BS7671:2018 Requirements for Electrical Installations.
A typical procedure for the design of an electrical circuit is:
Ib Calculation of design current
In Selection of protective device rating
It Selection ofcable from tabulated current rating
calculation of voltage drop
Zs Calculation of Earth Fault loop impedence
Io Calculation of Earth fault loop current
Determination of Circuit protective conductor cpc size
Ipf Assessment of prospective fault current