Cable Derating Calculator — Correction Factors Made Simple

Cable derating is the process of reducing the tabulated current carrying capacity of a cable to account for installation conditions that are less favourable than the reference conditions assumed in the BS 7671 tables. The current carrying capacity tables in Appendix 4 of BS 7671 assume a specific set of conditions: an ambient temperature of 30°C, a single circuit with no grouping, no thermal insulation in contact with the cable, and protection by an MCB or HRC fuse with a fusing factor close to 1.

When real-world conditions differ from these assumptions — and they almost always do — correction factors must be applied. Each factor adjusts the effective current carrying capacity of the cable. The combined effect of all applicable factors determines the minimum tabulated current (It) that the selected cable must have. This directly affects the cable sizing calculation and can mean the difference between a cable that runs safely within its thermal limits and one that overheats, degrades its insulation, and ultimately poses a fire risk.

Understanding and correctly applying these correction factors is one of the fundamental skills in electrical design. It is tested extensively in the City and Guilds 2391 examination and is a daily requirement for any electrician designing or verifying circuits to BS 7671.

The ambient temperature correction factor (Ca) adjusts the cable rating for installations where the surrounding air temperature differs from the reference temperature. For cables in air, the reference temperature is 30°C. For cables buried directly in the ground, the reference temperature is 20°C.

The Ca values are found in BS 7671 Tables 4B1 (for cables in air) and 4B2 (for cables in the ground). Higher ambient temperatures reduce the cable rating because the cable cannot dissipate heat as effectively when the surrounding air is already warm. Conversely, temperatures below the reference value increase the permitted current slightly.

In most UK domestic installations, the ambient temperature is close to 30°C and Ca does not significantly affect the cable size. However, in plant rooms, boiler cupboards, roof spaces in summer, and commercial kitchens, temperatures of 35-45°C are common and Ca becomes a significant factor. The voltage drop calculator can also apply a temperature correction to the mV/A/m values for more accurate results on lightly loaded cables.

The grouping correction factor (Cg) accounts for the mutual heating effect when multiple cables or circuits are installed together. Cables generate heat when carrying current, and when they are grouped close together, they heat each other up. This reduces the ability of each cable to dissipate its own heat, so the current carrying capacity must be reduced.

The Cg values are found in BS 7671 Tables 4C1 through 4C5, depending on the installation arrangement. The factor depends on the number of circuits grouped together and the installation method (conduit, trunking, cable tray, clipped direct, etc.).

Grouping is one of the most impactful correction factors. Six circuits in a single conduit reduces each cable's capacity to just 57% of its tabulated value. This is why cables in heavily loaded trunking runs often need to be larger than you might expect. The conduit fill calculator and trunking fill calculator work alongside the derating calculator to ensure both physical space and thermal capacity are adequate.

The thermal insulation correction factor (Ci) applies when cables are in contact with or enclosed by thermal insulating material. Modern building regulations require high levels of thermal insulation, which means cables increasingly pass through or are surrounded by insulation in walls, ceilings, and loft spaces.

BS 7671 Regulation 523.9 provides the rules for cables in thermal insulation. The derating depends on the extent of contact with the insulation:

In practice, the most common scenario in domestic installations is flat twin and earth cable running through joist spaces where loft insulation lies over the cable. This is treated as one side in contact, and Reference Method 100 values are used from the tables. The Ci = 0.5 factor is reserved for cables that are genuinely enclosed in insulation — for example, a cable run through a fully insulated cavity wall where the insulation wraps around the cable on all sides.

The semi-enclosed fuse factor (Cf = 0.725) applies when the circuit is protected by a BS 3036 rewirable fuse. These fuses have a fusing factor of approximately 2, meaning they may not blow until the current reaches twice their rated value. This means the cable could be carrying significantly more current than the fuse rating for an extended period before the fuse operates.

To compensate, the cable must be rated higher. The Cf factor of 0.725 effectively increases the required tabulated current carrying capacity by approximately 38%. For example, if the protective device is a 30A BS 3036 fuse, the required It = 30 / (Ca x Cg x Ci x 0.725), which gives a significantly larger cable than would be needed with an MCB or HRC fuse.

BS 3036 fuses are still found in older installations but are rarely specified for new work. If you encounter them during an EICR inspection, you need to verify that the cables are adequately rated with the Cf factor applied. The Elec-Mate derating calculator makes this straightforward — select BS 3036 as the protective device type and the 0.725 factor is applied automatically.

The current-carrying capacity requirements that the correction factors serve sit in Section 523 of BS 7671. During periodic inspection and testing (Chapter 65), an inspector assessing an installation has to satisfy themselves that conductors were selected with adequate current-carrying capacity for the conditions — which in practice means that all applicable correction factors (Ca, Cg, Ci, and Cf) were correctly applied at the design stage. Correct cable derating is therefore not just a design obligation but a judgement that feeds directly into the EICR coding decision when an undersized circuit is found.

When more than one correction factor applies — which is the case in almost every real-world installation — the factors are multiplied together to give the overall correction. The required tabulated current carrying capacity is then:

For example, consider a circuit protected by a 32A Type B MCB, with 4 circuits grouped in trunking (Cg = 0.65), at an ambient temperature of 35°C (Ca = 0.94), with no thermal insulation (Ci = 1.0) and no BS 3036 fuse (Cf = 1.0):

The Elec-Mate electrical calculator suite links the derating calculator directly to the cable sizing tables, so once you have your It value, the recommended cable size is displayed immediately.