About this cable derating calculator
This free cable derating calculator applies the four correction (de-rating) factors from BS 7671:2018+A2:2022 Appendix 4 and checks whether a chosen cable still carries its design load once installation conditions are accounted for. It is built for electricians, designers and inspectors who need a quick cable-selection check, combining the ambient, grouping, thermal-insulation and protective-device factors and confirming the Ib ≤ In ≤ Iz rule. All calculation runs in your browser. Nothing is uploaded.
How cable derating is calculated
The combined correction factor is the product of the four de-rating factors a circuit's tabulated capacity must be adjusted by: Cf total = Ca × Cg × Ci × Cf, where Ca is the ambient-temperature factor (Table 4B1), Cg the grouping factor (Table 4C1), Ci the thermal-insulation factor (Reg 523.9 / Table 52.2) and Cf the BS 3036 semi-enclosed-fuse factor (0.725 = 1.45/2, otherwise 1.0). The minimum tabulated current-carrying capacity the cable must have is It ≥ In ÷ (Ca·Cg·Ci·Cf), and the installed (derated) capacity of the chosen cable is Iz = It × (Ca·Cg·Ci·Cf). The selection passes when the device rating In is at least the design current Ib and the derated capacity Iz is at least In, i.e. Ib ≤ In ≤ Iz, per BS 7671 Appendix 4.
Frequently asked questions
Which correction factors should I include?
Include every factor that applies to the installation method: ambient temperature (Ca, Table 4B1), grouping with other circuits (Cg, Table 4C1) and contact with thermal insulation (Ci, Reg 523.9). Set any factor that does not apply to 1.0. The fuse factor Cf is only 0.725 where a BS 3036 semi-enclosed fuse protects the circuit.
Why is the BS 3036 fuse factor 0.725?
Semi-enclosed rewireable fuses to BS 3036 have a poorer fusing factor than cartridge fuses or MCBs, so BS 7671 requires the tabulated capacity to be de-rated by 0.725 (which is 1.45 ÷ 2) to keep the cable protected against overload. For BS 88 fuses and circuit-breakers the factor is 1.0.
What is the difference between It and Iz?
It is the cable's tabulated (uncorrected) current-carrying capacity straight from the Appendix 4 tables. Iz is the effective capacity once the combined correction factor is applied. Iz = It × Cf total. The cable is suitable only if Iz is at least the protective-device rating In.