Cables in ducts - Derating factor for cable grouping - Wiring and the Regulations BS 7671 - IET EngX - IET EngX

Cables in ducts - Derating factor for cable grouping

Hello all. I'm having one of those moments where everything I thought I knew is wrong.

I need some advice on cable derating factor (Cg) when running cables in underground ducts. I'll provide a hypothetical example to help with my question:

I have a switchboard at one end of a set of 4 power cable ducts, and 9 items of equipment requiring power at the other end. 

Lets assume the ducts are:

  • Approx 25m long (voltage drop and Zs are unlikely to be an issue).
  • 150mm diameter.
  • Are in a cluster, 2 ducts on top, 2 on the bottom, seperated 100mm in each direcfion.
  • All circuits will be 'ON' at the same time.
  • We don't need to discuss soil temperature or anything like that - just the grouping factor, Cg

Let assume the 9 circuits are spread evenly across 3 of the ducts, with the 4th duct left spare - so 3 cables in 3 of the ducts.

For my derating factor Cg for each cable, is this based on 3 cables in a group, or 9 cables in a group?

Many thanks in advance for any asistance