Derating Cables / Grouping Factors / Cable Size

Here's a quick one for you 16th edition guru's:
I need to run 75% of the cables from my consumer unit together in a bundle for approx 1 meter, (as far as I can see I have no other practical physical choice for the routing). I'll need to derate the cables, which will mean I'll need a larger cable for my rings, (I'll do the full calculation later, but suffice to say it's going to be larger than 2.5mm^2)
The question is; is it acceptable to use the larger cable just for the legs from the CU to the first/last socket, (i.e. through the 'grouped' section), and then wire the rest of the ring in 2.5mm^2 ? [after of course checking on other derating issues for the rest of the circuit etc etc etc etc etc]
Mike
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Yes, you can use 4mm for just the deratable bit. However, do the calcs properly. Your lighting cables are likely to be overrated by 2x already, which means they don't count for the calcs for the other cables, either. 2.5mm is likely to be rated for around 27A in the likely installation conditions, whilst only needing a 20A rating for a ring. Find out if this is enough.
Christian.
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