What is the maximum fill ratio for trunking?

The generally accepted maximum fill ratio for trunking is 45% of the internal cross-sectional area, as recommended by BS 7671 and trunking manufacturers. This allows cables to be laid flat without excessive stacking, provides adequate ventilation for cooling, and leaves space for future additions. Some specifications require 40% for long runs or where cables may be added later.

What are cable factors in BS 7671 Appendix C?

Cable factors are the space (in mm²) that each cable type and size occupies in trunking, as tabulated in BS 7671 Appendix C. These factors account for the cable's overall dimensions plus a small clearance. For example, a 2.5mm² single-core 6491X cable has a factor of 13.9mm², while a 2.5mm² twin and earth flat cable has a factor of 48mm². Using factors rather than raw OD gives more accurate results for non-circular cables.

Can I mix different cable types in the same trunking?

Yes — you can mix cable types provided the total of all cable factors does not exceed the fill limit. However, BS 7671 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.

Does grouping factor derating apply in trunking?

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.

What trunking size do I need for a typical domestic consumer unit?

For a standard domestic installation with 8-12 circuits using single-core cables from the consumer unit to the first accessory: 50×50mm trunking (2,500mm² internal area) typically accommodates 20-30 single-core 2.5mm² cables at 45% fill. For larger installations or where twin and earth cables are used, 75×50mm or 100×50mm may be needed. Always calculate rather than guess.

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Trunking Fill Calculator — BS 7671 Appendix C

Calculate cable fill ratio for trunking using BS 7671 cable factors. Ensure compliance with the 45% maximum fill ratio for easy cable installation and maintenance.

Cable TypeCable construction type

Cable SizeConductor CSA

Number of Cables

Total cables in trunking

Trunking SizeInternal dimensions

How We Calculate This

Trunking fill is calculated using cable factor tables from BS 7671 Appendix C, which give the space occupied by each cable type in mm².

Fill % = (Sum of cable factors ÷ Trunking internal area) × 100

Common Trunking Capacities at 45%

25×16mm: ~180mm² usable — 13× 1.5mm² singles
50×25mm: ~562mm² usable — 40× 1.5mm² singles
50×50mm: ~1,125mm² usable — 80× 1.5mm² singles
100×50mm: ~2,250mm² usable — 161× 1.5mm² singles
100×100mm: ~4,500mm² usable — 321× 1.5mm² singles

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.