T&E in conduit, grouping, derating questions

[luusac]

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

 

[PrenticeBoyofDerry]

It's highly unlikely that you will get that number of T&E cables within a 20mm or even 25mm conduit.

 

[luusac]

Hi,
I didn't mean 4 or 5 cables within the one conduit, more likely two, but that is a physical constraint that I can see myself whether they will fit or not - my question relates to the grouping / derating / heat considerations and the wiring regs. As the chases are not filled I can still do something about the number of cables and placement of them in one or more conduit. My main concern at the moment is the 2.5mm T&E drops and the 6mm T&E/2x1.5mm T&E/1.5mm 3 core drop, as I want to get those chases filled.
Thanks,
Lu

 

[ban-all-sheds]

Shouldn't you have copies of the Wiring Regulations, and GN1?

 

[JohnW2]

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: ...

I presume that you're talking about plastic conduit, in which case you might want to consider whether you need it at all - the cables could simply be clipped within the chases and then plastered over.

You may perhaps be thinking that use of conduit might facilitate replacing cables at some point in the distant future. However, even with one T+E cable within a conduit, that's probably wishful thinking, and if you managed to get more than one T+E into the conduit, then your chances of ever being able to 'replace' the cables would, IMO, be very small.

Kind Regards, John

 

[luusac]

Yes, plastic conduit. I know what you are saying, but already existing conduit has saved several chasings when extending circuits and replacing. I don't inderstand the wishful thinking bit because I would think by looking that I could possibly get 4x2.5mm T&E in a 25mm oval conduit, but again, I am not wanting to stuff the conduits full, but rather be mindful of the constraints in the regs. I can't be reopening a chase in a years time when converting the loft if it is decided that what is installed now is insufficient. As to why not plan correctly in the first place ... I need flexibility to allow for change. E.g if it is decided a shower is needed in a future conversion the 6mm T&E won't be sufficient.

 

[JohnW2]

Yes, plastic conduit. I know what you are saying, but already existing conduit has saved several chasings when extending circuits and replacing. I don't inderstand the wishful thinking bit because I would think by looking that I could possibly get 4x2.5mm T&E in a 25mm oval conduit, but again, I am not wanting to stuff the conduits full, but rather be mindful of the constraints in the regs. I can't be reopening a chase in a years time when converting the loft if it is decided that what is installed now is insufficient. As to why not plan correctly in the first place ... I need flexibility to allow for change. E.g if it is decided a shower is needed in a future conversion the 6mm T&E won't be sufficient.

As I implied before, I understand your intent, but it sounds as if you have been much more fortunate than myself and others. If you do go down the conduit route, I can but wish you luck if/when you attempt to make any changes in the future, particularly with conduits containing more than one T+E cable. T+E cable is far from the ideal for use in conduit.

Kind Regards, John

 

[luusac]

Thanks John. Yes, it is one thing to pull T&E through loose conduit, but quite another to pull it through fixed and buried conduit. It is now too late for me to go for singles though ...

Can anybody help with the derating/grouping question?

 

[ban-all-sheds]

You'll find the answer in Appendix 4 of BS 7671.

I'm not trying to be funny - if YOU have decided to take it on yourself to do electrical design work, then it is YOU who has to actually do it, because it will be YOU who has to sign the EIC for design, certifying that YOU exercised reasonable skill and care and that to the best of YOUR knowledge and belief the work for which YOU were responsible complied with BS 7671.

Nobody here can sign the EIC for you, so nobody here can (should) do the design for you. If they do, how can you truthfully (and safely) sign the EIC?

 

[JohnW2]

Thanks John. Yes, it is one thing to pull T&E through loose conduit, but quite another to pull it through fixed and buried conduit. It is now too late for me to go for singles though ...

Fair enough. As I said, good luck if you ever wantto change any of the T+E cables or pull any additionalones through - I suspect that you might need it.

Can anybody help with the derating/grouping question?

I'll try ...

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,

For a ring final circuit, the current carrying capacity (CCC) must be at least 20A. In conduit in a wall (Ref Method B), the CCC of a single 2.5mm² cable is 23A (Table 4D2A of regs). The problem I have (hopefully others can help) is in interpreting Table 4C1 of the regs. It gives derating factors according to "the number of circuits or multicore cables". For a single cable, this factor is obvioulsy just 1.0 (i.e. no derating), for 2 "circuits or multicore cables" is 0.80, which would reduce the CCC of your cable to an unacceptable 18.4A. What I'm not sure of is whether having two cables from the same ring final circuit in the conduit constitutes one or two "circuits or multicore cables". Given that both cables could theoretically be carrying substantial currents, common sense tends to suggest that it should count as "two", but it's also easy to interpret it as "one circuit". The difference is clearly important to you, since it makes the difference between your proposal being acceptable and not, so hope others can offer their opinions.

