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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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    You can ignore lightly loaded cables (such as lighting circuits) - see Note 9 of Table 4C1.

    This grouping is all very well, but suppose that a ring circuit is in a ceiling void, or even surface mounted above doors and drops down and back up again to each socket. Is that one circuit or two? Of course it is one circuit, but there could be 20 A going up and down and there seems to be a good argument for applying the grouping factor for 2 circuits.

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    You can ignore lightly loaded cables (such as lighting circuits) - see Note 9 of Table 4C1.

    This grouping is all very well, but suppose that a ring circuit is in a ceiling void, or even surface mounted above doors and drops down and back up again to each socket. Is that one circuit or two? Of course it is one circuit, but there could be 20 A going up and down and there seems to be a good argument for applying the grouping factor for 2 circuits.

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