T&E in conduit, grouping, derating questions

[JohnW2]

If, as seems to be the case, you believe that there is a substantial possibility that you will have a need to upgrade the 6mm² cable within the next 12 months, why on earth don't you put in something bigger than 6mm² now - there's never a problem in having a cable which is over-specified!

I take your point. Cost would be one reason, another would be that as I didn't design the installation to start with.

Cost surely isn't an issue. The small marginal difference between price of 6mm² and 10mm² cable is surely trivial in relation to the time, effiort, hassle (and potential cost) of having to dig around in the walls within 12 months, isn't it??

Kind Regards, John

 

[ban-all-sheds]

You're welcome. You may be right to have had that concern since, as you will see, both BAS and myself are inclined to consider the two 2.5mm² cables of the ring as 'separate cables', in which case having them both in the same conduit would not be compliant with the regulations. It remains possible that others will disagree with this interpretation of the regs.

Given that it says "Number of circuits or multicore cables", I've no doubts that it's correct to count the number of multicore cables. After all, a circuit not using multicore would consist of 3, possibly 2, singles.

 

[JohnW2]

You're welcome. You may be right to have had that concern since, as you will see, both BAS and myself are inclined to consider the two 2.5mm² cables of the ring as 'separate cables', in which case having them both in the same conduit would not be compliant with the regulations. It remains possible that others will disagree with this interpretation of the regs.

Given that it says "Number of circuits or multicore cables", I've no doubts that it's correct to count the number of multicore cables. After all, a circuit not using multicore would consist of 3, possibly 2, singles.

Yes, I'm pretty sure that's the right interpretation. On reflection, the reference to 'circuits' is probably there to cover singles - i.e. a pair of singles consitutes 'a circuit', and is hence equivalent to one 2-core cable. If that's the intent, they probably could have expressed/explained it a bit more clearly (after all, as we all know, a 'circuit' is a collection of things which shares a common OPD )!

Kind Regards, John

 

[ban-all-sheds]

Indeed.

What if it was a ring wired in singles? 6 of them. Counting it as 2 circuits seems correct.

 

[ban-all-sheds]

"Number of permanently live L/N pairs" would catch everything.

 

[JohnW2]

Indeed. What if it was a ring wired in singles? 6 of them. Counting it as 2 circuits seems correct.

That would certainly be my view.

"Number of permanently live L/N pairs" would catch everything.

I'm not so sure about the 'permanently live' bit - I can see that raising all sorts of issues/deabtes!! Wouldn't it be far better without it (i.e. just 'number of L/N pairs')? Mind you, one would presumably also have to make some provision for 3-phase situations, for which 'L/N pairs' would fall flat on its face!

Kind Regards, John

 

[JohnW2]

MK helpfully list cross sections of oval conduit here, but I can't find any corresponding tables anywhere dealing typical cross section areas of flat T&E.

Did you seeing the following when I posted it and, if so, was it of any assistance:

If one takes the 'typical' external dimensions of T&E from the forum's wiki (which I mentioned before) and assumes that the shape is such that the cross-section can be represented as a rectangle with a semi-circle on each end, then what one can calculate is as follows:

Untitled

• JohnW2
• 17 Aug 2013
• 1

However, guidelines in relation to CSA's is obvioulsy only part of the story when deciding what cables will even fit in what conduit (particularly if oval)!

Kind Regards, John

 

[SimonH2]

Interesting thread ...
Given that white oval conduit is a) quite thin, and b) a 2.5T&E is a fair good fit, then does that count as derating the cable as much as for "generic" conduit/cable combinations ? The T&E will be in physical contact with one or other walls of the conduit for much of it's length, and where it isn't in physical contact, then the air space will be quite small. I'd suggest that the thermal effect will be substantially less than some arbitrary cable in some arbitrary large conduit.

And as a matter of observation, where a property has been done with white ovals, has anyone seen a situation where the sparky didn't put both cables to a socket in the one conduit ?
EDIT: for the normal "two cables up/down to a socket" and leaving aside special cases like spurs and jumping between floors.

Also, I find the negative comments rather odd ? Unless the conduit is undersized so as to make the cable a tight fit, then pulling out and replacing cables is normally not that hard. Of course, where the conduit stops short of the box and the cable has to do a (plastered in) dogleg then that changes things. But that's not something I'd allow on any job I was paying for, ditto, any mention of clipping direct and plastering in - but then that's just my preference, based on having been able to pull out and replace cable in conduit many times over the years.

