Spacer between CU and trunking

[SimonH2]

When setting up a CU alongside metal trunking, what do people use as a spacer and as protection from the metal edges ? That's as in, make a large slot shaped hole in both parts, make the hole in the spacer slightly smaller, so the spacer ensures that the cables cannot contact the metal edges as well as spacing the parts apart so the trunking kid will still fit.

I know Paxolin (aka Tufnol or SRBP) is commonly used, but in the past I've seen a sheet of something white (don't know what, and the sparky responsible is long since retired), and I've seen PVC sheet or even ply suggested. I've had a rummage in my late father's box of offcuts and there's a piece in there that would just about do, but is not really the right size. But I've also realised that there's a few offcuts of splashback left over from Mum's kitchen that would be about about the right thickness, much easier to work, but are made of MDF.
I can't really see anything wrong with using that - if something is in contact with it and hot enough to set it charring, then I think there's bigger issues than having a small piece of MDF there.

PS - no I don't want to use a number of bushes etc. I think it looks untidy, and provides a limited space to get cables through. The last install I witnessed, they completely filled the bushes with the initial install - I pity anyone who needs to add a circuit later.

 

[Harry Bloomfield]

You can use a U profile plastic called 'grommet strip' along the metal edge.

 

[333rocky333]

You have mentioned the most common ways in your initial post, usually paxolin, plywood or bushes, bear in mind anything spaced between can affect the earth continuity between the two, one of the roughest i have seen is the Pvc outer sheath of T and E used as grommet strip.
50mm bushes and lockrings, with one lockring as a spacer look fine.

Also when spacing, test fit the lid asap, as your spacer can sometimes hinder the lid fitting properly.

 

[SimonH2]

... one of the roughest i have seen is the Pvc outer sheath of T and E used as grommet strip.

At least they used something rather than leaving a bare metal edge - that probably looks like it was cut with a blunt can opener !

 

[crystal ball]

At least they used something rather than leaving a bare metal edge - that probably looks like it was cut with a blunt can opener !

Seen a few of those, sometimes the installer has bent over the jagged edge to form a rounded entry, must have taken ages, why not do it right?

 

[SimonH2]

You have mentioned the most common ways in your initial post, usually paxolin, plywood or

Seems odd doesn't it - in a domestic environment we now have to use ferrous metal CUs, but it's considered fine to have a layer of ply that the cables go through in other environments. Hmm, as a mind experiment, I find it interesting to try and work out if that would be allowed in a domestic environment - the CU would still have a non-combustible enclosure

But it sounds like the spare splashback should do the job fine, and I know from past experience that it's nice and easy to work.

... anything spaced between can affect the earth continuity between the two

Yes, had thought of that.

Also when spacing, test fit the lid asap, as your spacer can sometimes hinder the lid fitting properly.

Ha ha, yes I've been caught out by things like that in the past. It's "annoying" to create something with great care and precision - only to end up hacking it about when you realise you've missed some detail like that

 

[OwainDIYer]

Silicone rubber sponge sheet?

https://www.polymax.co.uk/silicone/silicone-rubber-sponge-sheet-closedcell/

Sandwich an uncut piece between CU and trunking, then pierce as required for cables to pass through.

 

[ban-all-sheds]

Would a couple of tee couplers next to each other give enough space to take all the cables through?

Obviously if they fit in the trunking they will fit through the trunking sized aperture, but I was thinking about manageability...

 

[SimonH2]

That is indeed one way of doing it - tee off the trunking, short length of trunking, then flange coupler. It's a lot more work to do, and doesn't look half as neat.

 

[ban-all-sheds]

tee off the trunking, short length of trunking, then flange coupler.

I didnt think there was that big a gap between the trunking and the CU.

What is the spacing?

 

[SimonH2]

There normally isn't - just a small gap between the side of the trunking and the box.
But if you use a tee, then there will be. You don't just stick the branch of the tee through a hole in the box - to properly connect it you need a flange coupler. And you can't just bolt the flange coupler to the tee - so you need a short (few inches) length of trunking. Altogether you need around 4-5 inch.
You can "roll your own" - perhaps bending the tabs on the branch of the tee to form a mounting flange. But you then have several metal edges to protect the cables from, and it's going to be really hard to maintain IPxxD or IP4X performance - I assume you know about 521.10.1 ?
By contrast, cutting a slot in both trunking (some manufacturers even make pre-slotted trunking sections for this) and box, and using a "cable friendly*" spacer with a slightly smaller slot, gives a small and neat connection. As long as everything stays flat and is bolted together properly, then cables are protected and IP performance is maintained. Similarly using couplers and male bushings.
* Typically something electrically insulating and reasonably easily worked is used. Being at least ¼" thick (so the trunking lid will go back on), it's easy to radius the edges of the hole so cables are protected.

 

[SimonH2]

Well I did go with the splashback offcut. Got a chance to use an air-nibbler I bought cheap from Aldi a few months ago - certainly made light work of opening up the slots in the metalwork It's a bit rough in places - would have helped to have the garage/workshop built instead of making do in the shed using a workmate as a bench
I don't think I'll ever fill those holes up

 

[donrkebab]

I would use 4mm flame proof mdf, easy to cut, won't damage cable insulation, no need for grommet strip.

 

[ban-all-sheds]

I didn't know it was trunking between and servicing 2 CUs.

Given that they will be securely fixed and the trunking will be, why not have just had a gap in it but not in the cover?

[EDIT]Typo corrected[/EDIT]

 

[donrkebab]

I didn't know it was trunking between and servicing 2 CUs.

Given that they will be securely fixed and the trunking wil be, why not have just had a gap in it but not in the cover?

This all a bit beyond you isn't it.