Current-carrying capacity: 2.5mm T&E in conduit in solid wall?

Question

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).

Andy Wade · Accepted Answer

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

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.

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!

neverwas · Answer

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.

Very tempting.

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

Andy Wade · Answer

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):

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= 18.4 A]

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.

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.

Current-carrying capacity: 2.5mm T&E in conduit in solid wall?

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