Cable de-rating for trunking

South Spark · Apr 24, 2017

Just doing a re-wire on a community hall/scout hut. There's a actually a surprising amount of cable gone into a 25 x 10 m one storey building. 6 x 3kw heaters, sockets everywhere, water heaters , LED Panels, downlighters etc etc.

All cable runs are being done in the loft space and due to the shape of the building and the location of the meter all start by running along one wall before heading out along the ceiling joists.

In an effort to tidy this up and prevent damage to cable I've put everything on that side into 100 x 50 mm trunking. As the rest of the install is clipped direct should I be worried about the cable grouping de-rating factors?

Andy-1960 · Apr 24, 2017

i think this is definitely something you should be taking into consideration especially with the number of simultaneous circuits your talking about.

D Skelton · Apr 25, 2017

Should you be worried? No, not in the slightest. Should it be something you take in to account? Absolutely.

It's worth remembering that the appendices in BS 7671 after 1 are only informative. Also worth remembering is the fact that when it comes to the grouping factors shown in appendix 4, these only apply to cables under prolonged and steady-state operation at 100% of load factor.

If you as the designer decide that after taking this into account certain derating factors need to be applied so be it. If not, so be it.

This is a call you and you only can make as none of us here are the ones designing the installation.

All I will say is that nine times out of ten, when grouping factors have been applied, they're normally wrong, and grossly overcalculated.

That being said, my advice should not be taken as carte blanche to sling cables together willy nilly without a care in the world.

Ian1981 · Apr 25, 2017

Grouping factors can be ignored for cables if cables are carrying no more than 30% of it's grouped rated current.
It's worth remembering that it's the amount of circuits which contribute to the grouping calculations not the amount of cables present.
As already pointed out it's your call.

Toneyz · May 13, 2017

What type of cable are you installing singles, T&E ?.

Toneyz · May 14, 2017

I should of read the post properly sorry it is just the run from the distribution board, I think it will depend upon the length to which there will be a detrimental effect on the cables CCC and grouping factor needing to be applied ,however assuming if the cables are run in the loft space they will be in contact with insulation and be de-rated/CCC increased therefore you will need to see which one is the more arduous condition. I would go for 100 x 100 trunking (there would be no reason why you could not fit cable spacers inside the trunking to reduce grouping factor).

HandySparks · May 14, 2017

anthonybragg said:
...there would be no reason why you could not fit cable spacers inside the trunking to reduce grouping factor.

Separating the cables into more than one piece of trunking with an air gap between would improve heat dissipation, but how would fitting spacers in trunking reduce the grouping factor?

Toneyz · May 14, 2017

HandySparks said:
Separating the cables into more than one piece of trunking with an air gap between would improve heat dissipation, but how would fitting spacers in trunking reduce the grouping factor?

As all the cables would not be grouped in all one lump together

Rpa07 · May 14, 2017

Especially if it was a 100x100 room for spacing and a little bit of dancing too! One gets hot when one dances, no different with cables and electrons!