T&E in conduit, grouping, derating questions

[luusac]

Hello again,

I take the point that with a ring final you have two cables, but one circuit.
Does 2.3.1 of 17ed clarify matters at all? Does the note to 2.3.1 help? Does this help http://www.tlc-direct.co.uk/Book/4.3.5.htm (referring specifically to circuits and not cables in their example).

So I will be permitted to run 2x1.5mm T&E + 1x1.5mm 3 core in the same conduit, provided I adhere to the 43% rule? 4D2A references both 2&3 cores under installation method B. Can you clarify which table is being referred to in note 4 of 4C1. It doesn't mean group factor 0.8 for the 2xT&E and factor 1 for the 1x3core does it? Your interpretation of note four is to treat the combination of 2 core and 3 core cables as being just the same type of multicore cable therefore giving a grouping factor of 0.7? All three 1.5mm circuits will be on individual 6A MCBs so well within permitted corrected CCCs I think, but I would like to understand how to apply the correct calculation for other situations.

Where can I find the cross section area of the cables to assist in calculating space/filling requirments of the (oval) conduit?

Thanks again.

Lu

 

[JohnW2]

I take the point that with a ring final you have two cables, but one circuit. Does 2.3.1 of 17ed clarify matters at all? Does the note to 2.3.1 help?

I'm not sure what you are referring to - those are not 17th edition regulation numbers - can you clarify?

Does this help http://www.tlc-direct.co.uk/Book/4.3.5.htm (referring specifically to circuits and not cables in their example).

That relates to the '16th Edition', which became obsolete in 2008. Frankly, as BAS and I have both said, common sense suggests that the two 'legs' of a ring final circuit should probably be considered as 'two cables', not 'one circuit', since both could be carrying substantial currents. However, others may disagree.

So I will be permitted to run 2x1.5mm T&E + 1x1.5mm 3 core in the same conduit, provided I adhere to the 43% rule? 4D2A references both 2&3 cores under installation method B. Can you clarify which table is being referred to in note 4 of 4C1. It doesn't mean group factor 0.8 for the 2xT&E and factor 1 for the 1x3core does it? Your interpretation of note four is to treat the combination of 2 core and 3 core cables as being just the same type of multicore cable therefore giving a grouping factor of 0.7? All three 1.5mm circuits will be on individual 6A MCBs so well within permitted corrected CCCs I think, but I would like to understand how to apply the correct calculation for other situations.

To be frank, I would not personally get too worried about details as regards the grouping of three 1.5mm² cables (where T+E or 3C+E) on 6A circuits (for lighting and alarms) - since there is no way that the derated CCCs are going to be less than 6A. Indeed, by invoking Note 9 of $C1 (as found by BAS!), you can probably 'ignore' the alarm circuit cables for the purpose of grouping calculations!

Where can I find the cross section area of the cables to assist in calculating space/filling requirments of the (oval) conduit?

External sizes of cables vary a bit, but approximate external dimensions are given in the forum's wiki click here , from which you could attempt to calculate/estimate approximate external cross-sectional-areas.

Kind Regards, John

 

[luusac]

I take the point that with a ring final you have two cables, but one circuit. Does 2.3.1 of 17ed clarify matters at all? Does the note to 2.3.1 help?

I'm not sure what you are referring to - those are not 17th edition regulation numbers - can you clarify?

I am pretty sure that this is the 17ed, but not necessarily the most recent amendment. I think the number refers not to a reg number, but to a para in appendix 4 (from memory), see p. 306.

Does this help http://www.tlc-direct.co.uk/Book/4.3.5.htm (referring specifically to circuits and not cables in their example).

That relates to the '16th Edition', which became obsolete in 2008. Frankly, as BAS and I have both said, common sense suggests that the two 'legs' of a ring final circuit should probably be considered as 'two cables', not 'one circuit', since both could be carrying substantial currents. However, others may disagree.

I follow your logic. Does diversity have anything to say on the matter, because both legs won't necessarily be carring the maximum permitted load. Don't misunderstand I am not trying to make the regs fit what I want, but would like to clarify things (I assume that the team who reviewed the regs thought that there wasn't any ambiguity). I guess that is what you refer to re note 9, but you couldn't argue that any part of a ring final would be down to 30%.

