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 · 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 standard estimate is Vdrop = (factor × K × I × L) ÷ CM, where K is resistivity (copper 12.9, aluminum 21.2), I is amps, L is the one-way length in feet, and CM is the wire's circular mils. The factor is 2 for single-phase and 1.732 for three-phase.
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. Copper drops about 39% less for the same gauge.
- Raise the voltage. The same load at 240V has half the percent drop of 120V.
Estimates for planning only. Follow the NEC and local code, and have a licensed electrician size and install circuits.
FAQs
How do you calculate voltage drop?
Voltage drop = (factor × K × I × L) ÷ CM. K is the conductor resistivity (12.9 for copper, 21.2 for aluminum), I is the current in amps, L is the one-way wire length in feet, CM is the wire's circular mils (from its AWG size), and the factor is 2 for single-phase or 1.732 for three-phase. Divide the drop by the source voltage for the percentage.
What is an acceptable voltage drop?
The NEC recommends a maximum of 3% voltage drop on a branch circuit and 5% total (feeder plus branch). Staying at or below 3% keeps lights bright, motors happy, and electronics within spec. This calculator flags results above the 3% guideline.
How do I reduce voltage drop?
Four levers: use a larger wire (lower AWG number = more copper = less drop), shorten the run, use copper instead of aluminum, or raise the system voltage (240V drops half the percentage of 120V at the same load). Upsizing the wire one or two gauges is the most common fix for long runs.
Is copper or aluminum better for voltage drop?
Copper has lower resistance, so for the same gauge it drops less voltage (about 39% less than aluminum). Aluminum is cheaper and lighter, so it's common on large feeders, but you typically need to go up one or two wire sizes to match copper's voltage drop.
Does voltage drop matter on a short run?
Rarely. On short runs (under ~50 ft) at normal loads, drop is usually a fraction of a percent. It matters most on long runs, high-current circuits, and low-voltage systems (12V solar, landscape lighting), where even a small absolute drop is a large percentage.