Cable Derating (Factors)

Last updated on 2023-03-31 2 mins. to read

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= the greater of  I b  or  I n

Ir        - required current rating, A
Ib        - cable design current (that intended to be carried in normal service, load current), A
In        - rated current or current setting of the protective device (if required), A

Each standard will maintain a table of base cables ratings, It,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:

Ca        - for ambient temperature (either air or ground)
Cc        - for circuits buried in the ground (BS 7671 factor only for buried circuits)
Cd        - for depth of burial (circuits directly buried or buried in ducts)
Cf        - for semi-enclosed fuse to BS 3036 (BS 7671 factor for this type of protective device)
Cg        - for grouping (number of cable circuits run together)
Ci         - for thermal insulation (if a circuit is encased inside any thermal insulating material)
Cs        - 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 I r C a C c C d C f C g C i C s







Mar 10, 2020 4:47 AM

For the derating factor, what if I assume it as 1?
Is there have any consequences?

Steven McFadyen Mar 10, 2020 10:50 AM
If you assume 1, you are ignoring derating.  If the installation conditions match the base conditions of the standard, you will be ok.  If they are different, then you could would like by undersized (or in some instances oversized).
Rocco Rims Oct 06, 2023 11:57 AM
What sort of deratings are applied for the HV cable sizing if you are running say 7 conductors (cable) per phase and have two circuits in the same tench touching at 1.5m deep with 1.5thermal resistivity and 25-degree ground temprature (from TF1 - INV1 and TF2 - INV2) what would be the deratings for the 7 conductors in parallel?
Steven McFadyen Oct 06, 2023 12:20 PM
It would depend on the sizing standard you are using.  In general, you must consider factors for the number of circuits (grouping), soil thermal resistivity and ground temperature.  

The cables in parallel will change the electrical current used in looking up the cable base capacity.  It may also be necessary to allow for multiple parallel cables grouped. Although this may already be considered if you are using commercial software.