BS 7671 is the UK requirements for electrical installations standard, also known as the Wiring Regulations. Appendix 4 (BS 7671:2018) gives guidance determining the current capacity and voltage drop for cables.
Cable current capacity calculations in BS 761 are based on the derating factor method, see Cable Derating (Factors). The derating tables have been derived from IEC 60287 and aligned with IEC 60364 (although there is some fundamental difference).
The parameters considered in the standard are, ambient temperature, soil thermal resistivity, the installation methods and grouping of cables.
Calculation of voltage drop uses tables of resistive and reactive voltage drop. The tables give the voltage drop in mV/A/m for various installation conditions. These are related to the line voltage for three phase circuits and phase voltage for single phase circuits.
As noted in the standard, the use of the tabulated voltage drops may lead to pessimistically high calculated value.
Note: myCableEngineering does not use the BS 7671 tables, but instead calculations the circuit impedances in accordance with IEC 60909 and directly calculations voltage drop from this.