As the IET itself states: “The tables are a starting point, not the final answer.” Ignoring that principle is the fastest route to a non-compliant – and dangerous – installation. This piece is for educational and reference purposes. Always refer to the latest BS 7671 and consult a qualified electrical engineer for live designs.
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
[ Z_s = Z_DB + (R_1 + R_2) \times L ]
[ S = \frac\sqrtI^2 tk ]
[ V_d = \frac(mV/A/m) \times I_b \times L1000 \quad \text(in volts) ] As the IET itself states: “The tables are