 

[JohnW2]

Interesting thread ... Given that white oval conduit is a) quite thin, and b) a 2.5T&E is a fair good fit, then does that count as derating the cable as much as for "generic" conduit/cable combinations ? The T&E will be in physical contact with one or other walls of the conduit for much of it's length, and where it isn't in physical contact, then the air space will be quite small. I'd suggest that the thermal effect will be substantially less than some arbitrary cable in some arbitrary large conduit.

I agree - but I suppose it would be impossible for the regs to attempt to cover all possible situations. Anyone is, of course, (unless constrained by employers, scheme operators etc.!) free to deviate from BS7671 if they can produce their own satisfactory engineering justification.

Also, I find the negative comments rather odd ? Unless the conduit is undersized so as to make the cable a tight fit, then pulling out and replacing cables is normally not that hard. Of course, where the conduit stops short of the box and the cable has to do a (plastered in) dogleg then that changes things. ....

It seems that 'experiences vary'. FWIW, my experience is that even a single T&E in a plastic conduit plenty big enough for it can be difficult, particularly if the run is not dead straight. T&E seems to me to be really too stiff for this purpose - the slightest kink/bend in the cable can cause it to 'jam' in the conduit. Again, just IME, more than one T*E in any plastic coduit is definitely bad news in this respect. However, others (seemingly like yourself) seem to have much happier experiences.

Kind Regards, John

 

[SimonH2]

Yes, it does vary.
Worst case us pushing cable up a conduit, easiest is pushing it down.
If feeding in an extra one alongside existing cables then it depends whether the original cables were kept straight or are full of kinks. I've had to resort of jiggling the other cables - push everything down a bit together, pull the existing cables back a bit - to work a new one in. Or pull them all out and feed the new set in together.

And some cables are "stickier" than others.

 

[DetlefSchmitz]

I watched someone in Germany trying to push cable through a conduit and wondered whether they realised that pulling it is infinitely easier. I presume you meant this!

I agree that if you want to add an extra it is usually easier to take everything out and pull them through together.

 

[lxboy]

I have read on some earlier posts that when enclosing cables in conduit grouping and derating need to be considered so I wanted to check the following: 

Is it ok to enclose 2 x 2.5mm T&E (Ring circuit) in 20 or 25mm oval conduit?  The conduit is buried in the wall, is approx 83" long and only encloses that part of the circuit.

Same question for a 4 cable run, approx 88" conduit length.

1 x 3 core 1.5mm (interconnected smoke alarms)
2 x 1.5mm T&E (2 lighting circuits)
1 x 6mm T&E (supply to loft junction box for future use, e.g. Radial etc).

How about the 6mm in one 20mm oval and the other three in a separate 25mm oval conduit? 

and finally...

Switch drops (3 way) & fitting: 5x1.5mm T&E / 4x1.5mm + 1.5mm 3 core.  If I remember correctly, these will all fit in a 1 1/2" capping.  Can I do this, or can I do 3 cables in one 20/25mm conduit and 2 cables in another one.  these drops aren't so long (approx 58").

Thanks!


Lu

I would use 25mm conduit for all drops and max 3x 1.5mm 2x 2.5mm 1x 6mm in each.

regards,

lxboy

 

[riveralt]

I would use 25mm conduit for all drops and max 3x 1.5mm 2x 2.5mm 1x 6mm in each.

How did you work that out?
Roughly Area inside conduit is 490mm². (not sure of internal measurements)
45% of 490mm² = 220mm² which is maximum allowed.
Using charts supplied in this post your cables come out at 300mm².

 

[ban-all-sheds]

How did you work that out?
Roughly Area inside conduit is 490mm². (not sure of internal measurements)

For 25mm oval? Probably not much more than 200mm²

45% of 490mm² = 220mm² which is maximum allowed.

90-100mm².

Using charts supplied in this post your cables come out at 300mm².

Won't even fit, let alone fit at 45%.

Round?

Assume 2mm wall thickness, internal csa = approx 350mm².

45% = 158mm²

So no way will that many cables go in anything described as 25mm conduit.

 

[lxboy]

I would use 25mm conduit for all drops and a max of 3x 1.5mm of 2x 2.5mm 1x 6mm IN EACH.

That's 3 tubes ?

Lxboy