First question is does this fall under BS 7671? If it part of a machine or control panel then other standards likely take precedence - and they sometimes have differing approaches to cable installation methods.

Unsheathed singles in slotted trunking wouldn't be an normal BS 7671 installation method - as the trunking typically wouldn't meet the IP4X requirement (although it might technically pass IPXXD, but expect raised eyebrows about the spirit of that). That might not be relevant if its inside an electrical enclosure though - again pointing to BS 7671 not really being the ideal standard.

   - Andy.

I agree with Andy - unlikely BS7671 applies inside a control panel.

If you are concerned about grouping, then can you provide more diverse routing inside the panel to reduce these factors? 

I have had a similar issue but the other way round - the grouping/sizing on the field cables was the cause of my problems. 

Jason.

I agree that BS7671 isn't the correct approach, rather BS EN 61439-1/2 for control assemblies is, and this standard often makes reference to BS EN 60204-1 (safety of electrical machines) ... it is a control panel for a machine i'm working on.

However, BS EN 60204-1 doesn't help with my question on slotted trunking either. The cable calculations are very similar (if not identical) to BS7671 of which i have Trimble software to back up my long hand attempts! And I think ultimately all these electrical standards come from the same source (physics) and are similar in many aspects, especially cable calculations.

The slotted trunking, the purpose of which is more for cable tidiness than mechanical protection, has gaps both sides greater than 30% of its surface area  (the requirement for perforated tray)  albeit it comes with a lid. I guess I was hoping someone has had come across this issue before and might argue it is reasonable to reference slotted trunking the same as perforated tray because the air flow can easily pass through it.

I have increased my conductor size to 50mm and run the cables in separate lengths of trunking to avoid grouping of more than 2 circuits. It just would have been great if I could have used 35mm cable, i've been lucky this time with the few circuits i am dealing with but I  might have had a real problem.