grouping factor question

[thekingiam]

i was thinking of the installation i have nearly completed and i was thinking about the grouping factor issue, the cables are grouped in pvc trunking leaving the consumer unit and travel about 4 meters before entering ceiling before dispersing into seperate circuits.would a correction factor be needed?
it says in a book i have that no correction factor need to be aplied if entering cu as vairying size i won't mention the authors name

 

[Farmelectrics]

I would of thought there would be a grouping facto calc but not 100% as I'm awaiting to start design course my self

 

[thekingiam]

thanks for response hopefully someone knows.

 

[telectrix]

if in trunking, with plenty of space, then forget grouping factors.

 

[Jimmy Boy]

What are the grouped circuits supplying
J

 

[johnboy6083]

If any of these circuits are carrying less than 30% of the cables rating (taking account of derating for temp, insulation ect), then they can be ignored for grouping.
it's down to your judgment really. As long as you can justify it, backed up with best practice ect.
Another relevant point is that if you have to apply derating factors, for environmental conditions, but they apply at different parts of the circuit, the. Only the. most severe derating may apply.

 

[Sparky Ninja]

It says beneath table 4C1 "These factors are applicable to uniform groups of cable, equally loaded". - Which would suggest the theory you have found to be true.

It also says "If, due to known operating conditions, a cable is expected to carry not more than 30% of its grouped rating, it may be ignored for the purpose of obtaining the rating factor for the rest of the group" - which kinda contradicts the first point as it is suggesting different loads within the grouping..

Interpretation is a wonderful thing. I will flick through some of my books to see what examples are given of this to see if it helps.. personally, I would take the same opinion as Tel.

 

[Jimmy Boy]

Above is why I was asking what they were supplying ..tables tend to assume heating circuits are not grouped.
J

 

[thekingiam]

lighting and sockets.
anyways thanks for the response and for once for me a good one.......happy days! and a good nights sleep tanks

- - - Updated - - -

by the way the book was brian scaddans fifth eddition.

 

[Silly Sausage]

I will flick through some of my books to see what examples are given of this to see if it helps.. personally, I would take the same opinion as Tel.

I seen this on I&Ts in hotels, DBs feeding 20 odd bedrooms with bundles of T&E tie wrapped together spewing out from the DB, but within a few metres going their separate ways. Lightly loaded I should imagine.

I/We await your verdict widdler.

 

[johnboy6083]

I think that if the cables run through insulation further on in the circuit ect, and have been derate for that, but there is no insulation where the cables are bunched near the DB, then You can ignore the grouping. Obviously this can't be a blanket rule, and you must use common sense and your experience. What is the cables current carrying capacity, compared with the MCB size? This can be another tool to help you come to a decision on whether it's acceptable.

 

[kingeri]

Grouping can often be disregarded in domestic, as above, use common sense.