Lime plaster and PVC sheathed cables

[Flyfisher]

That's was my feeling too, but I wasn't entirely sure.
I've since found a table of cable ratings and things are much clearer. Taking 2-core 2.5mm2 cable as an example, the ratings seem to be:

In wall in thermal insulation: 18.5A
In conduit on wall: 23A
Clipped direct: 27A

So, a single spur socket protected by a 15A fuse or a 30A ring would seem to be fine under any conditions. Which makes sense in terms of observed practice, but it's nice to know why.

 

[Penners]

As you say, those figures imply no problem under normal domestic usage. Nevertheless, it's interesting to see what a considerable effect burying the cable in insulation has.

 

[NT]

(18.5/27)^2 means about half the heat transfer rate.


NT

 

[Flyfisher]

Yes, although presumably these figures are at 'normal' temperatures.

I've also read that de-rating considerations are different when running cables around the apex of ceilings because the typically higher temperatures reduce the rate of heat loss from the cables. Even long runs of vertical conduit, where airflow is possible, can have a big heating effect as all the warm air rises, apparently.

But I think these sorts of 'refinements' are more concerned with commercial installations, so I'm not going to lose sleep over it for my purposes.

 

[DaveBrigg]

We've just been rewired and replastered. Sparky ran cables all over, held on at intervals with cable clips and occasionally channelled into the brickwork where the original plaster was intact but thin. The plasterers then covered the lot in lime, and you'd never know it was there. Much more of a problem was running cables down lath and plaster walls and fitting sockets - holes the size of dinner plates or bigger