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