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Voltage Drop Calculator

Calculate the voltage drop along a copper or aluminium cable for single- or three-phase circuits.

Result

Enter your values and select Calculate to see the result.

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How the Voltage Drop Calculator works

Current flowing through a cable loses some voltage to the wire's resistance. Enter the supply voltage, current, one-way cable length and wire size, and choose copper or aluminium and single- or three-phase. The result shows the drop in volts and percent, and the voltage at the load.

Formulas

Single-phase or DC

Vd = 2 × L × I × ρ ÷ A

Three-phase

Vd = √3 × L × I × ρ ÷ A

L is the one-way length in metres, A the conductor area in mm² and ρ the resistivity.

Examples

A 30 m run of 12 AWG at 15 A, 120 V

2 × 30 × 15 × 0.017241 ÷ 3.31 ≈ 4.69 V — a 3.91% drop, above the common 3% guideline, so 10 AWG would be better.

The 3% guideline

On 230 V that allows about 6.9 V of drop; on 120 V about 3.6 V.

Frequently asked questions

Why is the length doubled?

Current flows out on one conductor and back on the other, so a single-phase circuit has twice the one-way length of wire.

What is an acceptable drop?

Guidance commonly suggests 3% for a branch circuit and 5% overall; your local rules may differ.

Does temperature matter?

Yes — copper's resistance rises about 0.4% per °C, so hot cables drop a little more than this estimate.

Information:

Good to know

Estimates for planning. Follow local building codes and the manufacturer's instructions, and use a qualified professional for structural or electrical work.

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