Voltage Drop Calculator

Estimate the voltage drop in copper and aluminium cables. Free, fast and easy to use.

How it works

Voltage drop grows with cable length and current and falls with a larger cross-section. The calculator uses conductor resistivity at about 20 °C and ignores reactance, which is accurate enough for typical small installations. Many standards recommend keeping the drop below 3–5 %.

Formula

DC, AC 1~: ΔU = 2 × L × I × ρ / A
AC 3~: ΔU = √3 × L × I × ρ / A
ρ Cu = 0.0178, ρ Al = 0.0282 Ω·mm²/m

Worked examples

Circuit type: AC single-phase · Voltage (V): 230 · Current (A): 10 · Cable length, one way (m): 30 · Conductor cross-section (mm²): 1.5 · Conductor material: Copper → Voltage drop: 7.12 V
Circuit type: AC three-phase · Voltage (V): 400 · Current (A): 32 · Cable length, one way (m): 50 · Conductor cross-section (mm²): 6 · Conductor material: Copper → Voltage drop: 8.22 V
Circuit type: DC · Voltage (V): 12 · Current (A): 10 · Cable length, one way (m): 5 · Conductor cross-section (mm²): 4 · Conductor material: Copper → Voltage drop: 0.45 V

Calculations run locally in your browser. Results are for information only — for engineering, electrical or financial decisions, verify the assumptions and applicable standards.

Frequently asked questions

What voltage drop is acceptable?

Many standards recommend at most 3 % for lighting circuits and 5 % for other circuits, from the supply point to the load.

How can I reduce voltage drop?

Use a larger cross-section, a shorter cable or a higher voltage. Copper has a lower voltage drop than aluminium of the same cross-section.