Voltage Drop Calculator (UK Cable)
Choose the cable size, run length and current. You get the volts lost, the percentage, and whether it is inside the BS 7671 limit.
This is a resistance-only estimate for copper conductors. For a design, use the tables in BS 7671 Appendix 4 and the cable maker's data. It does not choose the cable size for current rating.
Worked examples
Socket circuit
2.5 mm2, 20 m, 16 A, single phase, 70 C
5.7 V, 2.5% of 230 V, within the 5% limit
17.78 mV/A/m x 16 x 20 / 1000.
Lighting circuit
1.5 mm2, 10 m, 6 A, single phase, 70 C
1.74 V, 0.76%, within the 3% limit
29.04 mV/A/m x 6 x 10 / 1000.
About Voltage Drop Calculator (UK Cable)
Every cable has resistance, so some of the supply voltage is lost along its length. The longer the run and the higher the current, the more is lost, and the equipment at the end gets less than it should. In UK wiring, BS 7671 recommends that the drop from the origin of the installation to any load stays within 3% of the nominal voltage for lighting and 5% for other uses. This calculator estimates the drop for a copper cable and checks it against those limits.
It works from the maximum conductor resistance at 20 degrees Celsius for each standard size, set out in BS EN 60228, corrected for the temperature the cable runs at. The result is also shown as millivolts per amp per metre, the figure used in the tables of BS 7671, and the longest run that would stay inside the limit at your current. The page uses resistance only. Reactance matters for larger cables over long runs, so check against the standard for a design.
How to use Voltage Drop Calculator (UK Cable)
- Choose the copper cable size in square millimetres.
- Enter the length of the run, one way along the cable, and the current in amps.
- Choose single phase or three phase, the cable temperature rating, the nominal voltage and the circuit type.
- Read the volts lost, the percentage, whether it is within the limit and the longest permitted run.
Common uses
- Checking a long garden or garage supply cable
- Deciding whether to step up to a larger cable size
- Estimating the drop on a socket circuit before installing
- Comparing 2.5 and 4 square millimetre cable for a long run
Frequently asked questions
What are the voltage drop limits in the UK?
BS 7671 Appendix 4 recommends a maximum voltage drop of 3% of the nominal supply voltage for lighting circuits and 5% for other uses, measured from the origin of the installation to the load.
Why is the length counted twice?
Current flows out along the live conductor and back along the neutral in a single phase circuit, so the drop is across both. The page doubles the resistance for single phase and direct current, and uses 1.732 times for three phase.
Does this choose the cable size for me?
No. The cable must also carry the current without overheating, which depends on the method of installation, insulation, grouping and temperature. This tool checks the voltage drop only.
Is this suitable for a real installation design?
It is a good guide, but resistance only. A design should follow BS 7671 and the cable manufacturer's tables, and be carried out and signed off by a competent electrician.