2 x 2.5mm² T&E in 25mm round uPVC conduit?

[bonebill]

I read a thread recently ("T & E in steel 20mm conduit") about a chap who had ended up with 2 x 2.5mm² T&E in (about) 20mm steel conduit as part of a ring circuit on a 32A MCB.

ban-all-sheds pointed out that in this situation the current rating of the cable would have to be reduced and that as such it would be pushing it to say that it was still to code, and that to keep things comfortably within the regs, it would mean using a lower rated MCB, probably 25A.

I am wanting to come up with a cabling design for my new house that will allow me to have 2 x 2.5mm² T&E, in conduit, for ring circuits, and use a 32A MCB, and be comfortably within the regs.

Nearly all of the time the cables will be in the safe zones, and it seems that uPVC would be easier to work with than steel, so I thought it would be nice if I could go with uPVC.

So I was wondering about using 25mm round uPVC conduit.

Assuming a 1mm wall thickness, 20mm conduit would have an inner cross-sectional area of about 254mm², and 25mm conduit would have a inner cross-sectional area of about 415mm² (63% larger).

www.tlc-direct.co.uk says that 2.5mm² T&E has cross-sectional dimensions of 10.3mm x 6mm, which would have a cross-sectional area of about 55mm².

So for 20mm conduit, 2 x 2.5mm² would occupy some 22% of the space, whereas with 25mm conduit it would only occupy 13% of the space.

Would that be sufficient to make it work?

I have spoken with asterisk and it seems that the thing we are dealing with is the grouping factor. The grouping factor seems to be expecting singles in conduit rather than T&E, and it doesn't appear to take into account the amount of free space in the conduit.

I am wondering though about in what scenarios the grouping factor actually applies. Imagine you have a 500mm(!) diameter conduit with 2 x 2.5mm² T&E, randomly positioned, running through it. It would seem that the cables would be considered to be independent and no special factor would be applied. But in some a technical sense, the grouping factor might still apply. For practical purposes, would anyone apply the grouping factor? If not, then where do we draw the line?

Below is a drawing showing a cross section of the 20mm and 25mm scenarios. Everything is drawn to the same scale. The conductors are 2.5mm² for the phase and neutral and 1.5mm² for the earth.

Could it not be argued that for the 25mm case we are into "sausage down a railway tunnel" territory?

Is the problem that the cables are unclipped, and therefore we can't guarantee how much space is between them, so we have to assume the worst?

______________________________
Moderator

please note 10a

 

[ban-all-sheds]

If it's a ring main, why have you got two cables in the conduit?

And if you're using conduit, why are you using T/E in the first place?

 

[bonebill]

If it's a ring main, why have you got two cables in the conduit?

It is a single-storey building with a loft space. The wiring would be in the ceiling so the "in" and "out" cables both go upwards from the socket. Since both cables are running the same route between the socket and the ceiling, the logic is that we might as well save effort and make a single channel for both of them.

And if you're using conduit, why are you using T/E in the first place?

If I'm using conduit then in theory I could use singles.

But I was only planning on using conduit in the wall chases, to provide a nice (potentially threadable) path for the cables. In the loft space I was planning to just have T&E clipped on surface.

With singles I would need to have conduit for their entire length, which would mean putting in conduit everywhere complete with joints which (for a DIY-er whose experience and time are both limited) sounds like a lot more work than just running a bit of T&E.

So I am still wanting to go down the route of putting the T&E in some kind of channel for the wall chase.

If we're not happy with a single 25mm round conduit then I could put in two (smaller) separate channels; could be 20mm round conduit or even oval conduit.

I guess I have some design parameters:

· Compliance with Regs
· Want to be able to re-thread cable in wall channel
· Reasonably low installation time (I only have weekends)
· Do not care so much: cost of materials

 

[ennell]

If you plan to have socket outlets with metal back boxes, using 25mm conduit will mean a messy drilling job to convert the 20mm knockouts to 25mm clearance holes.

I assume you'll be making every point a double socket outlet. I know it means a wider chase, but two separate vertical conduit drops, one into each of the two knockouts on the top edge of the box, will make sure it's easy to pull in or change the cable (but how often do you reckon you'll be changing it?)

