installation method slotted PVC trunking - Wiring and the Regulations BS 7671 - IET EngX - IET EngX

installation method slotted PVC trunking

What installation method would you pick for running single core circuits inside slotted PVC trunking (horizontal and vertical) mounted to a steel gear-plate all enclosed within an electrical control panel assembly that has forced air ventilation (ambient temp controlled <40°C). 

I'm unsure if reference method B (specifically method 6 and 7) is correct being that it says 'enclosed trunking' on masonary or wooden surface, for one that my trunking is slotted (offers some ventilation maybe?) and is mounted to steel (maybe thats worse!?).

Is there an installation method i'm overlooking? hopefully something that will give me more juice!  Could one not get away with saying these cables are clipped direct or in free air on unperforated tray (to the steel gearplate) or is that too ambitious!?

I ask because if i apply all my correction factors (Ca, Cg, 0.97 for class 5 strands) my singles CSA come out at 70mm2 (protection is a 100Amp B type MCB and my design current 87Amps) ...  the SSR's (switches) for these circuits can only take max 50mm2! 

As a comparison, the armoured multicore taking these circuits to the load in the field only needs to be 25mm2 !! i feel i've gone horribly wrong somewhere.

  • 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. 


  • 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.