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?

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.

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).


--
Robin

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

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

Thanks. I wasn't at all sure I could use any of those for T&E rather
than the special (fudge factor for ring main?) Table 4D5A.

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.

Very tempting.

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!

Yes, thanks, I'll see if I can finds space for two conduits - although
the Cat5e, PF100 and alarm cables have first dibs on conduit space.

--
Robin

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

Thanks. I wasn't at all sure I could use any of those for T&E rather
than the special (fudge factor for ring main?) Table 4D5A.

4D5A is a relatively recent addition (2002 amendment) that came out of
just this issue - T&E ratings for ring circuits, particularly in
insulated walls. It introduced 'new improved ratings' for the specific
cases it covers, which were the result of some test work done at ERA.
There were a couple of articles in the /Wiring Matters/ magazine, IIRC.
Notwithstanding all this, nothing stops you using the older and more
general 4D2A where it's more relevant.

There are more new ratings in the 17th edition (due any day now):
http://www.wiki.diyfaq.org.uk/index....es#Cable_Sizes

[Iz = 18.4 A]
Very tempting.

Yes, if this is in the middle third of a circuit which is rarely going
to be heavily loaded, and the protection is _not_ by rewireable fuse,
I'd say there's little to worry about.

Yes, thanks, I'll see if I can finds space for two conduits - although
the Cat5e, PF100 and alarm cables have first dibs on conduit space.

And those are more likely to need replacing with something new in 15
years' time, whereas the 2.5 T&E will last for ever.

--
Andy