UK Standard
BS 7671 — IET Wiring Regulations
BS 7671 — Requirements for Electrical Installations (IET Wiring Regulations)
In short: BS 7671, the IET Wiring Regulations, is the UK national standard for electrical installations — it sets the rules for selecting cable size based on current, installation method, voltage drop and run length.
What BS 7671 covers
Cable sizing must satisfy current-carrying capacity, voltage drop and protection requirements under BS 7671; undersized cable is a fire and safety risk.
The standard accounts for installation method and grouping (de-rating factors) as well as the load, which is why a longer run may need a larger cross-section.
Apply this standard
Calculators that use BS 7671
Official source: IET — BS 7671 Wiring Regulations. Figures on this page are a plain-English summary for estimating purposes — always refer to the published standard for compliance.