UK Cable Trunking Sizes & Capacity

The IET On-Site Guide (Appendix E) and IET Guidance Note 1 recommend that cables should not occupy more than 45% of the internal cross-sectional area of the trunking. This allows space for heat dissipation and future additions. For example, 25 x 16 mm trunking has an internal area of approximately 340 mm² — usable cable space is about 153 mm². Cable fill is calculated by adding up the cross-sectional area of each cable including insulation.

For a single twin and earth cable (e.g. one 2.5 mm² T&E to a socket), 16 x 16 mm is usually sufficient. For 2-3 cables, 25 x 16 mm is standard. For 4-6 cables or a mix of data and power, 38 x 25 mm or 38 x 16 mm is needed. Always calculate the actual cable fill — do not guess, as overfilling causes overheating and makes future maintenance difficult.

Not in the same compartment. BS 7671 requires segregation of circuits at different voltages. Use compartmentalised trunking with separate chambers for power and data/telecommunications cables, or use separate trunking runs. The segregation barrier must be continuous and the compartments must not be interconnected.

Mini trunking: fix at maximum 300 mm centres (some manufacturers specify 250 mm). Maxi trunking: fix at maximum 600 mm centres for sizes up to 75 x 75 mm, and 750-900 mm for larger sizes. Always fix within 150 mm of each end, at every joint, and either side of bends. Use appropriate fixings for the surface — screws for timber/plasterboard, rawl plugs for masonry.

Mini trunking covers the smaller sizes, from 16 x 16 mm up to around 40 x 25 mm, and is used for single circuits or small groups of cables run on the surface to an accessory, such as a socket or light switch extension. Maxi trunking starts at 50 x 50 mm and goes up to 150 x 75 mm or larger, and is used for distribution runs carrying many circuits together, such as along a commercial corridor or plant room wall. Both use the same 45% fill principle; maxi trunking is chosen once the cable count or cable sizes are too large for mini trunking to hold within that limit, or where compartmentalised segregation of several circuit types is needed.

Both are standard. Most domestic and light commercial cable trunking, including mini, maxi and dado/perimeter trunking, is rigid PVC-U (unplasticised PVC), which is lightweight, flame retardant and self-extinguishing, and easy to cut and fix on site. Both plastic and metal trunking are made to the same product standard, BS EN 50085-1 with BS EN 50085-2-1 for wall and ceiling mounting, with fire performance assessed by glow-wire testing to BS EN IEC 60695-2-11. For industrial, commercial or fire-rated installations, galvanised steel trunking is used instead, giving greater mechanical protection and higher temperature tolerance, and on some installations it can serve as the circuit protective conductor (BS 7671 Regulation 543.2.1). Both materials are available in a similar range of widths and depths, so the fill and space-factor calculations described on this page apply equally to plastic and metal trunking.

Perimeter trunking, also called dado or skirting trunking, runs around a room at skirting or desk height rather than along a wall or ceiling like mini and maxi trunking. It is a separate product range from the mini and maxi trunking above, commonly available from about 100 x 50 mm as a single compartment up to around 170 x 50 mm with two or three internal compartments, so that power, data and voice cabling can be segregated within the same enclosure. Sizing follows the same 45% fill principle as mini and maxi trunking, calculated separately for each compartment since cables in different compartments cannot be counted together.

Add up the overall cross-sectional area of every cable you plan to run, including insulation and sheath, then check the total against 45% of the internal area of the trunking size you are considering (the tables and worked example on this page show how). If the total is under the 45% limit for a given size, that size passes; if not, move up to the next size or split the cables across two runs. Our Trunking Fill Calculator carries out this calculation for any mix of cable types and trunking sizes, and the Conduit and Trunking Fill Calculator compares conduit and trunking options side by side.

The 45% fill figure is not written into BS 7671 itself. Cable capacity tables for conduit and trunking were removed from the Wiring Regulations in 1991 and now sit in the IET On-Site Guide (Appendix E) and IET Guidance Note 1, which is why 45% is described as an IET convention rather than a BS 7671 clause; BS 7671 Regulation 522.8 sets the general requirement for cables to be adequately supported and protected against damage, without stating a fill percentage. Trunking size does affect current rating in a different way: cables grouped together in trunking need a BS 7671 Table 4C1 grouping correction factor applied to their current-carrying capacity, for example 0.80 for 2 circuits and 0.70 for 3 circuits, so a trunking size that comfortably passes the 45% fill check can still need a larger cable to carry the same load once grouping is applied. Use the Cable Derating Calculator to apply grouping, temperature and thermal-insulation correction factors to your circuit. Final cable and circuit design should always be confirmed by a qualified electrician against BS 7671 for the actual installation.