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