Bs7671 Cable Sizing May 2026

For any electrical installation operating in the UK, compliance with BS 7671 (IET Wiring Regulations) is not optional—it is a legal benchmark under the Electricity at Work Regulations 1989. At the heart of this compliance lies a process often misunderstood as simple table-lookup: cable sizing .

[ Z_s = Z_DB + (R_1 + R_2) \times L ]

[ S = \frac\sqrtI^2 tk ]

Where ( L ) is the cable length in metres (line + neutral – so for single-phase, use the tabulated mV/A/m directly; for three-phase, note correction). bs7671 cable sizing

For a final circuit:

If voltage drop exceeds the limit, the cable size must be increased – often overriding the thermal sizing for long runs. Even if a cable is correctly sized for load current, it must survive a short circuit fault without insulation damage. BS 7671 provides the adiabatic equation: For any electrical installation operating in the UK,