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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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    broadgage: 
     

    A couple of factors might help.

    What size is the upstream protective device, probably the DNO cut out fuse. If it is 100 amps or less, then obviously the sustained total current on all the cables can not exceed 100 amps.

    A simplified case would be three ring finals each 32 amps. That would give 6 cables each loaded to 16 amps, or allowing for some imbalance, 3 cables loaded to 20 amps and another 3 each carrying 12 amps. If the three ring finals are fully loaded, then ALL the other cables can be ignored as they cant be carrying more than 4 amps, sustained in total.

    Very lightly loaded cables can be ignored. 

    Ah I see, you're saying that based on the design calculation with diversity, there's likely only 6 cables being heavily loaded at one time, so the grouping factor for 6 cables should be used??

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  • 0

    broadgage: 
     

    A couple of factors might help.

    What size is the upstream protective device, probably the DNO cut out fuse. If it is 100 amps or less, then obviously the sustained total current on all the cables can not exceed 100 amps.

    A simplified case would be three ring finals each 32 amps. That would give 6 cables each loaded to 16 amps, or allowing for some imbalance, 3 cables loaded to 20 amps and another 3 each carrying 12 amps. If the three ring finals are fully loaded, then ALL the other cables can be ignored as they cant be carrying more than 4 amps, sustained in total.

    Very lightly loaded cables can be ignored. 

    Ah I see, you're saying that based on the design calculation with diversity, there's likely only 6 cables being heavily loaded at one time, so the grouping factor for 6 cables should be used??

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