Grouping Factors

[jdmjrmdam]

I'm I correct in thinking that when calculating the current carrying capacity of a cable the grouping that can occur as multiple final circuits enter a CU can be ignored?

If that is true what is a reasonable distance the cables could remain grouped outside the CU before the cables would have to be de-rated by a grouping factor?

 

[revjames]

I looked into this when doing a design course recently. I think that the final runs into a CU can be overlooked but there is a limit on how long that run can be. Not very helpful to not find the reg I know...

 

[netblindpaul]

rev,

Knowing which course you are on about, and having done the same one, we did not consider grouping at the entry to the CU either!

Mind if you look into grouping, you will find that circuits whose loading is less than x% of max can be ignored for grouping.

e.g. a 1.5 mm radial on a 6A mcb feeding 2 100W outside lights could probably be ignored!

Extrapolate this and there are significant savings to be made unless you do the right thing and allow for "load creep".

 

[revjames]

Paul

Quite right, it would be impossible to do if you considered the grouping at every CU and would end up with massive cables! In this case I wondered where the reg was that mentioned the grouping where the runs went to the CU

James

 

[Lurch]

Pretty sure I mentioned something about this on revs thread, can;t remember what, sure Patch will pop a link in before long.

 

[jdmjrmdam]

Thanks folks, I'm struggling to find any reference to this in the BRB, OSG or any other publications.

 

[ian3012]

not entirely relevant, but when considering derating factors for cable runs through thermal insulation... i remember the brb always mentioning runs of over 0.5M... just a thought.. :Blushing