[SillyMode=on]The simplest solution seems to me to be fix the conduit in the channel, pull the 2x2.5mm through, then fill the conduit with plaster or cement , surely that would kind-of comply with installation method c! Or even better a vertical cable tray screwed to the surface of the wall, t&e clipped to that, with the socket dangling in mid-air - will give me all the amps I need![/SillyMode]

 

[JohnW2]

I take the point that with a ring final you have two cables, but one circuit. Does 2.3.1 of 17ed clarify matters at all? Does the note to 2.3.1 help?

I'm not sure what you are referring to - those are not 17th edition regulation numbers - can you clarify?

I am pretty sure that this is the 17ed, but not necessarily the most recent amendment. I think the number refers not to a reg number, but to a para in appendix 4 (from memory), see p. 306.

Right. Thanks for clarifying - I didn't realise you were talking about Appendix 4 (not the least because you didn'ttell me ). Yes, I suppose the note to 2.3.1 helps a bit, but not much, since it's so vague. It says that if the cable is less that '100% loaded', the group rating factor may be higher (i.e. less de-rating), but it gives no indication as to 'how much higher' for various degrees of loading, so I'm not really sure how one is meant to make any use of that note! In any event, as I'm about to say, there is potential for the pair of cables, between them, to be loaded to the tune of 32A total, which really represents a pretty high potentail 'loading' for both (e.g. 80% loading for both if both were carrying 16A and '100% loading' were 20A) - so probably not much potential relaxation of de-rating, even if we were told how to work it out!

That relates to the '16th Edition', which became obsolete in 2008. Frankly, as BAS and I have both said, common sense suggests that the two 'legs' of a ring final circuit should probably be considered as 'two cables', not 'one circuit', since both could be carrying substantial currents. However, others may disagree.

I follow your logic. Does diversity have anything to say on the matter, because both legs won't necessarily be carring the maximum permitted load.

That's not really diversity in the normal sense. As I said above, there is potential for both cables to be carrying 16A, which is nearly 80% of the 'full' (not derated) CCC of each of the cables in conduit - which I personally feel is plenty high enough to regard the cables as if they related to different circuits..

Don't misunderstand I am not trying to make the regs fit what I want, but would like to clarify things (I assume that the team who reviewed the regs thought that there wasn't any ambiguity).

That is an oft-questioned concept in these parts, since there are so many aspects of the regs that lack 'total clarity'!

I guess that is what you refer to re note 9, but you couldn't argue that any part of a ring final would be down to 30%.

Sure. As I said, with you alarm circuit cables, I'm sure you would be well below 30%, so you would be able to ignore them for grouping de-rating purposes. However, as you say, one most certainly could not do the same for your ring circuit, since there is potential for both cables to be carrying considerably more than 30%.

[SillyMode=on]The simplest solution seems to me to be fix the conduit in the channel, pull the 2x2.5mm through, then fill the conduit with plaster or cement , surely that would kind-of comply with installation method c! ... [/SillyMode]

In concept, it's not silly at all. If you recall, my very first contribution to this thread was to question the value of your use of conduit. If you simply buried the cables (slightly apart!) and plastered over them (i.e. Installation Method C), you would have no CCC problems to worry about (and the cable would be no more difficult to 'change' than would one in a conduit full of plaster or cement!!)!

Kind Regards, John

 

[luusac]

[SillyMode=on]The simplest solution seems to me to be fix the conduit in the channel, pull the 2x2.5mm through, then fill the conduit with plaster or cement , surely that would kind-of comply with installation method c! ... [/SillyMode]

In concept, it's not silly at all. If you recall, my very first contribution to this thread was to question the value of your use of conduit. If you simply buried the cables (slightly apart!) and plastered over them (i.e. Installation Method C), you would have no CCC problems to worry about (and the cable would be no more difficult to 'change' than would one in a conduit full of plaster or cement!!)!

I just meant that if you are going to fill in your conduit it is easier to clip directly to the wall . It seems easier to use capping, 21.6A after derating! saying that, as the conduit is not circular, and not part of a full conduit system, diagrammatically one might try and argue that no.58 (4A2) is closer to the proposal than no.60, giving the same CCC as capping. Admittedly, cables would be insulated from the wall on the internal (wall) side which they wouldn't be with capping.
Lu

 

[JohnW2]

I just meant that if you are going to fill in your conduit it is easier to clip directly to the wall . It seems easier to use capping, 21.6A after derating!

