Free Cable Size Calculator
Estimate twin-and-earth cable size from load, length and install method: voltage drop guidance to BS 7671 principles.
Cable Size Calculator results
Fill in the details and select Calculate to see your figures.
Enter design current or load, circuit length, system voltage and install method to get a recommended copper CSA, voltage drop and maximum length at that size. Guidance only to BS 7671 18th Edition principles: not a compliant design. Final selection must be by a qualified electrician.
How to use the Cable Size Calculator
- Enter design current Ib, or switch to load (kW) and let the tool convert using 230V or 400V.
- Enter the one-way circuit length in metres.
- Choose single-phase or three-phase, install method, and 3% (lighting) or 5% (power) limit.
- Read the smallest CSA that meets both current-carrying capacity and voltage drop.
- Treat the result as guidance only. A full design needs grouping, ambient, insulation, Zs and adiabatic checks.
How the Cable Size Calculator works
Cable size is the smallest CSA where current-carrying capacity Iz ≥ design current Ib and voltage drop stays within the percentage limit.
- Ib = 32A, length 20m, 230V power (5% limit = 11.5V).
- 2.5mm² at 18 mV/A/m: Vd = 18 × 32 × 20 ÷ 1000 = 11.5V → exactly 5%.
- If the run is longer or Iz is reduced by insulation, step up to 4mm² (11 mV/A/m).
- Always re-check against the live BS 7671 Appendix 4 table for your cable type.
Estimates to BS 7671 18th Edition principles. Not a compliant design. Final cable selection must be done by a qualified electrician accounting for install method, grouping, ambient temperature, thermal insulation, and Zs/adiabatic checks.
Typical UK benchmarks
- Lighting voltage-drop limit
- 3% of nominal voltage: BS 7671 guidance
- Power voltage-drop limit
- 5% of nominal voltage for other uses
- Ring final T&E
- Common 2.5mm² twin-and-earth ring
- Shower ~9.5kW
- Often 6mm² (length/method dependent)
See it in your business.
Frequently asked questions
What size cable do I need for a ring final circuit?
Domestic ring finals commonly use 2.5mm² twin-and-earth on a 32A protective device when the design satisfies current capacity and voltage drop. Long runs or adverse install methods may need a larger CSA. This tool is guidance only.
How is voltage drop calculated in BS 7671?
Voltage drop in volts is (mV/A/m × design current × length) ÷ 1000, using the mV/A/m value from the Appendix 4 table for that cable size and type. Lighting is often limited to 3% and other circuits to 5% of nominal voltage.
What cable for a 9.5kW electric shower?
A 9.5kW shower at 230V draws about 41A. Many installs use 6mm² or 10mm² depending on length and method. Always design to BS 7671 with the correct protective device and install conditions.
Does install method change cable size?
Yes. Cable buried in insulation or bunched in conduit carries less current than the same size clipped direct. That is why a full design applies correction factors from Appendix 4. This calculator only applies a simplified derating.
Is this a substitute for an electrical design?
No. It is a teaching and estimating aid based on simplified BS 7671 figures. A qualified electrician must produce the compliant design, including Zs, disconnection times and adiabatic checks.
