[ 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
[ Z_s = Z_DB + (R_1 + R_2) \times L ]
For a final circuit:
| 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) | [ S = \frac\sqrtI^2 tk ] Where (
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 . The 18th Edition Amendment 2 also clarified requirements
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.