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Voltage Drop Calculator (copper wire, single phase)

Enter cable length, current and wire size (mm²) to see how much voltage drops. For choosing the right cable size.

Quick answer

30 m, 16 A, 2.5 mm²: drop 2.87 %.

How it is calculated

  1. Drop (volts) = 2 × length × current × 0.0172 ÷ wire size (2 for out and back; 0.0172 is copper’s resistivity).
  2. 2 × 30 × 16 × 0.0172 ÷ 2.5 = 6.6 V.
  3. Percent = drop ÷ supply voltage × 100.

Worked example

16 A through 30 m of 2.5 mm² copper cable on 230 V: drop 6.6 V, i.e. 2.87 %.

Example values
Cable length (one way)30 m
Current16 A
Wire size2.5 mm²
Supply voltage230 V
Result
Voltage drop2.87 %
Drop in volts6.6 V
Formula

Copper resistivity 0.0172 Ω·mm²/m, single-phase

vd = 2×Lm×Ia×0.0172/Am
Voltage drop = vd/Vs×100
Drop in volts = vd

Good to know

Questions people ask

How much voltage drop is OK?

Up to 3% is usually fine; up to 5% on long runs.

How do I reduce the drop?

Use a thicker cable, shorten the run or split the load.

Which cable for a motor?

Enter the motor current and distance; pick the size that keeps the drop under 3%.

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Updated 13 Nov 2026 · Desktop N Calculator v1.2.0