How to Size a Cable (BS 7671)
Updated June 2026 · Estimates only — verify before quoting
Cable sizing is the calculation electricians do most often — and getting it wrong means nuisance tripping, overheating or a failed EICR. BS 7671 sets out a clear method, and once you know the steps it takes seconds.
This guide walks through the full process, then you can run the numbers instantly with the free Cable Size Calculator.
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Cable Size Calculator
Full BS 7671 workflow — capacity, correction factors and voltage drop in one.
Open the Cable Size CalculatorThe BS 7671 cable sizing steps
Sizing a cable is really five checks. Work through them in order:
- Design current (Ib) — the load the circuit will actually carry
- Protective device (In) — must be ≥ Ib and protect the cable (In ≥ Ib)
- Correction factors — ambient temperature (Ca), grouping (Cg), insulation (Ci) and device type (Cf)
- Tabulated capacity (It) — It = In ÷ (Ca × Cg × Ci × Cf); pick a cable whose rating Iz ≥ It
- Voltage drop — check the volt drop over the run is within 3% (lighting) or 5% (power)
Why correction factors matter
A cable's tabulated rating assumes ideal conditions (30°C, on its own, not in insulation). Real installations are hotter, bunched together, or buried in loft insulation — each of which reduces how much current the cable can safely carry. Dividing In by the product of the correction factors gives the rating the cable must have in those conditions.
Don't forget voltage drop
A cable can be big enough on current capacity but still fail on voltage drop over a long run. The calculator below checks both and tells you which factor decided the final size — exactly as you would on a design.
Frequently asked questions
What size cable for a 32A circuit?
It depends on the installation method and correction factors, but a 32A ring or radial on twin & earth is commonly 2.5mm² or 4mm²/6mm² depending on length, grouping and insulation. Use the calculator to confirm against current capacity and voltage drop.
What are the correction factors in BS 7671?
Ca (ambient temperature), Cg (grouping of circuits), Ci (thermal insulation) and Cf (protective device type — e.g. 0.725 for a BS3036 rewireable fuse). You divide the device rating by their product to get the required cable rating.
What is the maximum voltage drop allowed?
BS 7671 recommends 3% for lighting and 5% for other uses, measured from the origin to the furthest point of the circuit.
From estimate to winning quote
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