Discuss Cable tray installation in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

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spud1

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Hi there,

Im carrying out a commercial tray installation soon and need some guidance as to cable sizing and grouping;

When I refer to table 7.1 in the onsite guide (page 44) it indicates 'allowed installation' in terms of reference method for a given circuit type/cable size/overcurrent protective device type and size.
If I want to install a ring main circuit protected by a 32A Type B or C RCBO wired in 2.5mm/1.5mm PVC/PVC it does not state ref method E (in free air or on perforated cable tray) as an allowed installation method ?? Am I being stupid..

Also where can I gain guidance for cable spacings and grouping for my circuits on this tray installation so, more than anything, I can select the correct width tray?

Thanks again compadraes !:cool:
 
For cables spacings and groupings see the 17th Edition (Full version) - Appendix 4.

The use of T&E on trays is not usual, hence not included in the list. In which case you would have to spend soom time looking at derating of cable for groupings and maybe compare how installing on a tray differs from clipped direct.

Can't you use other cable types?
 
Steel cable tray will generally and rapidly assume ambient temperature whatever that may be. Its thermal conductivity means it readily soaks heat away from cables fixed to it. That is generally a good thing but... The flip side of that (and something to bear in mind in industrail/process situations particularly) is that it will also effectively "import" heat if one leg of it branches off into a hot area. The heat travels back up the tray thereby having implications on cables fixed to it in an otherwise notionally "cool" area!

I never group power cables on tray any more than in trefoil form. Once you start stacking 4 or more together you rapidly reduce the amount of cable surface area available to shed heat.
 
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For cables spacings and groupings see the 17th Edition (Full version) - Appendix 4.

The use of T&E on trays is not usual, hence not included in the list.

Not usual in industrial installations but not unusual in commercial builds in my experience. In a factory there is a much elevated risk of mechanical damage hence the reluctance to deploy unarmoured/reinforced cable types. I have though used lidded tray in the past and also improvised the same when needed by inverting a length and using it as a lid.

In which case you would have to spend soom time looking at derating of cable for groupings and maybe compare how installing on a tray differs from clipped direct.

Can't you use other cable types?

Singles in conduit perhaps? steel or plastic. Tray work usually takes a bit more bracket work and good route finding if it's of any size but if it's just to carry a pair of twin 'n skins then 3" light duty would do and you can form that stuff fairly quickly using just tin snips and your knee. Then along comes a data man and straps a shed load of fibres and Cat5 to it. "Ta very much Sparks"
 
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