Many standards, manufacturers and power authorities give guidance on determining how much current flow in a cable by using the idea of derating factors.  While each standard has variations and particularities, the general principals are the same. 

Other knowledge base notes discussed particular standards in more detail.  This note outlines the general principals relevant to the method.

Derating By Factors Method

Depending on whether overload protection is required or not, the required current rating of a cable is given by:

$Ir=\text{ the greater of }{I}_b\text{ or }{I}_n$

where:
I_r        - required current rating, A
I_b        - cable design current (that intended to be carried in normal service, load current), A
I_n        - rated current or current setting of the protective device (if required), A

Each standard will maintain a table of base cables ratings, I_t,for a cable under a defined set of installation and operating environmental conditions.  Depending on the standard, several derating factors are then applied to the base rating.  The following are the most commonly applied derating factors:

C_a        - for ambient temperature (either air or ground)
C_c        - for circuits buried in the ground (BS 7671 factor only for buried circuits)
C_d        - for depth of burial (circuits directly buried or buried in ducts)
C_f        - for semi-enclosed fuse to BS 3036 (BS 7671 factor for this type of protective device)
C_g        - for grouping (number of cable circuits run together)
C_i         - for thermal insulation (if a circuit is encased inside any thermal insulating material)
C_s        - for thermal resistivity of soil (for direct buried or cables buried in ducts)

The required current rating and base rating of the cable are related by:

$I_t\ge \frac{I_r}{C_a{C}_c{C}_d{C}_f{C}_g{C}_i{C}_s}$