Trunking Fill Calculator — 45% Cable Fill

The generally accepted maximum is 45% of the internal cross-sectional area. This 45% space factor comes from the IET On-Site Guide (Appendix E) and IET Guidance Note 1, and from trunking manufacturers — not from BS 7671 itself, which sets requirements (Reg 522.8) but no fill figure. The 45% allowance lets cables be drawn in and laid without excessive stacking, gives air space for cooling, and leaves room for future additions. Some specifications use 40% for long runs or where cables may be added later.

Cable-capacity tables were removed from the Wiring Regulations in 1991 (16th Edition), so they are NOT in BS 7671:2018. The authoritative tables now sit in the IET On-Site Guide Appendix E (Tables E5 and E6) and IET Guidance Note 1. This calculator uses the geometric method: it works out the actual cross-sectional area each cable occupies (a single-core 6491X by its overall diameter, a flat twin-and-earth or three-core by its overall width × height from manufacturer datasheets) and compares the total against 45% of the trunking's internal area. For example a 2.5mm² 6491X single (overall diameter about 3.7mm) occupies roughly 10.8mm², and a 2.5mm² flat twin and earth about 71mm² (its 6.2 × 11.5mm overall envelope). As an alternative, the IET On-Site Guide cable-factor method assigns each single-core cable a tabulated factor (2.5mm² = 12.6) and compares the total against a tabulated trunking factor (50×50mm = 1037); both methods give similar trunking sizes.

Yes — you can mix cable types provided the total cable area does not exceed the 45% fill limit. However, BS 7671 (Reg 528) requires segregation of circuits at different voltages — for example, ELV (extra-low voltage) must be separated from LV (low voltage) by a physical barrier or partition within the trunking. Data cables should also be segregated from power cables to avoid electromagnetic interference.

Yes — cables installed in trunking are grouped, and BS 7671 Table 4C1 correction factors apply. The grouping factor depends on the number of circuits: 2 circuits = 0.80, 3 = 0.70, 4 = 0.65, 5 = 0.60, 6 = 0.57, 7 = 0.54, and so on. This means each cable can carry less current than its standalone rating. Always check both trunking fill AND current derating. For conduit installations, use our Conduit Fill Calculator, or see our Cable Tray Sizing Calculator for open tray installations.

For a standard domestic installation with 8-12 circuits using single-core 6491X cables from the consumer unit to the first accessory, 50×50mm trunking (2,500mm² internal area) has ample room — at 45% fill it holds well over 100 single-core 2.5mm² cables, so in practice the cable count is never the limiting factor at that size. Where flat twin and earth is run inside trunking the cables take far more space (a 2.5mm² T&E occupies about 71mm² versus 11mm² for a single), so 75×50mm or 100×50mm is more typical. Always calculate for your actual cable mix rather than guess.