Cable Voltage Drop Calculator
Check the voltage drop on a copper cable run against the UK wiring regulations' 3% and 5% limits, with the longest run and smallest cable that pass.
New: please check your results.
For guidance only. This is not a substitute for a qualified engineer, and important designs should be checked by one.
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- Voltage drop
- 7.2 V
- As a share of the supply
- 3.13 %
- Result
- Within the 5% limit
- Longest run within the limit
- 31.9 m
- Smallest cable within the limit
- 2.5 mm²
- mV/A/m used
- 18
What it does
Check how much voltage is lost along a copper cable before it reaches the load, and whether it's within the limits recommended by BS 7671, the UK wiring regulations: 3% for lighting and 5% for other circuits. Enter the current, the length of the run and the cable size. It also shows the longest run that stays within the limit and the smallest standard cable that would.
How to use it
- Current
The design current of the circuit: what the load draws, or the rating of the protective device (MCB) to be safe. For watts, use the watts to amps calculator.
- Cable length (one way)
From the consumer unit or distribution board to the load, along the route the cable takes. Don't double it: the calculation allows for the return conductor.
- Supply
Homes are single-phase. Three-phase is for larger motors, workshops and commercial supplies.
- Supply voltage
230 V in the UK.
- Line voltage
400 V between phases in the UK.
- Cable size
The cross-section of each conductor, printed on the sheath. Ring and radial socket circuits are usually 2.5 mm², lighting 1.0 or 1.5 mm², cookers and showers 6 or 10 mm².
- Insulation
PVC for twin and earth and most flex; XLPE for most armoured (SWA) cable. Hotter-running cables have higher resistance, so a slightly bigger drop. Check the cable's datasheet if unsure.
- Voltage drop per amp per metre
From the cable maker's datasheet or the BS 7671 Appendix 4 tables, for aluminium, flexible cords or sizes over 35 mm².
- Circuit for
BS 7671 recommends these limits for installations fed from the public supply. Private supplies allow 6% and 8%.
Common uses
- Checking a long run to a garage, garden office or outbuilding.
- Sizing the cable for an electric shower, cooker or car charger.
- Checking a submain or armoured cable run.
- Working out the current from an appliance's watts. Try the Watts to Amps Calculator
- Working out volts, amps or ohms from the other two. Try the Ohm's Law Calculator
The formula
Voltage drop (V) = mV/A/m × current (A) × length (m) ÷ 1,000Share of supply = voltage drop ÷ supply voltage × 100Longest run = (limit × supply voltage) × 1,000 ÷ (mV/A/m × current)Where:
- mV/A/m
- millivolts lost per amp per metre of run, for that cable size, from BS 7671 Appendix 4
- length
- one way: the tabulated value already includes the return conductor
Uses tabulated values for copper conductors at full operating temperature, which is cautious: a lightly loaded cable runs cooler and drops a little less. Reactance is ignored, which is accurate up to 35 mm².
Worked example
A 20 m run of 2.5 mm² PVC cable carrying 20 A:
- From the BS 7671 tables, 2.5 mm² cable drops 18 mV/A/m on single-phase
- Voltage drop: 18 × 20 × 20 ÷ 1,000 = 7.2 V
- As a share: 7.2 V ÷ 230 V = 3.13 %, within the 5% limit (11.5 V)
Questions people ask
What voltage drop is allowed in the UK?
BS 7671 Appendix 4 recommends no more than 3% for lighting and 5% for other uses, from the origin of the installation, when it's fed from the public supply. On 230 V that's 6.9 V and 11.5 V.
Is voltage drop the only thing that sets cable size?
No. The cable must also carry the current without overheating (its current rating depends on how it's installed, insulation around it and grouping with other cables), and the circuit must disconnect fast enough in a fault. Use this for the voltage drop check; a qualified electrician should design and certify new circuits.
Should I measure the length both ways?
No. Enter the one-way route length. The mV/A/m figures already include both the line and neutral (or all three phases).
Last checked 3 November 2026 · Last rev. 31 October 2026 · 3f85