Cable DeratingCalculator

Cable derating is the process of reducing the rated current-carrying capacity (Iz) of a cable to account for installation conditions that increase its operating temperature. Cables are rated at specific reference conditions — typically 30°C ambient, single cable clipped to a surface, no surrounding insulation. When any of these conditions are more onerous (higher temperature, multiple cables grouped together, cables buried in insulation), the cable cannot dissipate heat as effectively and must be operated at a lower current to avoid overheating the insulation, which can cause fire or premature failure.

Thermal insulation dramatically reduces cable current-carrying capacity because it prevents the cable from losing heat to its surroundings. A cable totally surrounded by thermal insulation (e.g. embedded in mineral wool in a wall or ceiling) must be derated to 50% of its clipped-to-surface rating (Ci = 0.5). A cable touching insulation on one side is derated to 75% (Ci = 0.75). This is why cables in loft insulation must be rated substantially higher than the circuit design current — or better, run in conduit above the insulation level. BS 7671 Regulation 523.9 specifically addresses thermal insulation.

Cables running through loft insulation are subject to the thermal insulation correction factor (Ci). If the cable is totally enclosed in the insulation, Ci = 0.5, meaning the cable must have a tabulated current rating at least twice the design current. The practical options are: (1) route cables above the insulation level and clip to rafters; (2) use a substantially larger cable size to compensate; (3) run cables in conduit that is itself above the insulation level. The IET On-Site Guide provides detailed guidance on loft cable installations. Never lay cables directly in or under insulation without applying the derating factor.

When multiple cables are grouped in trunking, conduit, or run alongside each other on a cable tray, they restrict each other's ability to dissipate heat. The grouping factor Cg (BS 7671 Table 4C1) reduces the current rating accordingly. Two cables together: Cg = 0.80; three cables: 0.70; five cables: 0.60; ten cables: 0.48; twenty cables: 0.38. In practice, electricians should spread circuits across multiple trunkings or tiers rather than running all cables through a single duct, both to improve current ratings and to allow for future circuit additions.

The ambient temperature correction factor (Ca) adjusts the cable rating for environments hotter or colder than the reference temperature of 30°C (for 70°C cables) or 30°C (for 90°C XLPE cables). In most UK domestic installations, 30°C is a reasonable assumption and Ca = 1.0. However, in plant rooms, commercial kitchens, loft spaces in summer, or industrial environments, ambient temperatures of 40–55°C are common and can significantly derate the cable. Always survey the actual installation environment — a cable near a boiler or in a hot ceiling void may be in an ambient of 50°C+ without any obvious indication.

The combined derating factor Ct = Ca × Cg × Ci × Cc. When multiple derating factors apply simultaneously, they multiply and can result in a very low combined factor. A cable at 45°C ambient (Ca = 0.79), grouped with 5 other cables (Cg = 0.60), in thermal insulation on one side (Ci = 0.75), and protected by a BS 3036 fuse (Cc = 0.725) has a combined Ct of 0.79 × 0.60 × 0.75 × 0.725 = 0.258. The cable must then be rated at almost 4× the design current. Combined Ct below 0.5 should trigger a review of the installation method.

BS 7671 (18th Edition) derating factors are: ambient temperature Ca — Table 4B1 (Appendix 4); grouping Cg — Table 4C1; thermal insulation Ci — Regulation 523.9 and Appendix 4; BS 3036 semi-enclosed fuse correction Cc = 0.725 — Regulation 433.1.1 note. The combined correction is applied to the tabulated current-carrying capacity (Iz) from the cable manufacturer or BS 7671 Appendix 4 tables. The final requirement is that the cable's tabulated Iz ≥ (design current Id) / (Ca × Cg × Ci × Cc).