Cable voltage drop calculator
Enter the one-way length from the panel to the load; the tool accounts for the current going out and back.
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How to use it
- Enter supply type, conductor, one-way cable length, conductor cross-section, current and supply voltage.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
ΔV = k × L × I × ρ ÷ A, with k = 2 for DC and single-phase and k = √3 for three-phase; ρ is 1/58 Ω·mm²/m for copper and 0.028264 for aluminium at 20 °C, corrected by ρ × (1 + α × (T − 20)).
Example
Inputs: Supply type = AC single-phase, Conductor = Copper, One-way cable length = 30 m, Conductor cross-section = 2.5 mm², Current = 16 A. Result: Voltage drop: 6.621 V; Drop as percent: 2.88%; Voltage at the load: 223.38 V.
Limits and caveats
- Uses resistance only and ignores reactance; for large cables (roughly above 25 mm²) the real drop is a little higher.
- Conductors heat up in service; enter the operating temperature (70 °C for PVC, 90 °C for XLPE) for a conservative result.
- The allowed drop is set by your local wiring code.
Questions about this tool
What voltage drop is acceptable?
Many codes based on IEC 60364 allow about 3% for lighting and 5% for other loads from the origin. Check the code used in your country.
Do I enter the out-and-back length?
No, enter the one-way length only. The factor 2 or √3 in the formula covers the return path.
Why does aluminium give more drop?
Its resistivity is about 64% higher than copper, so it needs a larger cross-section for the same drop.