Rating factors are divided into the nominal rating of the circuit protective device (I_n) to determine theminimum current-carrying capacity (I_z) of a cable in a circuit. For example:
I_z = I_n/(C_a x C_g x C_c x C_i x C_h)
Rating factors:
C_a –ambient temperature
C_c –buried installation methods 70 to 73 (Cc = 0.9)
C_d –for depth of burial
C_f –BS3036 protective device (Cf = 0.725)
C_g –grouping
C_i –cable run through insulation
C_s –for thermal resistivity of soil
C_h –3rd harmonic current