However as you suggest it's a new-build, presumably to be plastered after the conduit is installed, you shouldn't need to feed every socket outlet with two cables from the roof space. In each room, one vertical conduit drop to the first s/o and then a horizontal conduit run to the next one, and to the next one etc, and then a vertical conduit up to the roof space from the 'last' s/o should suffice - with just a single 2.5 T&E in each run of conduit.

To make going round the 90 degree corner from one wall to another easier, make sure you put a pull wire in the conduit when the bends are being fitted.

ennell

 

[has]

why not use 25mm capping?
http://www.tlc-direct.co.uk/Products/MTCAP25.html

 

[Big_Spark]

Bonehill, if your simply using the conduit as a drop from cieling to socket, then 20mm is sufficient for your needs and avoids many of the probl;ems you would encounter with 25mm conduit.

Grouping factors only really come into play in this situation if the whole circuit were in conduit and the cables were going round bends etc. Of course there is am effect on the cables with it being in conduit, but to be honest this effect is negligable.

However as has been suggested above, what is the matter with using capping?

 

[ban-all-sheds]

Or installing a 4mm radial...

 

[bonebill]

If you plan to have socket outlets with metal back boxes, using 25mm conduit will mean a messy drilling job to convert the 20mm knockouts to 25mm clearance holes.

I'm pretty sure the 35mm deep double box I looked at had four knock-outs along the long sides: 25, 20, 20, 25, although other boxes had only 20mm holes.

However as you suggest it's a new-build, presumably to be plastered after the conduit is installed, you shouldn't need to feed every socket outlet with two cables from the roof space. In each room, one vertical conduit drop to the first s/o and then a horizontal conduit run to the next one, and to the next one etc, and then a vertical conduit up to the roof space from the 'last' s/o should suffice - with just a single 2.5 T&E in each run of conduit.

Sounds good to me.

To make going round the 90 degree corner from one wall to another easier, make sure you put a pull wire in the conduit when the bends are being fitted.

Good plan. This would defeat the threadability objective, although as you imply that would not be an issue for everyone.

 

[bonebill]

why not use 25mm capping?
http://www.tlc-direct.co.uk/Products/MTCAP25.html[/QUOTE]

Ordinarily it would be fine, but at the minute I am fairly keen on threadability.

1. One side of the space would be rough brickwork - not the best for threading
2. The space behind capping isn't very deep - again not the best for threading
3. Plaster can get through gaps and leak through to the space

 

[bonebill]

Bonehill, if your simply using the conduit as a drop from ceiling to socket, then 20mm is sufficient for your needs and avoids many of the problems you would encounter with 25mm conduit.

Grouping factors only really come into play in this situation if the whole circuit were in conduit and the cables were going round bends etc. Of course there is an effect on the cables with it being in conduit, but to be honest this effect is negligible.

Sounds good to me.

 

[andy]

why not use 25mm capping?
http://www.tlc-direct.co.uk/Products/MTCAP25.html[/QUOTE]

Ordinarily it would be fine, but at the minute I am fairly keen on threadability.

1. One side of the space would be rough brickwork - not the best for threading
2. The space behind capping isn't very deep - again not the best for threading
3. Plaster can get through gaps and leak through to the space

your doin this the hard way. just use cpaain like every1 else does. how often do you plan on re-wiring?

 

[bonebill]

Or installing a 4mm radial...

Interesting.

That does have a certain appeal.

I'd need almost as much 4mm as 2.5mm, so it would cost more for the cable, but that's not necessarily a show stopper.

4mm is thicker and therefore I guess it would be less easy to put in, but again not necessarily a show stopper.

Then there's the pros and cons of radials and rings and their failure modes.

I'll think about it.

 

[bonebill]

your doin this the hard way. just use capping like every1 else does. how often do you plan on re-wiring?

Good point. I'll think about it some more.

 

[Big_Spark]

Bonebill, if this is a new build, and you want rewirability, then you should use conduit for the entire circuit and use singles for the cables instead of T&E.

Lets be honest here, modern PVC T&E has a shelf life of 25 years under normal conditions, once you wire this place you should not be worrying about it for at least 15 years, so youe method of wiring is in essence a moot point.

If you want a rough guide, this was done a year ago for rewires not new installs, but many of the elements are the same, try THIS

 

[ban-all-sheds]

your doin this the hard way. just use cpaain like every1 else does. how often do you plan on re-wiring?

Q: How often do we get people on here asking questions about joining cables because they want to extend or re-route the circuit?

A: Quite frequently.

It's not about rewiring, it's about future modifications.