Hi,
I'm using twin and earth cables for a office building refurb. In the BS 7671 regs i can only see cable current carrying capacities for these cables in thermal ceilings and walls, this will not be the structure for the office building internally. How can i size the twin and earth cables? I.e are there alternative tables? I hope you can answer soon.
Regards
I'm a consultant so wasn't show these days how the cables will be installed in the walls
Surely it's a requirement of the design (to BS 7671) to know this ... or specify it ... otherwise it may change the Installation Method - for example, if you have thermal insulation in walls, and cables are simply dropped down, can you tell whether the cable will be touching the wall or not?
Some installation methods require care, especially where T&E or similar is used - for example, can T&E be run in Basket without damage (yes, but it depends how it's run) ... and the fact that tray and basket are the same product standard ...
Hi, the cables will not be installed in thermal insulation, i already stated this. When we specify cables, we tend to errr on the side of caution.
It was an example ... there are others. Most important, though, is that the installation needs to be validated against the design, so you can't really do the design unless you know - or specify - the installation method(s).
In addition, as I said, some installation methods are unsuitable for certain cables: either completely unsuitable (like installing T&E in ducts that cannot be guaranteed to be free of water) or without special provisions (like installing T&E - or data cable for that matter - in basket, where it will deform over time unless cable mats are used).
the cables will not be installed in thermal insulation
Having said what I've said, it is possible to do parts of the design without knowing the installation methods - but you'd need to account for worst-case.
That is how the csa for the range of installation methods is arrived at in the OSG ... but it effectively specifies the installation method(s) the cable can be used with. Which effectively aligns with the statement I make 'you can't really do the design unless you know - or specify - the installation method(s)'.
completely unsuitable (like installing T&E in ducts that cannot be guaranteed to be free of water) or without special provisions (like installing T&E - or data cable for that matter - in basket, where it will deform over time unless cable mats are used).
Oh dear, someone should have told the M and E consultants in a major hospital refurb about that. A trillion miles of data cable in basket! I note many office refurbs being done the same way with many using T/E in basket above the suspended ceiling. Far better trade-off than a spaghetti mess of cables lying over the suspended ceilings!
It all rather depends who is the design authority signing on the dotted line that it will be OK.
I presume the original poster is not the main consultant, as it seems they are not in charge of the design - the snippy answer of course is to go and ask the design authority - after all this is what they are paid for, but it maybe that in this case the person 'in the hot seat' is not that clued up on electrics.
Someone will be deciding he construction of the partition walls - and if for example those partitions might be have sharp metal inserts that may damage T and E, then the correct cable and or routing method needs to be defined. You need to find that person, or to be that person dictating the spec. (this sort of thing is common but needs great care - pulling wires in with abandon can strip it to the copper and create a live wall .. )
In un-insulated partitions the cable would be pretty much in free air, so I'd use the clip direct figures perhaps less 10% for luck (but note that indoor meeting rooms often have insulation in the walls, not for heat but to provide acoustic privacy - so unfilled partitions are not always a certainty.)
Unrelated but I think I have seen plenty of TnE in baskets without a mat , and the only instance of deformation damage I can think of is when there was a huge bundle of it so the compression force was from the weight of many cables above, but maybe avoid the cheapest basket with really thin wires widely spaced.
Mike.
Electrical installation methods have moved on now, sometimes driven my speed and cost.
Do those cable installation methods comply with the manufacturer's intentions?
BS 7671 is intended to be a standard, not a manual.
In fact, BS 7671 does give a consultant enough information to complete the design, but you will need to consider the requirements of other (referenced) standards, rather than looking up the data in a table in BS 7671.
Unrelated but I think I have seen plenty of TnE in baskets without a mat , and the only instance of deformation damage I can think of is when there was a huge bundle of it so the compression force was from the weight of many cables above, but maybe avoid the cheapest basket with really thin wires widely spaced.
Sadly, I;'ve seen the worst of this, as well as the option of keeping the T&S close to a horizontal part of the basket ... although I still couldn't assume the T&E would be OK even in the latter case, because the diameter horizontal wire of the basket is less than the width of the T&E.
All I can say, is that the cable is not likely to be designed for that purpose, and may be subject to mechanical damage.
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