Isn't that what I've been saying all along?

Kind Regards, John

 

[riveralt]

Isn't this a classic case of coming up with a bad idea - receiving advice - ignoring advice - going full circle and finally accepting that it was a bad idea and yet still not getting the right answer.
Why do you need capping? You wont be able to pull it through - Just clip it direct - as suggested eons ago.
If your mad enough to use buried conduit then use singles otherwise do as above.

 

[JohnW2]

Isn't this a classic case of coming up with a bad idea - receiving advice - ignoring advice - going full circle and finally accepting that it was a bad idea and yet still not getting the right answer. Why do you need capping? You wont be able to pull it through - Just clip it direct - as suggested eons ago. If your mad enough to use buried conduit then use singles otherwise do as above.

I agree totally - and, as you indicate, I said all this right at he start of the thread. However, I do admire the OP's desire (and consdierable efforts) for the work to be fully compliant with the regs, even if I think that his 'plan' is ill-conceived.

One interesting thing (at least, interesting to me) which has emerged out of all this is that appears that to have both legs of a 2.5mm² 32A ring final in the same buried conduit (whether T+E or singles), even for a short distance, is probably not complaint with the regs.

Kind Regards, John

 

[ban-all-sheds]

18-19A, I reckon.

 

[JohnW2]

18-19A, I reckon.

Quite - which, when I last looked, is less than 20A!

Kind Regards, John

 

[luusac]

Isn't this a classic case of coming up with a bad idea - receiving advice - ignoring advice - going full circle and finally accepting that it was a bad idea and yet still not getting the right answer.
Why do you need capping? You wont be able to pull it through - Just clip it direct - as suggested eons ago.
If your mad enough to use buried conduit then use singles otherwise do as above.

Yes, I agree John & BAS were saying that all along. I didn't dispute that. I wasn't saying that I was going to do it that way (clipping), but agreeing that it would indeed solve the problem after my comment about filling the conduit. Having read around it seems some sparks do use 2x2.5T&E in oval, but given the explanations recieved here I am reluctant to do that (otherwise I wouldn't have asked in the first place)! I already explained that it is too late to use singles and that I am probably already looking at within the next 12 months reopening a chase (with all of the hassle, mess, inconvenience etc that will cause), or in some other way laying a new or additional cable as I think that there is a real possibility that the 6mm will be insufficient. So it is not about just the 2.5s....
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.

 

[JohnW2]

...I am probably already looking at within the next 12 months reopening a chase (with all of the hassle, mess, inconvenience etc that will cause), or in some other way laying a new or additional cable as I think that there is a real possibility that the 6mm will be insufficient. So it is not about just the 2.5s....

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!

Kind Regards, John

 

[securespark]

If you think the 6 milli will not be sufficient in years to come, fit 10 milli now.

I did a bathroom for my Mother in 1986. I wired the shower in 10 milli, with a 2 gang sized MK 45A switch and stand-alone RCD. Turned out very handy to be able to replace the shower yesterday with a Mira 10.8kW with no alterations!

On the advice of a Norweb engineer, I had upgraded to 6 milli (and subsequently 10 milli) for the MEB's and done the supplementary.

 

[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.

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

 

[luusac]

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. I am a bit reluctant to say to the spark that I dumped the 6mm for a 10+ and that I hope that it doen't interfere with any other of his specs or calcs. I don't know whether he specified and laid a 6mm for sockets only because of v drop (I haven't looked at this yet to see whether it is relevant in the given circumstances) given the cabling route, grouping and distance from the CU, and had it in mind to lay another cable purely for shower in the future. Another would be that I haven't actually planned in detail the requirements after a loft conversion, e.g. can I get by with a 10mm for shower and sockets and ... etc, will cabling route and distance dictate a larger size etc. The job was fully discussed with the spark, including future requirements, but his reasoning on this, his calcs and specs are is in his head, not mine! I suspect that he had it in mind to lay another dedicated cable purely for a future shower and via a different route, particularily given that 10mm is difficult to work with and won't necessarily lend itself to the route the 6mm has taken.
BTW, the 2x2.5mms were in the conduit before he left site