Remove t&e sheaving for short conduit drops?

[Achillies]

I am installing a new ring main into part of a basement in my house . The ring main will be connected to its own MCB.
I am wanting to use t&e for the main runs, which will be in the ceiling void, but use round conduit for the drops to the various 2G sockets. The drops are only 32" in length but each will carry the two 2.5mm^2 t&e of the ring.
The reason is beacuse the walls are not uniform and flat, and clipping t&e would look very untidy and would not give adequate cable support.
Following the guidance from earlier posts, I am concerned of the conduit de-rating factor so thought it sensible to bare back the sheaving of the t&e for the drops.
I cannot use conduit and singles throughout, i.e. a complete conduit installation, as the ceiling void restricts its use due to the nature of original building construction.
So, my question?:
Is it sensible to bare back the sheaving for just the drops and if so how do I make the step from singles to t&e at the open end? Do I need some 'special' termination box or is it ok to gunk the open end with silicone sealer etc?
I appreciate that any conduit system should be protected against the ingress of solids and moisture so the quation, I suppose, would apply to whether is was t&e in the drops or bared back t&e singles.

thanks ...Achillies

 

[holmslaw]

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[PrenticeBoyofDerry]

Removing the sheath from the T&E will not alter the de-rating factor of the cables. I assume the cables are being surface mounted.
So your alternatives are decrease MCB size, that could effect the load, increase size CSA of cable to 4.00mm.

Plus why conduit? I would prefer trunking or chase in and plaster.

 

[Achillies]

Hi Holmslaw & PrenticeBoy and thanks for your replies.

Holmslaw: Yes, plastic 20mm.
Ok, so i'm ok to add a bit of silicone to prevent dirt ingress. Thinking about my original post the chances of moisture ingress is zero anyway.
Are you suggesting running two conduits per 2G box?

PrenticeBoy: I understood, probably misunderstood!, de-rating was to do with the heat dissipation of the cables based on the available free space in the conduit for air circulation. My idea was by removing the sheaving I would increase the amount of free space for the size of the cable whilst still retaining the same size of cable and level of physical protection (individual core insualtion + conduit, instead of core insulation + sheave).
The reason for the conduit is part aesthetic, I think it looks good, as the basement is my workshop, and the walls are rough brick without plaster/plasterboard and quite dusty as old brick and mortar is. This makes it impossible to use the self-adhesive trunking, and I have tended to find nail-in cable clips coming loose. Securing conduit with saddle clamps fitted with Rawlplugs does the trick.

Ideally, as I have already mounted the single-knockout 2G boxes, and 20mm conduit boxes with saddle clamps ready for the cable runs, removing the part of the sheaving within th econduit, or feeding 2x 2.5mm^2 into one conduit would be my preferred options, but if you advise that this is non-preferred/prohibited etc. I will have to think again.

Regards ...Achillies

 

[JohnW2]

I am installing a new ring main into part of a basement in my house . .... The reason is beacuse the walls are not uniform and flat, and clipping t&e would look very untidy and would not give adequate cable support.

I suppose it depends upon how your basement is used, and how beautiful you want it to be, but in my cellar I have dealt with similar issues by attaching a length of timber to the wall and clipping the cables to that. If you don't like that then, as have been suggested, you're probably down to two conduits (or trunking) or moving up to 4mm² cables, if you want compliance with the regs.

As others have said, in conduit, using singles would give you virtually no advantage over T&E (same grouping factor and virtually the same current-carrying capacity) - and, in any event, what were you planning on doing with the CPC of stripped T&E - leaving it bare?

Kind REgards, John.

 

[holmslaw]

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[Achillies]

Thanks Holmslaw, that makes sense.

Hi JohnW2.
I wanted the basement to really look the part, hence my first choice for conduit, but I appreciate everyones comments on getting the spec correct.
With regards to the CPC it was my intention to use a green/yellow sleeve. I know that this would take up a bit of room freed up by stripping back the t&e sheaving but I can't leave it bare.

I really appreciate everyone's comments and the next sentence isn't meant to be challenging the advice, its just that I don't understand. If running singles into 20mm conduit has no appreciable advantage over t&e into conduit, and therefore making it impossible to use the 2.5mm^2 for ring mains, why do, say, MK make 2G boxes with just 1x 20mm knockout? Like I said, I ask the question respectfully so I can understand and it's not made in any way to 'argue' the point.

...Achillies

 

[holmslaw]

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[Achillies]

So to sum up, what I need if I want to keep the short runs of conduit is:

1. 2G surface boxes with 2x knockouts.
2. Only one 2.5mm^2 in each conduit
3. Seal against dust ingress by a blob of silicone sealer

Many thanks ...Achillies

 

[JohnW2]

The reason for the conduit is part aesthetic, I think it looks good, as the basement is my workshop, and the walls are rough brick without plaster/plasterboard and quite dusty as old brick and mortar is. This makes it impossible to use the self-adhesive trunking, and I have tended to find nail-in cable clips coming loose. Securing conduit with saddle clamps fitted with Rawlplugs does the trick.

It sounds as if you're quite keen on the aesthetics of conduit. If that were not the case, there are many methods available for 'clipping/tying/attaching cables to a brick wall which don't rely on nails or adhesive - e.g. cable tie plugs and bases which can be attached (with direct insertion, or screwed into plastic plugs) to holes drilled in the bricks. Indeed, there are plastic wall plugs available which are designed to take 'nail-in' cable clips.
Examples of cable tie plugs/bases are: this and this, and the 'pin plugs' here.

Kind Regards, John.

 

[sparkwright]

Don't remove the sheath from the cable. It's rough.

Either use mini trunking, two round conduits, or just put two whole cables in one round conduit.

 

[JohnW2]

Either use mini trunking, two round conduits, or just put two whole cables in one round conduit.

I think the OP's concern with the first and third of those options is that it would probably not be technically compliant because of the grouping factor; how I might personally view the situation could be a different matter I think we're all agreed that to strip the sheath off T&E (or, indeed, to use proper singles) would not actually achieve anything, quite apart from not being a nice idea.

Kind Regards, John.

 

[janner10]

If you going to have conduit on the wall may as well do one drop and then horizontal across to the other sockets and make it a 20A radial. Just a though, may make it a little easier.

 

[sparkwright]

It seems to me in the real world no one worries about putting two cables in one conduit or trunking.

 

[JohnW2]

It seems to me in the real world no one worries about putting two cables in one conduit or trunking.

I think the OP's concern with the first and third of those options is that it would probably not be technically compliant because of the grouping factor; how I might personally view the situation could be a different matter

Kind Regards, John.