!!top!! — Bs7671 Cable Sizing

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

[ S = \frac\sqrtI^2 tk ]

[ I_z = I_t \times C_a \times C_g \times C_d \times C_i \times C_c ] bs7671 cable sizing

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).

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: [ Z_s = Z_DB + (R_1 + R_2)

The 18th Edition Amendment 2 also clarified requirements for (soil thermal resistivity default 2.5 K·m/W) and thermally insulated walls . Conclusion: Tables Are Not Enough BS 7671 cable sizing is a system-level constraint problem. A cable that works thermally may fail on voltage drop, fault current withstand, or loop impedance. The competent designer moves beyond the quick table and applies the full set of correction factors, adiabatic validation, and regulatory limits.

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. BS 7671 provides the adiabatic equation: The 18th

| Factor | BS 7671 Ref | Applies to | |--------|-------------|-------------| | ( C_a ) | Table 4B1 | Ambient temperature ≠ 30/40°C | | ( C_g ) | Table 4C1 | Grouping of circuits (mutual heating) | | ( C_d ) | Table 4B2 | Buried cables (soil thermal resistivity) | | ( C_i ) | Table 4B3 | Thermal insulation (e.g., in a stud wall) | | ( C_c ) | Regulation 433.1 | Protective device type (e.g., BS 3036 semi-enclosed fuse: 0.725) |