Cable Derating Calculator — Temperature, Grouping & Insulation

Cables are rated at a reference temperature (30°C ambient for most BS 7671 tables) in free air or clipped direct. Real installations often differ: higher ambient temperatures (loft spaces at 40°C+), cables grouped together (reducing ability to dissipate heat), or cables passing through thermal insulation. Each of these factors reduces the cable's ability to shed heat, so the current-carrying capacity must be reduced (derated) to prevent the insulation from overheating and degrading.

BS 7671 Table 4B1 lists Ca factors for different ambient temperatures. For PVC cables (70°C rated): 25°C = 1.03, 30°C = 1.00, 35°C = 0.94, 40°C = 0.87, 45°C = 0.79, 50°C = 0.71. For XLPE cables (90°C rated), the factors are higher because the cable can operate at higher temperatures: 35°C = 0.96, 40°C = 0.91, 45°C = 0.87. Loft spaces in summer can reach 40-50°C — always check the ambient conditions.

BS 7671 Table 4C1 gives grouping factors (Cg) based on the number of circuits grouped together: 2 circuits = 0.80, 3 = 0.70, 4 = 0.65, 5 = 0.60, 6 = 0.57. These apply when cables are touching or in the same conduit/trunking. If cables are spaced at one cable diameter apart, higher factors apply. For cables on perforated cable tray in a single layer, touching: the factor is similar. Grouping is the most commonly forgotten derating factor.

BS 7671 Table 52.2 requires derating where a cable is enclosed in thermal insulation. The factor (Ci) depends on the LENGTH enclosed: 50mm = 0.89, 100mm = 0.81, 200mm = 0.68, 400mm = 0.55. A cable that passes through a joist with 100mm loft insulation on top is 'enclosed in 100mm insulation' (Ci = 0.81). For a cable totally surrounded by insulation for 0.5m or more (e.g. buried in wall insulation), Reg 523.9 requires Ci = 0.5 (half the clipped-direct rating). This significantly reduces capacity — a 27A cable at Ci 0.5 becomes only 13.5A. Important: where a cable is in a thermally insulating wall or above a thermally insulating ceiling and is in contact with the insulation on ONE side only, BS 7671 states it is NOT necessary to apply a Table 52.2 factor — that case is already covered by the Reference Method tabulated capacities (Installation Methods 100-103, Table 4D5). The best approach there is to enter the Method 100-103 tabulated It and leave thermal insulation set to 'none'; the calculator's 'one side' option is only a conservative rule-of-thumb (0.75), not a Table 52.2 value. Table 52.2 itself applies to conductors up to 10mm² in insulation with thermal conductivity greater than 0.04 W/m/K.

The overall correction factor is the product of all individual factors: Overall = Ca × Cg × Ci. The derated current capacity = Tabulated capacity × Overall factor. For example: 27A cable at 40°C (Ca = 0.87), grouped with 2 others so 3 circuits in total (Cg = 0.70 per Table 4C1), enclosed in 100mm insulation (Ci = 0.81): Overall = 0.87 × 0.70 × 0.81 = 0.493. Derated capacity = 27 × 0.493 = 13.3A. The design current must not exceed this derated value. Note Table 4C1 counts the cable plus the others around it — so 4 circuits in a group would use Cg = 0.65, not 0.70. Then use our Cable Sizing Calculator to select the right cable, and verify with our Voltage Drop Calculator.