Trunking on consumer unit group fating factor

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. 

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.

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.

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.

Yes it's a 100A distributors fuse. It's a domestic rewire. The circuits are SPD, 9.5kW shower, 8.5kW shower, hob / oven, kitchen ring, down ring, up ring, down lights, up lights, boiler, smokes.

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??

Yes, I would ignore the two lights, boiler, and smokes. As Mike has suggested, the showers, cooker,  and rings are unlikely to be loaded for a significant period all at the same time, but what view you take on that requires “engineering judgment”.

Question.. Does reference method B apply to multicore cables?

What is your starting point for Current carrying capacity when the only offerings for Reference method B in 2 cables Single phase or 3-4 cables three phase?