Voltage Drop Calculator

How this voltage drop calculator works

Voltage drop is the voltage lost along a wire because the wire itself resists the current. This voltage drop calculator, sometimes called a voltage loss calculator, shows the drop in volts, the drop as a percentage of the source, and the voltage left at the load. It covers DC, single-phase AC, and three-phase AC circuits, in feet, meters, miles, or kilometers.

Choose where the wire data comes from:

• NEC wire data uses the AC resistance and reactance values from National Electrical Code Chapter 9, Table 9, for copper or aluminum wire in PVC, aluminum, or steel conduit. These apply to three-phase, 60 Hz, 75°C conductors. For a DC circuit, only the resistance is used.
• Estimated resistance works from the wire's size alone, for copper, aluminum, silver, or gold, from 40 AWG up to 600 kcmil. Copper values are for annealed copper at 20°C. Aluminum is taken as building-wire grade aluminum, which conducts about 61% as well as copper, so it has about 1.64 times the resistance of copper the same size.
• Custom resistance takes a resistance or impedance per length, such as the value on a manufacturer's data sheet.

Voltage drop formula

Ohm's law says voltage equals current times resistance. Current travels out and back, so a DC or single-phase circuit counts the one-way distance twice. A balanced three-phase circuit uses √3 instead of 2.

DC or single-phase: Vdrop = 2 × I × Z × L ÷ n

Three-phase: Vdrop = √3 × I × Z × L ÷ n

I is the load current, Z is the wire's impedance per unit length, L is the one-way distance, and n is the number of conductors in parallel per phase. With NEC data on an AC circuit, Z depends on the power factor:

Z = R × PF + X × sin(arccos PF)

How to calculate voltage drop: an example

Take a 240 V single-phase circuit with 10 AWG copper wire in PVC conduit, a 0.9 power factor, a 20 A load, and a 150 ft run. NEC data gives R = 3.9 Ω/km and X = 0.164 Ω/km, so Z ≈ 3.58 Ω/km. The 150 ft run is 0.04572 km.

2 × 20 A × 3.58 Ω/km × 0.04572 km ≈ 6.55 V

That is about 2.7% of 240 V, leaving roughly 233.45 V at the load.

What affects voltage drop?

• Wire material. Aluminum resists current more than copper of the same size, so it drops more voltage.
• Wire size. Thicker wire has less resistance. In AWG, a smaller gauge number means a thicker wire.
• Distance. Drop grows in step with the length of the run.
• Current. Drop also grows in step with the load.

Frequently asked questions

What is voltage drop?

It is the difference between the voltage at the source and the voltage at the load. Too much drop can dim lights, weaken heaters, and make motors run hot.

What is the maximum voltage drop allowed?

A common target is 5% or less at full load. The National Electrical Code suggests 3% on a branch circuit and 5% in total for feeders and branch circuits combined, as informational notes rather than strict requirements. Check your local code for what applies to you.

Should I use one-way or round-trip distance?

Enter the one-way distance from the source to the load. The formula already accounts for the return path.

How do I reduce voltage drop?

Use a thicker wire, shorten the run, reduce the load, raise the system voltage, or run conductors in parallel where code allows it.

Does a DC or 12V circuit need a different calculation?

No. This also works as a dc voltage drop calculator: DC uses the same formula as single-phase, with no power factor. Low-voltage circuits feel drop more: the same 1 V drop is about 8% on a 12 V circuit but under 1% on 120 V.

Limitations

This is an estimate for planning, not an electrical design or a code compliance check. Real drop varies with temperature, connections, frequency, and wire condition. A wire must also have enough ampacity for the load, which this calculator does not check. Have a licensed electrician confirm any installation.