Voltage Drop Calculator — wire run loss

How it works

A wire is not a perfect conductor — it has resistance, and pushing current through it loses some voltage before the power reaches the load. The lost voltage is Ohm's law applied to the wire's own resistance:

voltage drop = current × wire resistance

The wire's resistance comes from its material, length and thickness:

R = ρ × (2L) ÷ A

• ρ (resistivity) is a property of the metal — copper is 1.72×10⁻⁸ Ω·m and aluminum 2.65×10⁻⁸ Ω·m at 20 °C, so aluminum of the same size drops more.
• 2L is the conductor length: the current leaves the source and returns, so a run of length L uses twice that length of wire.
• A is the conductor's cross-sectional area — a thicker wire (larger A) drops less.

The percent drop compares the loss to the supply voltage. A common rule of thumb keeps drop under about 3% on a branch circuit, but always follow your local code.

Worked example

A 15 A load 30 m away on 4 mm² copper: R = (1.72×10⁻⁸ × 60) ÷ (4×10⁻⁶) = 0.258 Ω, so the drop is 15 × 0.258 = 3.87 V — about 3.2% of a 120 V supply. Move the same load to 60 m and the drop doubles to 7.7 V; use 2 mm² wire instead and it doubles again.

Frequently asked questions

How do I calculate voltage drop?

Multiply the current by the wire's resistance: voltage drop = I × R, where R = ρ × 2L ÷ A. Use the one-way run length L (doubled for the out-and-back conductor), the resistivity ρ of the metal, and the conductor's cross-sectional area A. A 15 A load on 30 m of 4 mm² copper drops about 3.9 V.

Why is the length doubled?

Because current flows in a loop — out to the load along one conductor and back along another. A load 30 m away needs 30 m of wire each way, so 60 m of conductor carries the current and contributes resistance. That is why the formula uses 2L for a one-way run of L.

Why does aluminum drop more voltage than copper?

Aluminum has a higher resistivity — 2.65×10⁻⁸ Ω·m versus copper's 1.72×10⁻⁸ Ω·m — so an aluminum wire of the same length and thickness has more resistance and drops about 54% more voltage. Aluminum conductors are therefore sized larger than copper for the same run.

How much voltage drop is acceptable?

A widely used guideline keeps voltage drop under about 3% on a branch circuit, and around 5% total from the service to the load, so equipment gets close to its rated voltage. These are recommendations — the enforceable limits are set by your local electrical code, which this calculator does not encode.

How do I reduce voltage drop?

Use a thicker wire (larger cross-sectional area), shorten the run, switch from aluminum to copper, or raise the supply voltage so the same power draws less current. Because drop is proportional to length and current and inversely proportional to area, doubling the wire area halves the drop.

What conductor area should I enter?

Enter the cross-sectional area of the metal conductor in square millimetres, from the wire's specification. Metric wire is sold by area directly (2.5 mm², 4 mm², 6 mm²…); for AWG wire, look up the gauge's area — the number is on the spec sheet — and enter that.