[ V_d = \frac(mV/A/m) \times I_b \times L1000 \quad \text(in volts) ]
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. bs7671 cable sizing
For a final circuit:
[ Z_s = Z_DB + (R_1 + R_2) \times L ]
[ I_z = I_t \times C_a \times C_g \times C_d \times C_i \times C_c ] [ V_d = \frac(mV/A/m) \times I_b \times L1000
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). The competent designer moves beyond the quick table
| 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) |
