rating of ring main cable?

Need to pass the two 2.5mm^2 cables of a ring main through a 25mm hole in a 90mm timber joist.
The installation methods in IEE regs (table 4A) don't mention timber.
What reference method does this count as?
Are the cables counted as touching or bunched?
What is concerning me is that if method 4 (enclosed in conduit in thermally insulated wall) applies, then the rating falls to 16A (ie below the 20A limit for a ring).
It would be possible to insert a separator (eg a piece of plastic trunking) to keep the cables about 1.2x De apart, but would that improve things?
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please tell me this is a wind up
if it's not - then every electrician in the country should be locked up for reckless endangerment
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    yorks_dales snipped-for-privacy@hotmail.com (the yorkshire dalesman) writes:

I don't have the regs on me, but ISTR it says cables have to pass through half a metre of insulation before derating starts having to be applied. I don't recall what it says about grouping, but I would have thought a similar assumption would be perfectly valid.
--
Andrew Gabriel

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It's not quite as generous as that; if there's half a metre or more you're required to derate by 50%. For shorter lengths there's a Table roughly along the lines of:
50mm - 0.89 100mm - 0.81 200mm - 0.68 400mm - 0.55
In practice, as Mr Oates has said, no-one ever considered the couple of inches over which a pair of ringmain 2.5s are in close contact as they pass through the typical underfloor joist as giving rise to Grouping or a different Reference Method...
Stefek
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