Voltage Drop Calculator guide
Voltage Drop Calculator: Cable Voltage Drop and BS 7671 Limits
Use our voltage drop calculator to check a cable run against the BS 7671 limits of 3% for lighting and 5% for other circuits, and find the smallest cable size that works.
By the AllCalculator editorial teamUpdated 15 October 20262 min read
When current flows through a cable, some voltage is lost along the way. Too much, and lights dim, motors struggle and equipment may not work properly. Our Voltage Drop Calculator works out the drop for a cable run and checks it against the BS 7671 limits.
Electrical work must be designed and carried out by a competent person. In England and Wales, much domestic electrical work is notifiable under Part P of the Building Regulations. Use this calculator for estimates and learning.
How to use the calculator (Voltage Drop Calculator)
- Choose single phase (230V) or three phase (400V).
- Enter the design current and the one-way cable length.
- Choose the conductor size and material.
- Choose whether it's a lighting or other circuit.
The formula
The calculator uses the conductor's resistance at a typical operating temperature of 70°C:
- Single phase: drop = 2 × length × current × resistivity ÷ size
- Three phase: drop = √3 × length × current × resistivity ÷ size
The factor of 2 accounts for the line and neutral conductors.
The BS 7671 limits
For installations supplied from the public network, BS 7671 suggests a maximum drop of:
- 3% for lighting circuits (6.9V at 230V)
- 5% for other circuits (11.5V at 230V)
A worked example
With the calculator's default settings, a 32A single-phase circuit run in 6mm² copper over 25m:
- Drop: 2 × 25 × 32 × 0.0225 ÷ 6 = 6.0V, or 2.6%
- That's within the 5% limit. 4mm² would also pass for this run.
Things it doesn't cover
Cable size also depends on current-carrying capacity, installation method, grouping, ambient temperature and fault protection. For cables of 25mm² and above, reactance becomes significant. For design, use the mV/A/m tables in BS 7671 Appendix 4.
For three-phase loads, see our Three-Phase Power Calculator.
Frequently asked questions
What's an acceptable voltage drop?
3% for lighting and 5% for other uses, measured from the origin of the installation.
Does a longer cable need a bigger size?
Often, yes. Voltage drop rises with length, so long runs to garages or outbuildings usually need a larger cable.
Is aluminium cable different?
Yes. Aluminium has higher resistance than copper, so it needs a larger size for the same drop.
Use the Voltage Drop Calculator for quick checks, and always confirm a design with a qualified electrician.
Sources and further reading
- IET: BS 7671 Wiring Regulationselectrical.theiet.org
- GOV.UK: Electrical safety standards in the private rented sectorgov.uk
Links open official and independent websites in a new tab.
This guide is general information to help you understand your options. It isn't financial, tax, legal or building advice and doesn't take your personal circumstances into account. Figures were checked when the guide was last updated and can change, so confirm anything important with GOV.UK, HMRC, a regulated adviser or a qualified tradesperson.