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What goes inside conduit, and how full it can be45 % full, or a 45 % gap?

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  • Conduit and other containment can carry most types of cable, but it's mostly used for single-core cables (singles). These have only one layer of insulation and no sheath.
  • Singles must be installed in containment, and conduit is perfect for it.
  • General rule for any containment, conduit or trunking: don't overfill it. For trunking, keep to the 45 % space factor in Appendix E of the On-Site Guide. For conduit, Appendix E uses cable factors and conduit factors instead.
  • There are calculations to work out exactly how many conductors fit in a given conduit or trunking. Those are beyond these notes.
air cables: max 45 % free space: at least 55 % Cable area ÷ internal area of the trunking ≤ 45 %. Room is left so cables can be drawn in without damage and heat can escape. See On-Site Guide Appendix E.
The idea behind the 45 % rule: cables take up no more than 45 % of the inside, so at least 55 % is left empty. (Drawing shows about 43 %.)
Watch out
A common mix-up is that Appendix E explains there should be "a 45 per cent gap" in the conduit. It's the other way round: cables may fill up to 45 % of the space (the space factor), leaving at least 55 % free. Also, Appendix E handles conduit with a cable factor / conduit factor method (which allows for bends and length of run) and uses the 45 % space factor for trunking. Check the tables in your own copy.

On-Site Guide · Appendix E · cable capacities of conduit and trunking (45 % space factor) Key reference: tab it

BS 7671 · Reg 521.10.1 · non-sheathed (single-insulated) cables must be enclosed in conduit, ducting or trunking

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