neverwas wrote:
I know I don't know BS7671 but I now find I can't even fathom the
current-carrying capacity if I put bog-standard 2.5mm T&E in plastic
conduit in an internal plaster/lathe wall and plaster over it. Is it
table 4D5A - despite the fact that methods 2 & 3 both involve insulated
walls which this ain't?

Installation method 7 is what you're describing, so use Table 4D2A
reference method 3: answer 23 amps.

It matters because the derating for grouping would mean I could not put
2 cables through one conduit and hit the magic 20A figure - unless, of
course, I've misunderstood that too. So I was rather hoping for a
rating which took account of the greater conductivity of the wall and/or
the greater surface area of flat T&E compared to round multicore.

From 4B1 the Cg for 2 cables bunched in conduit is 0.8, 0.8 * 23 =
18.4, so still non-compliant, but you might deem it close enough if it's
not at one end of a kitchen circuit.

And, while I could revert to just clipping and plastering over, it seems
a shame not to use conduit (especially given my hindsight is now so much
better than my foresight).

Two conduits would be OK, or clipped direct (touching) would give you 27
amps times a Cg of 0.85 which is 22.95 A - eureka!

--
Andy