Voltage Drop Calculator

Calculate the voltage drop and percentage loss along a cable run from the conductor size, current and length, for both UK and US wiring.

Part of:Electrical hub
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V
metres

How to Use This Calculator

How to Use This Calculator

  1. Wiring standard – Choose UK (conductor in mm², run length in metres) or US (AWG/kcmil, run length in feet).
  2. Conductor material – Copper is standard; aluminium has higher resistance so drops more voltage for the same size.
  3. Phase – Single phase uses a factor of 2 (out and return); three phase uses √3.
  4. Conductor size – The cross-section of the live conductor.
  5. Load current – The current the circuit carries, in amps.
  6. Run length – The one-way distance from the supply to the load.
  7. Supply voltage – Normally 230 V in the UK or 120/240 V in the US.

The calculator works out the conductor resistance, the voltage lost along the run, the percentage drop and the voltage left at the load. UK guidance (BS 7671) limits voltage drop to 3% for lighting and 5% for other circuits; the US NEC recommends a maximum of 3% on a branch circuit.

Common Questions

What is an acceptable voltage drop?

UK wiring regulations (BS 7671) recommend no more than 3% for lighting and 5% for other circuits. The US NEC recommends a maximum of 3% on a branch circuit and 5% overall including the feeder.

Why does voltage drop matter?

Excess voltage drop means appliances get less than their rated voltage, causing dim lights, slow motor starts, overheating and wasted energy. Long cable runs are the most common cause.

How do I reduce voltage drop?

Use a larger conductor, shorten the cable run, switch from aluminium to copper, or raise the supply voltage (for example wiring a load at 240 V instead of 120 V in the US).

Does this work for both UK and US wiring?

Yes. Switch the wiring standard to enter conductor size in mm² with the run in metres (UK) or in AWG/kcmil with the run in feet (US). The result is shown in volts and as a percentage either way.

What is the difference between this and the cable size calculator?

This tool tells you the voltage drop for a cable you have already chosen. The Cable Size calculator works in reverse – it finds the smallest conductor that keeps the drop within limits for your load.

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