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?

6mm² in its own conduit would obviously not have a de-rating factor. It would have a CCC of 38A if buried in conduit.
1.5mm² cable buried in conduit has a CCC of 16.5A. Three of those grouped has a derating factor of 0.7, reducing that to 11.55A. Asuming that this will all be on circuits with 6A MCBs, that's obviously fine.

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.

Again, a single 1.5mm² cable would have a CCC of 16.5A - so, as above, 3 of those in a conduit would be fine with 6A circuits (MCBs) (CCC 11.55A). If you somehow managed to get all 5 into one conduit, the derating facor would be 0.6, reducing the CCC to 9.9A, again fine for '6A circuits).

Hope that helps.

Kind Regards, John

 

[JohnW2]

You'll find the answer in Appendix 4 of BS 7671. ... I'm not trying to be funny - if YOU have decided to take it on yourself to do electrical design work, then it is YOU who has to actually do it, because it will be YOU who has to sign the EIC for design, certifying that YOU exercised reasonable skill and care and that to the best of YOUR knowledge and belief the work for which YOU were responsible complied with BS 7671.]

All true, but I would suggest that "exercising reasonable skill and care" can include seeking the advice of others as well as reading regulations oneself.

Nobody here can sign the EIC for you, so nobody here can (should) do the design for you. If they do, how can you truthfully (and safely) sign the EIC?

Well, as you will have seen, whether I "should have" or not, I've tried to help the OP. But I've also indicated what Tables in Appendix 4 I've based my comments on, so he can check for himself before signing anything.

As you'll have seen, there's one issue about which I'm uncertain - what is your view?

Kind Regards, John

 

[ban-all-sheds]

All true, but I would suggest that "exercising reasonable skill and care" can include seeking the advice of others as well as reading regulations oneself.

Indeed, but I'm not conviced that he is doing, or can do, the "as well as", and if he isn't/can't then certification becomes a bit of a snag.

But I've also indicated what Tables in Appendix 4 I've based my comments on, so he can check for himself before signing anything.

Ditto concerns about checking.

As you'll have seen, there's one issue about which I'm uncertain - what is your view?

My view is that if you think about the engineering reasons for having grouping factors in the first place, two cables forming part of a ring need the same consideration as two cables from different circuits.

 

[JohnW2]

All true, but I would suggest that "exercising reasonable skill and care" can include seeking the advice of others as well as reading regulations oneself.

Indeed, but I'm not conviced that he is doing, or can do, the "as well as", and if he isn't/can't then certification becomes a bit of a snag.

You may be right but, as I said, I've given him all the information he needs to look up and check the figures for himself. If he chooses not to then, as you imply, that is "his problem".

As you'll have seen, there's one issue about which I'm uncertain - what is your view?

My view is that if you think about the engineering reasons for having grouping factors in the first place, two cables forming part of a ring need the same consideration as two cables from different circuits.

As you will have seen, that was my inclinination, but it does rather beg the question as to when the 'number of circuits' and 'number of multicore cables' would not be the same thing. As for the 'engineering reasons', it obviously gets a bit more complicated with a ring final which, as we are agreed, is a rather odd animal. If one assumes (per circuit design) that the total current in the two cables will not exceed 32A, then if the current in one was 18.4A, the current in the other cannot exceed 13.6A - rather different from two separate circuits with potentially 18.4A in both, so I suppose one could try arguing that the same derating factor as for 'separate circuits' was not necessarily appropriate. This must have been an issue which has come up many times before, so I wonder what other think.

Kind Regards, John

 

[luusac]

Dear John,
Thanks for the figures and explanations, very helpful. I will have a look at the regs and tables you mention.
Dear ban-all-sheds I appreciate the concern and the issues raised. The history of the job is that a spark with trade association membership (by which I mean membership of a recognised body and the ability to issue certs, covering eg 'part p') designed the installation, specified the mcbs, laid the cable etc. I said at the time that I wanted to use conduit/capping for various reasons. At no time was using singles/grouping/derating discussed. The professional has now gone on an extended holiday with family, leaving me to make good. On his return he will inspect&test. But something I came across gave me concern re the grouping & derating in conduit and I can't get in touch with him. In the meantime I do need to make good on the chases and decorate.....

If I had the time again I would have asked him whether a different strategy would be suitable, e.g. a second CU in the loft to feed all of the upstairs lights (inc loft lights) and loft radial which would be fed by a single T&E cable e.g. 10mm+, but it is too late for that now.

Thanks,

Lu

 

[JohnW2]

Dear John, Thanks for the figures and explanations, very helpful. I will have a look at the regs and tables you mention. ... But something I came across gave me concern re the grouping & derating in conduit and I can't get in touch with him. In the meantime I do need to make good on the chases and decorate.... Lu

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.

Kind Regards, John