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Trunking on consumer unit group fating factor

Jimi

Hi, I have 10 circuits (14 multicore cables) coming out of the consumer unit into about 500mm of 50x50mm plastic trunking. This is reference method B which beneath says group rating factor in table 4C1 needs to be applied. This gives a factor of 0.41!!! Is this right? The rest of the install is either C (clipped direct), B (oval conduit in plaster or in a void), 100 (plasterboard ceiling), 102 (stud wall), or E (free air though joists). 

The cables are all back entry by spacing the consumer unit off the wall 20mm with metal bushes. Trunking looks neat and helps with the consumer unit IP rating and mechanical protection requirements but might be better taking the trunking away and spreading the cables out a bit through free air (more than 0.3 diameter of cable) so no rating factor?

I haven't done the calculations yet but I'm thinking the 0.41 group rating factor will make the CCC of some / most of my cables too small / less than In (protective device rating).

Any help appreciated, thanks. 

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    Further to that , it  is probably worth remembering that de-rating factors for grouping are really about dissipating heat, and how a cable surrounded by hot neighbours cannot sweat off anything like as much as a single cable rattling about on its own in the trunking.

    As above, the factors assume that all the cables are pretty much in full load at the same time, and while that is common in a factory, it may well be impossible, or at least very rare in a house with  a normal pattern of life, so the full load  de-rating need not always apply

    If this is an installation that has been in use for a few years, you could just look at a cable in the middle of the bundle and see if it looks distressed (softened insulation distorts and can become brittle and cracked), or indeed if it is hot to the touch. If it looks fine, then the diversity saves it. 

    In the day job I have sometimes used temperature indicating stickers that irreversibly change colour if they exceed some upper temperature, and you can leave these embedded in the installation and come back next year and say ‘ ah well, it never got above 60’ or whatever. (or of course it is burnt to a crisp and the hottets high tide marker is indicating, and then the recommendation is to split the wiring over 2 lots of tray or replace the hottest sections with thicker cable or something.) 

    The de-rating factor is right, but only if the load profile assumed is also right !!!

    Mike.

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  • 0

    Further to that , it  is probably worth remembering that de-rating factors for grouping are really about dissipating heat, and how a cable surrounded by hot neighbours cannot sweat off anything like as much as a single cable rattling about on its own in the trunking.

    As above, the factors assume that all the cables are pretty much in full load at the same time, and while that is common in a factory, it may well be impossible, or at least very rare in a house with  a normal pattern of life, so the full load  de-rating need not always apply

    If this is an installation that has been in use for a few years, you could just look at a cable in the middle of the bundle and see if it looks distressed (softened insulation distorts and can become brittle and cracked), or indeed if it is hot to the touch. If it looks fine, then the diversity saves it. 

    In the day job I have sometimes used temperature indicating stickers that irreversibly change colour if they exceed some upper temperature, and you can leave these embedded in the installation and come back next year and say ‘ ah well, it never got above 60’ or whatever. (or of course it is burnt to a crisp and the hottets high tide marker is indicating, and then the recommendation is to split the wiring over 2 lots of tray or replace the hottest sections with thicker cable or something.) 

    The de-rating factor is right, but only if the load profile assumed is also right !!!

    Mike.

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