annealed aluminum. Aluminum weighs and costs less, which is why it shows up on big feeders, sized up to make up the difference.
Does voltage drop matter on a short run?
It can. Even 50 ft of 12 AWG copper carrying 15 A at 120 V loses 2.9 V, 2.4% of the supply. Low-voltage systems suffer most: 25 ft of 10 AWG at 12 V and 10 A drops 5.1%.