Rating factors are divided into the nominal rating of the circuit protective device (In) to determine theminimum current-carrying capacity (Iz) of a cable in a circuit. For example:
Iz = In/(Ca x Cg x Cc x Ci x Ch)
Rating factors:
Ca –ambient temperature
Cc –buried installation methods 70 to 73 (Cc = 0.9)
Cd –for depth of burial
Cf –BS3036 protective device (Cf = 0.725)
Cg –grouping
Ci –cable run through insulation
Cs –for thermal resistivity of soil
Ch –3rd harmonic current