Conduit & Trunking Fill Calculator — BS 7671

The IET On-Site Guide and Guidance Note 1 (Appendix E) set a maximum space factor of 35% for conduit where the cable is not covered by the unit factor tables. This means the total cross-sectional area of all cables inside the conduit should not exceed 35% of the internal cross-sectional area of the conduit. The older 40% conduit figure was an arbitrary value later shown to be inappropriate (IET, Wiring Matters, November 2021). The 35% headroom allows for cable pulling, heat dissipation and future additions. Capacities of conduit and trunking have not been in BS 7671 itself since 1991 — the committee treats them as guidance, published in the IET On-Site Guide / Guidance Note 1.

Trunking has a higher recommended maximum space factor of 45% (IET On-Site Guide / Guidance Note 1, Appendix E). Because trunking is rectangular and cables can be laid flat, the arrangement can be more efficient than round conduit, so a 45% fill is permitted versus 35% for conduit. The 45% rule still ensures cables are not packed too tightly and can dissipate heat properly.

Each bend in a conduit run makes it harder to pull cables through, so the practical allowable fill is lower than for a straight run. It is recommended to have no more than two 90-degree bends between draw points; if more are needed, install additional draw-in boxes. The bend reduction used by this calculator is a simplified rule of thumb, not an IET figure — for runs with bends the definitive method is the IET On-Site Guide Appendix E reduced cable-factor and conduit-factor tables, which switch to a different factor set keyed on run length and number of bends.

Yes, you can mix different cable sizes, but you need to add up the cross-sectional areas of all cables and ensure the total does not exceed the space-factor limit for the enclosure — 35% for conduit, 45% for trunking. This calculator works with one cable type at a time, so add the CSA values for each cable type together manually if mixing sizes. For conduit runs with bends, the IET On-Site Guide Appendix E unit cable-factor and conduit-factor tables are the definitive sizing method.

Conduit is typically used for individual circuit runs and where mechanical protection is needed in walls, floors and ceilings. Trunking is used where multiple circuits run along the same route, such as along walls in commercial premises. Trunking is easier to add cables to later and provides better separation between circuits when using compartmented trunking. See our dedicated Conduit Fill Calculator and Trunking Fill Calculator for detailed sizing, or check standard UK cable trunking sizes before choosing a run.