Many standards, manufacturers and power authorities use derating factors to determine how much current a cable can safely carry under real installation conditions. Each standard has its own details, but the general principle is the same: start with a base cable rating and apply correction factors for the actual environment and installation method.
This article outlines the general derating-by-factors method. Specific standards should always be checked for the exact factor definitions, tables and calculation rules that apply to a project.
Derating by factors method
Depending on whether overload protection is required, the required current rating of the cable is given by:
| Ir | Required current rating, A |
| Ib | Cable design current, meaning the current intended to be carried in normal service, A |
| In | Rated current or current setting of the protective device, where required, A |
Each standard maintains tables of base cable ratings, It, for cables installed under defined operating and environmental conditions. Derating factors are then applied to account for differences between the tabulated conditions and the actual installation.
Common derating factors
| Ca | Ambient temperature factor, for air or ground temperature |
| Cc | Factor for circuits buried in the ground, used in BS 7671 for buried circuits |
| Cd | Depth of burial factor for circuits directly buried or buried in ducts |
| Cf | Factor for semi-enclosed fuses to BS 3036, where applicable |
| Cg | Grouping factor for the number of cable circuits run together |
| Ci | Thermal insulation factor, where a circuit is encased in thermal insulating material |
| Cs | Soil thermal resistivity factor for directly buried cables or cables buried in ducts |
The required current rating and base cable rating are then related by:
In practice, not every factor applies to every installation. For example, buried-cable factors are not normally relevant to a cable installed on tray in air. The applicable standard should be used to identify which factors are required and how they are combined.
If any derating factor is less than one, the cable’s tabulated current-carrying capacity is reduced. This may require a larger conductor size, a different installation method or changes to grouping, spacing, burial depth or thermal environment.
Related derating and current rating topics
For the wider current rating method, see IEC 60287 cable current capacity and cable thermal analysis. For installation-specific effects, see derating factors for cables grouped in air, solar radiation effects on cable current rating and cable troughs.