Suggestion uses Table 4D5 Method C (clipped direct) — other installation methods carry less current. Guidance only — always verify against BS 7671 and the On-Site Guide for the actual installation method.
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This cable sizing calculator applies the BS 7671 correction-factor method to work out the minimum tabulated current-carrying capacity a cable needs, then suggests a flat twin and earth size that meets it. It is built for UK electricians sizing final circuits where ambient temperature, grouping with other circuits, thermal insulation or a rewirable fuse derate the cable.
You enter the protective device rating In and select the conditions the cable will run in. The tool combines the correction factors — Ca for ambient temperature, Cg for grouping, Ci for thermal insulation and Cf for a BS 3036 rewirable fuse — and divides the device rating by the product to get the required tabulated capacity It. It then looks up the smallest twin and earth size whose Reference Method C (clipped direct) rating from Table 4D5 meets that figure.
Correction factors are where cable sizing goes wrong most often. A 2.5mm² cable that happily carries 27A clipped direct can be badly undersized once it is bunched with other circuits and buried in loft insulation — this tool makes that arithmetic explicit.
It ≥ In / (Ca × Cg × Ci × Cf)
It is the minimum tabulated current-carrying capacity the selected cable must have, in amps. In is the rating of the protective device. Ca is the ambient temperature correction factor (Table 4B1 — 1.0 at 30°C, falling to 0.71 at 50°C for thermoplastic insulation). Cg is the grouping factor for circuits bunched together (Table 4C1 — 0.8 for two circuits down to 0.57 for six). Ci is the factor for cable in thermal insulation (Table 52.2 — from 0.88 for 50mm of insulation down to 0.5 for 500mm or more). Cf is 0.725 where the device is a BS 3036 rewirable fuse, otherwise 1. Each factor derates the cable, so dividing In by their product raises the capacity the cable must be selected against.
Type in the protective device rating In in amps — for example 32 for a ring final MCB. The cable must be protected by this device, so sizing starts from it.
Select the expected ambient temperature around the cable. 30°C is the reference (factor 1.0); hotter locations such as boiler rooms or roof spaces reduce the cable's capacity.
Select how many circuits are bunched together on the same run and whether the cable passes through thermal insulation, and by how much. Both are common in domestic work and both derate the cable significantly.
The tool shows the combined factor, the required tabulated capacity It, and the smallest twin and earth size whose Method C rating meets it. If the answer exceeds 16mm² T&E it flags that SWA or singles are needed instead.
The method follows the coordination requirements of BS 7671:2018+A2: the cable's tabulated rating, after derating for its installation conditions, must be at least the rating of the device protecting it. The factors used are Ca from Table 4B1 (thermoplastic), Cg from Table 4C1 for bunched circuits, Ci from Table 52.2 for cable in thermal insulation, and the 0.725 factor applied where a BS 3036 rewirable fuse provides overload protection.
The suggested size uses the current-carrying capacities for 70°C thermoplastic flat twin and earth installed to Reference Method C — clipped direct — from Table 4D5 (for example 27A for 2.5mm², 37A for 4mm², 47A for 6mm²). Other installation methods carry less current: the same cable in conduit, in a stud wall or above a plasterboard ceiling has a lower rating, so treat the suggestion as a starting point and check the correct column for your actual method.
Where several factors apply, they multiply, and the required It climbs quickly — a 32A device with two grouped circuits at 40°C already needs It ≥ 46A. Note that grouping and insulation factors generally apply to the section of route where the condition exists; where conditions differ along the route, size for the worst section.
Current-carrying capacity is only one leg of the design. The chosen cable still needs checking for voltage drop, earth fault loop impedance and shock protection, and the adiabatic equation where relevant. Guidance only — always verify against BS 7671 and the On-Site Guide for the actual installation method.
Suggestion uses Table 4D5 Method C (clipped direct) — other installation methods carry less current. Guidance only — always verify against BS 7671 and the On-Site Guide for the actual installation method.
With no derating, 4mm² twin and earth clipped direct (Method C) is rated 37A and covers a 32A device; 2.5mm² at 27A does not on its own, which is why ring finals share the load over two legs. Once grouping, high ambient temperature or thermal insulation apply, the required capacity rises and 6mm² or larger may be needed. Run your actual conditions through the calculator and verify against BS 7671.
Ca corrects for ambient temperature (Table 4B1), Cg for grouping with other circuits (Table 4C1), Ci for thermal insulation around the cable (Table 52.2), and Cf is 0.725 where a BS 3036 rewirable fuse is used. They multiply together, and the device rating divided by the product gives the minimum tabulated capacity the cable must have.
Insulation stops the cable shedding heat, so its capacity drops sharply: the factors used here run from 0.88 for 50mm of surrounding insulation down to 0.5 where the cable is enclosed by 500mm or more. A cable buried in loft insulation can need to be two sizes larger than the same cable clipped in free air.
BS 3036 rewirable fuses are slow to operate on overload, so BS 7671 applies a 0.725 factor when they provide overload protection. Dividing the device rating by 0.725 effectively demands roughly 38% more cable capacity than the same rating on an MCB.
It is the BS 7671 installation method for cables clipped direct to a surface (or on a cable tray), which gives among the best heat dissipation and therefore the highest twin and earth ratings in Table 4D5. Cables in conduit, trunking, stud walls or above insulated ceilings fall under other methods with lower ratings — the suggestion in this tool is Method C only.
No — it satisfies current-carrying capacity only. You still need to confirm voltage drop over the run, maximum Zs for disconnection, and the CPC via the adiabatic check where applicable. A cable can pass on capacity and fail on volt drop over a long route, so complete all the checks before installing.
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