Conduit Fill Calculator

The proper way to size conduit in the UK is the IET On-Site Guide cable-factor / conduit-factor method (Appendix E), not a single fill percentage. Where a cable is not listed in those tables, the recognised rule of thumb is that the cables should occupy no more than 35% of the conduit's internal cross-sectional area (it is 45% for trunking, not conduit). Keeping below that leaves room to draw cables in and out without damage, allows air circulation for cooling and reduces pulling friction. This calculator works to that 35% space-factor default.

The conduit fill calculation is based on the overall outer diameter (OD) of the cable, including insulation and sheath, not just the conductor cross-sectional area. A 2.5mm² single-core cable (6491X) has an OD of approximately 3.65mm (per the manufacturer datasheet), while a 2.5mm² flat twin and earth (6242Y) is about 5.1 × 10.2mm — its wider dimension (~10.2mm) is used as the bounding-circle diameter. The actual space occupied in the conduit depends on the full cable diameter, which is why the same conductor size needs far more room as twin and earth than as singles.

For flat cables such as twin and earth (6242Y), this calculator uses the cable's wider overall dimension as a bounding-circle equivalent diameter, which is the conservative approach. The authoritative UK reference is the cable-factor / conduit-factor tables in the IET On-Site Guide Appendix E (and IET Guidance Note 1), which assign each cable and conduit a pre-calculated unit factor rather than working in raw cross-sectional area. For multi-core and flat cables in particular, that factor method is the one to follow for compliance.

For singles (6491X) in conduit: on a short straight run this calculator allows about 8× 2.5mm² singles in 20mm conduit at the 35% space-factor default (the IET Appendix E factor method gives roughly 10 — conduit factor 460 ÷ cable factor 43). Both figures fall once you add bends or a longer run. For flat twin and earth, 20mm conduit only really suits 1× 2.5mm² T&E — step up to 25mm or 32mm for more. A 20mm conduit is the usual domestic minimum, with 25mm preferred for future-proofing. For larger installations, consider using Trunking or Cable Tray instead.

Yes — circuits grouped in conduit must be derated per BS 7671 Table 4C1 (grouping rating factors). The factor depends on how many circuits are grouped and the installation method: for circuits bunched and enclosed, 2 circuits = 0.80, 3 circuits = 0.70, 4 = 0.65, 5 = 0.60. So three circuits in the same conduit can each carry about 70% of their tabulated capacity. This grouping factor applies on top of any ambient-temperature or thermal-insulation correction factors, so always check both the fill ratio and the derating.