following on from yesterdays post could any one second my thoughts and any comments
pipes shall be boxed in and laged, a compartment made at the bottom to carry cabble with concrete brick to 2 surfaces.the circuit is a ring so I have calculated cable using 21A,
I have used ref B (4D2A)
and factored for 50 degress temperature (table 4B1).

which comes out as 4.00mm T&e

does this sound OK can any one see a floor in this, i know it will be tight in the sockets.I will investigate possibility of a horizontal chase across wall but they were not to happy

Edited: 30 November 2012 at 12:18 PM by beaver74

I think 4.00mm is ott, 2.5mm will be fine I reckon, after all it is a ring. Are you expecting it to be really heavily loaded for long durations?

Dave.

the problem is when I visited the central heating was on and as a booste an electric fire was running and I counted 3 fires dotted around the house.
I do intend to put the ring on a 20A but at a latter date I dont want to have to tell them it can not be upgraded to a 32A if the electric heating is used and takes out the 20A breaker (BIBS AND BRACES)

does the calcs look OK you Dave

What sort of building type is it - if you are reasonably certain they wont have 4 or 5 x 2kW fan convectors plugged in, then 2.5mm2 will eat it (even at 50C ambient).

Last time I had to do anything like this, the chippy put a batten at low level, I ran a plastic conduit just above and then put T boxes and short double sets into chases up to the flush sockets at 400mm AFFL.

The pipe strangler then installed the F&R's with stubs up to the rads above the conduit to minimise the effects of convection, and added a bit of phenolic insulation to his runs. Chippy put an upper batten on and planted the skirtings (two piece in this case to get a bit of "height")

This was in a hair salon and the sockets did get a bit of use for dryers and the like - never had any concerns over it - it was wired in 2.5mm2 stranded drawn into the conduit (with a 4.0mm2 CPC).

Keep in mind that when we talk about temperatures (both ambient and of cables/conductors) they are rarely static - the numbers usually reflect the theoretical steady state condition - which usually never occurs - you need quite a long time to heat up a cable - loads rarely exist for long enough with varying (often substantially)

Regards

OMS

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Failure is always an option

And you won't aquire that "experience" if you always do everything by the book Beaver! (Start thinking outside the box)

Dave.

If it's of any help, your calculation looks OK to me.

Not that it makes a difference to the answer, but I think you could get away with using 20A rather than 21A - (see reg 433.1.103).

50 deg C sounds like a round figure - was that a guess? It's not too difficult to calculate if you want to - given the R-values (i.e. 1/U value) of the pipe insulation and the timber(?) boxing, and temperatures at each end - the temperature difference is proportional to the R value. i.e. it's just like applying Ohm's law to a potential divider. (You could add in R values for the copper walls of the pipes, and air/solid surfaces, but they're not likely to make a significant difference).

- Andy.

as in 'good judgement comes from experience ... and experience comes from bad judgement'?
- Andy.

Yup - you've got to f**k it up a few times to learn anything meaningful in this life - it's not a spectator sport -

Regards

OMS

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Failure is always an option