3% / 5%
Common UK design guidance
UK voltage drop calculator
Voltage Drop Calculator UK
Calculate voltage drop, percentage loss and the minimum standard cable size for DC, single-phase and three-phase circuits in seconds.
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Voltage drop calculator
Enter your circuit details. Results update instantly.
DC + AC
Three circuit modes
Metric sizes
UK cable cross-sections
The essentials
What is voltage drop?
Voltage drop is the reduction in voltage as electrical current travels through a cable. Every conductor has resistance, so part of the supplied electrical energy is lost as heat before it reaches the load.
Long cable runs, higher current and smaller conductors all increase the drop. If it becomes excessive, lights can dim, motors may run hotter, and sensitive equipment can behave unpredictably.
!
Design note This calculator is an estimating tool. Cable selection must also consider current-carrying capacity, installation method, grouping, ambient temperature, protective devices and the connected equipment.
Four key factors01—04
- 01Cable length
A longer route creates more resistance.
- 02Load current
More current produces a larger voltage loss.
- 03Cable size
A larger conductor reduces resistance.
- 04Conductor material
Copper has lower resistance than aluminium.
UK design guidance
How much voltage drop is acceptable?
For installations supplied directly from a public low-voltage distribution system, BS 7671 guidance commonly recommends these maximum values from the origin to the load.
3%
Lighting circuits
Equivalent to 6.9 V on a nominal 230 V supply.
5%
Other circuits
Equivalent to 11.5 V on a nominal 230 V supply.
Show your working
Voltage drop formulas
Our calculator uses conductor resistivity, cable cross-sectional area and operating temperature to estimate resistance, then applies the correct circuit factor.
DC & single-phase AC2 × I × L × R
The factor of two accounts for the outgoing and return conductors.
Three-phase AC√3 × I × L × R
The square-root-of-three factor accounts for the three-phase relationship.
Percentage drop(Vd ÷ V) × 100
Voltage loss expressed as a percentage of the supply voltage.
I Current (A)L One-way length (m)R Cable resistance (Ω/m)V Supply voltage (V)
Three simple steps
How to use the calculator
Choose the circuit
Select DC, single-phase AC or three-phase AC, then enter the supply voltage.
Enter your cable details
Add the one-way length, current, cable size, material and likely conductor temperature.
Review the result
Check the voltage loss, percentage, load voltage and suggested minimum cable size.
Knowledge base
Browse all articles →Practical voltage drop guides
Clear explanations for cable sizing, circuit calculations and common design decisions.
How to Calculate Voltage Drop: A Practical UK Guide
Understand the voltage drop formula, the values you need, and how to check a cable run before installation.
8 min readRead article →
3% vs 5% Voltage Drop: Which Limit Should You Use?
A clear explanation of common UK voltage-drop design limits for lighting and other final circuits.
6 min readRead article →
How to Choose Cable Size for a Long Cable Run
See how cable length, current and conductor size interact—and why voltage drop is only one part of cable selection.
9 min readRead article →
Common questions
Voltage drop FAQs
Clear answers for common cable-sizing questions.
What is an acceptable voltage drop in the UK?
BS 7671 guidance commonly uses 3% for lighting circuits and 5% for other circuits supplied from a public low-voltage system. The connected equipment and the complete installation design may require a lower value.
Does cable length mean one-way or return length?
Enter the one-way distance from the supply to the load. The calculator automatically includes the return path for DC and single-phase circuits.
Can I use this result to choose a cable?
Use it as a voltage-drop estimate only. Final cable selection must also satisfy current-carrying capacity, protective-device, fault-loop, grouping, insulation, ambient temperature and installation-method requirements.
Why does cable temperature matter?
A conductor's resistance rises as it gets hotter. Including an estimated operating temperature produces a more conservative and realistic voltage-drop estimate.
How can I reduce voltage drop?
Increase the cable cross-sectional area, shorten the route, reduce current where practical, and ensure every connection is correctly terminated and maintained.
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