What is the IEE regulations on space factors in conduits and ducts?

This topic is covered in the IEE's On-Site Guide, a supplement to BS 7671:2008 Regulations for Electrical Installations.

'Grouping' describes to the mutual heating effect of adjacent conductors. The effect of grouping acts to reduce the number of cables actually allowed into an enclosure because, as the number of circuits increases, the current-carrying capacity of the cables reduces -requiring the use of cables with larger cross-sectional areas! And using cables with larger cross-sectional areas may then require us to use larger-diameter conduit.

In practice, this might also mean distributing the cables between, say, two smaller conduits (or by using a small trunking system).

Guidance in this is provided by an equation called a space factor, where:

space factor = [(csa of all cables)/(internal csa of conduit)] x 100 %

...where the maximum permissible space factor is 40%. In other Words, the area of the cables must not exceed 40% of the internal cross-sectional area of a conduit. The 60% empty-space requirement is intended to enable the dissipation of the heat produced by the current-carrying conductors grouped inside the conduit.