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Topic Title: Sizing containment
Topic Summary:
Created On: 08 January 2015 08:18 AM
Status: Read Only
Linear : Threading : Single : Branch
 Sizing containment   - adammid - 08 January 2015 08:18 AM  
 Sizing containment   - nad - 08 January 2015 04:41 PM  
 Sizing containment   - AJJewsbury - 08 January 2015 05:05 PM  
 Sizing containment   - adammid - 08 January 2015 08:44 PM  
 Sizing containment   - nad - 08 January 2015 08:55 PM  
 Sizing containment   - cookers - 08 January 2015 08:56 PM  
 Sizing containment   - MrP - 09 January 2015 02:03 AM  
 Sizing containment   - Parsley - 09 January 2015 08:17 AM  
 Sizing containment   - adammid - 09 January 2015 08:32 AM  
 Sizing containment   - HarryJMacdonald - 11 January 2015 11:07 PM  
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 08 January 2015 08:18 AM
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adammid

Posts: 152
Joined: 02 November 2010

Please can anyone advice on how to size containment ie basket and tray utilising twin and earth cabling.

I have looked in the on site guide and it only refers to calculations for single core cables in conduit and trunking.

I assume that this is due because if you know the diameter of a conductor then you can ascertain how much area it covers in theory. My issue is that in relaity cables are not necessarily butted together and voids will be made between cables.

Can anyone also help with sizing containment ie trunking with utilises twin and earth cabling.
 08 January 2015 04:41 PM
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nad

Posts: 399
Joined: 14 January 2005

I've never found a quick way of sizing cable basket (except the common-sense combined with experience, best guess). Start by checking the containment manufacturer's instructions or call their technical department. With tray and basket you will find loading graphs and you will have to find out from the cable manufacture the mass of the cable per meter. With cable tray you can find out the external diameter of each cable (again from the cable manufacturer), sum them and reference that against the internal width of the tray. Remember to apply grouping factors accordingly when cable sizing.

-------------------------
Nad

*Regularly edited due to spell cheque misdiagnosis
 08 January 2015 05:05 PM
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AJJewsbury

Posts: 16117
Joined: 13 August 2003

It's also a bit unusual to have T&E on basket/tray. T&E tends to be used in domestics and similar where fault currents are relatively low, whereas basket/tray tends to be more industrial or large commercial where fault currents can be higher. The reduced c.s.a. c.p.c. of T&E (other than 1.0mm2 of course) can be problematic with higher fault currents & MCBs (see table A5 in GN3 for example). I'm sure it happens here and there, but maybe not the sort of thing the IET would want to be seen to be encouraging.
- Andy.
 08 January 2015 08:44 PM
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adammid

Posts: 152
Joined: 02 November 2010

I come across T&E installed on basket/ tray in many commercial installations, even new build supermarkets. T&E is cheaper than yy cabling and quicker to install than single core cables.
 08 January 2015 08:55 PM
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nad

Posts: 399
Joined: 14 January 2005

I worked on many Building Schools of the Future (BSF) projects where that was the installation method, I'm ashamed to say. And that was pre-credit-crunch. For the record I would have preferred steel tube.

-------------------------
Nad

*Regularly edited due to spell cheque misdiagnosis
 08 January 2015 08:56 PM
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cookers

Posts: 213
Joined: 10 February 2012

My experience is that de-rating cables by grouping factors and following the tray manufacturers advice will keep you out of trouble in most situations, however if the cables are fully loaded you can still get issues.

In simple terms a lot depends on how loaded the cables will be. Cables get hot when fully loaded, and put a lot of them together then things can get quite alarmingly problematic.

Solution is to space them out a bit, have a bigger tray, or de-rate the cables even more than the grouping factors mentioned above.
 09 January 2015 02:03 AM
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MrP

Posts: 955
Joined: 24 March 2006

Used t&e on basket lots of times on schools and commercials as long as you take a realistic approach and don't pack them in no problem afford as much space as possible if your installation had been designed for trunking tube and singles and it works I would think that t&e on basket would be less onerous.
they do an accessorie clip for the basket that will accept clix roses and wriggley conduit for your drops.
don't use trunking for your t&e install.
I remember subbing for Mitie on a city center hotel new build labour only where they used 6x6 metallic trunking with t&e with little or no design initiative seemed like a good idea before the cables were introduced sad to say all the lids had to be tie rapped on
a typical Mitie install
are they still around squeezing the last penny out of the subbie

Good luck buddy hope it goes well

MrP
 09 January 2015 08:17 AM
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Parsley

Posts: 1353
Joined: 04 November 2004

T&E basket/trunking is a common value engineering exercise that sometimes isn't properly though out.

Reduced CPCs, possible high fault currents (depending on supply characteristics & location from the tx) C type breakers, long final circuits resulting in high touch voltages.

But don't worry it's all protected by RCDs.

Regards
 09 January 2015 08:32 AM
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adammid

Posts: 152
Joined: 02 November 2010

Many thanks for your input.

Does anyone have any recommendations on how to size the containment for T&E though. Thats whats I am after really.
 11 January 2015 11:07 PM
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HarryJMacdonald

Posts: 409
Joined: 15 May 2002

When I had to do this for a large number of round multicore cables in trunking I assumed each one was square and added up the cross sectional area, i.e. if the diameter was 20mm, I added 400mmsq to the cross section required. You could do the same for T&E, taking the larger dimension and squaring it. That should leave enough for twists etc.
That still leaves the group derating factor to consider, but you know the likely circuit loadings.
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