Electrical runs

Voltage Drop Calculator

Pick a wire size and material, then enter the run length, the current and the source voltage. You get the voltage drop, the percentage, the voltage left at the load and the power the wire wastes as heat.

—Voltage drop
—Percent drop
—Voltage at load
—Power lost in wire
—Conductor resistance
—Smallest size for ≤ 3%

Calculated on your device. Nothing is uploaded to a server.

How to use the voltage drop calculator

  1. Choose the wireSelect the gauge and whether the conductor is copper or aluminum.
  2. Describe the runEnter the one-way length, the current and the source voltage.
  3. Check the resultUnder 3% is good for a branch circuit. The calculator suggests a size if the drop is too high.

How voltage drop is calculated

Every conductor has resistance, so current flowing through it loses some voltage as heat. For DC and single-phase circuits, the current flows out and back, so the drop is 2 × length × current × resistance per unit length. For balanced three-phase circuits, the line-to-line drop is √3 × length × current × resistance. AWG resistances come from standard tables for stranded conductors at 75 °C. Metric sizes use a copper resistivity of about 0.0210 Ω·mm²/m at 75 °C. Aluminum has about 1.64 times the resistance of copper.

Why it matters

Excess voltage drop makes motors run hot, lights dim and electronics misbehave. It also wastes energy for as long as the circuit runs.

Common limits

Many guides recommend no more than 3% on a branch circuit and 5% in total from the service panel to the load.

Voltage Drop Calculator FAQ

What is an acceptable voltage drop?

Up to 3% for a branch circuit and 5% total is a widely used guideline. Sensitive low-voltage DC systems often aim for 2%.

Is the length one-way or round-trip?

One-way. The calculator accounts for the return conductor (DC and single-phase) or the three-phase factor itself.

Why is aluminum worse?

Aluminum conducts about 61% as well as copper, so the same size has about 1.64 times the resistance and drops more voltage.

What is the power loss figure?

It's the heat produced in the wire itself: current squared times the total conductor resistance. That energy is wasted and is billed on your electricity meter.

Does it include temperature or reactance?

It uses resistance at 75 °C and ignores inductive reactance, which is fine for most building wire and DC runs. Very large AC feeders may need a detailed calculation.

Try other free tools

More handy calculators from My Panda Toolbox.

AWGWire Gauge CalculatorAWG wire size for current and distance V=IROhm's Law CalculatorSolve V, I, R or P from any two values ΩResistor Color Code CalculatorDecode resistor bands or find colors for a value kWhElectricity Bill EstimatorMonthly power cost from appliance usage