grouping factors in metal trunking

[sparkyork]

hi everyone got a querry regarding above.

through approx 1.5m om the installation all the main cables run through a 4" x 4" metal trunkning, was wondering if anynone is knowledgable at working the grouping factors out??

got a lot of pvc/pvc running though

cheeers

 

[Spudmiester]

Dont know, but have found lots of 2.5 t&e in 2 inch trunking before and it was a tad warm. It was running rows discharge lights in a warehouse, however it was in trunking all the way, and was proper full !

 

[spark-doctor]

Let me know how many cables and what csa and i will tell you how its done

 

[sparkyork]

hey doctor, theres gonna be at least 35, ive looked in the regs and got scared as when i look at 20 cables its got a derating factor of .38 or something,

do you want me to add the total CSA of all the line condunctors??

 

[spark-doctor]

What you will need to know is how many circuits you have, not the amount of cables.

You can discount circuits that are carrying less than 30% of its grouped rated factor.

And the load on each cable.

I must say, 35 circuits in a piece of 4x4 seems quite a lot and you may get problems with the cables in the center as they will not be able to dissipate the heat as well as the cables on the outside.

sorry, forget the csa, i was thinking of something else

 

[benji]

total up &size all cables as best you can ,apply what ever factors needed ,download from trunking manufacturer capacities of ,you will then have the recomended trunking size.My advice then is to get the next size up because you'll need it.
benji

 

[sparkyork]

right will get on it, 4 by 4 is huge tho i aint gonna be anything like near fillling the trunking

 

[spark-doctor]

are you doing it in singles?

 

[sparkyork]

will try and post a schedule of circuits

any help appreciated, alot of the above circuit will be operating at no where near there max limits, does this mean some of them can be ommited from any calcs?

 

[jibspark]

without looking anything up i seem to remember as a rule of thumb you just dont fill it over 55%

 

[sparkyork]

that should be fine then i thought it was something like that but was also aware of grouping factors. dont think it will be anywhere near 55% also i might make something that seperates the trunking into 2 compartments to aid heat disipation? any one think thats a good idea?

 

[tony.towa]

Having made a couple of rough calculations in theory you should just about be ok. You are running quite close to your 45% space factor when you take the insulation size into account. If you can seperate the cables it will make heat disapation better as you have some potentially fairly hefty loads passing through (as a total).

Instead of creating two seperate compartments in the trunking think about some form of hanger so that some of the cables can be "suspended" within the trunking, allowing better air movement around the cables and not creating such a large "core" of cables.

Hope it helps.

Tony

 

[sparkyork]

cheers tony your a star! ill get you a pint in when we all meet up somewhere!

yeah i could drill some 6mm holes in the sides and drop some sort of hanger down im sure, good idea. the trunking is only 2m max in length then the cables seperate and some go upstairs and the rest go through the flooring to the various other parts of the building, like you say im prettly much at the limits of everything really. including the main supply!! only got 45kva

 

[spark-doctor]

Sorry i did not reply Rich. I snagged a trip to the Italian GP. have just got back in.

As Tony says, you are just in on the cable spacing factor of 45% and are way under for the cable factor.

If you PM me a Email address i will send you a word doc the has all the calcs. Very easy to understand.