Why do cables need to be derated?

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.

How do I find the ambient temperature correction factor (Ca)?

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.

What grouping factor applies to cables in conduit or trunking?

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.

What happens if a cable passes through thermal insulation?

BS 7671 Table 52.2 requires derating for cables in contact with thermal insulation. Touching one side = Ci 0.75, enclosed in 50mm = 0.89, 100mm = 0.81, totally surrounded = 0.5. A cable that passes through a joist with 100mm loft insulation on top is 'enclosed in 100mm insulation' (Ci = 0.81). If the cable run is entirely within insulation (e.g., buried in wall insulation), use Ci = 0.5. This significantly reduces capacity — a 27A cable at Ci 0.5 becomes only 13.5A.

How do I combine multiple correction factors?

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 3 others (Cg = 0.70), touching insulation one side (Ci = 0.75): Overall = 0.87 × 0.70 × 0.75 = 0.457. Derated capacity = 27 × 0.457 = 12.3A. The design current must not exceed this derated value.

TradeCalculator

Cable Derating Calculator — Temperature, Grouping & Insulation

Apply BS 7671 correction factors for ambient temperature (Ca), cable grouping (Cg), and thermal insulation (Ci) to determine the derated current-carrying capacity of a cable.

Tabulated Current Capacity (It)

From BS 7671 Table 4D1A-4D5A

Design Current (Ib)

Maximum expected circuit current

Cable Insulation Type

Determines temperature correction table

Ambient Temperature

Temperature where cable is installed

Number of Grouped Circuits

Circuits touching or in same enclosure

Thermal Insulation

Cable contact with thermal insulation

How We Calculate This

This calculator applies BS 7671 correction factors to determine the actual current-carrying capacity of a cable in its installed conditions.

Formula

Derated capacity = Tabulated It × Ca × Cg × Ci

Correction Factors

Ca (ambient temp): Table 4B1 — lower in hot environments
Cg (grouping): Table 4C1 — lower with more grouped cables
Ci (thermal insulation): Table 52.2 — lower when in insulation
Combined: Ca × Cg × Ci must be applied together
Requirement: Iz (derated) ≥ In (MCB rating) ≥ Ib (design current)

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.