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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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    in that sense (and not for the first time) the regs trip them selves up by defining circuits in a funny way. (the other is a ring final feeding a fused spur or MCB a final circuit, and if it is what is the bit beyond the fuse or MCB.)

    The real thing of course for grouping is ‘cables dissipating some power’ arguing the toss about which fuse supplies them is really neither here nor there.

    Personally  I do not like to see the two bits of T and E to a socket to share oval conduit or capping except near the very centre of the ring, but I have seen many examples that have ignored this completely and seen no issue and so I am probably being a fuss-pot. The real safety feature is the time constant - in at least one case I have seen a 2.5mm  T and E can supply 40A long enough for at least one family member to have a shower, with a cool down period before the next one, and that overload pattern can repeat every day for years, with no symptoms of trouble, until the pattern of life changes, and one day the whole family take showers one after the other... 

    In the same way parts of the ring appearing overloaded for the time it takes to boil a kettle and pop out a couple of slices of toast will not be an issue either, because the load is off again long before things have reached maximum temperature. Long duration loads like heating big tanks of water or charging large batteries are far more testing than short duration high loads.

    Mike.

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  • 0

    in that sense (and not for the first time) the regs trip them selves up by defining circuits in a funny way. (the other is a ring final feeding a fused spur or MCB a final circuit, and if it is what is the bit beyond the fuse or MCB.)

    The real thing of course for grouping is ‘cables dissipating some power’ arguing the toss about which fuse supplies them is really neither here nor there.

    Personally  I do not like to see the two bits of T and E to a socket to share oval conduit or capping except near the very centre of the ring, but I have seen many examples that have ignored this completely and seen no issue and so I am probably being a fuss-pot. The real safety feature is the time constant - in at least one case I have seen a 2.5mm  T and E can supply 40A long enough for at least one family member to have a shower, with a cool down period before the next one, and that overload pattern can repeat every day for years, with no symptoms of trouble, until the pattern of life changes, and one day the whole family take showers one after the other... 

    In the same way parts of the ring appearing overloaded for the time it takes to boil a kettle and pop out a couple of slices of toast will not be an issue either, because the load is off again long before things have reached maximum temperature. Long duration loads like heating big tanks of water or charging large batteries are far more testing than short duration high loads.

    Mike.

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