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Topic Title: cable adjusted for heat
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Created On: 30 November 2012 12:07 PM
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 30 November 2012 12:07 PM
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beaver74

Posts: 361
Joined: 07 February 2007

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
 30 November 2012 12:26 PM
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daveparry1

Posts: 5888
Joined: 04 July 2007

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.
 30 November 2012 12:32 PM
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beaver74

Posts: 361
Joined: 07 February 2007

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
 30 November 2012 12:35 PM
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OMS

Posts: 18919
Joined: 23 March 2004

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

-------------------------
Failure is always an option
 30 November 2012 01:13 PM
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beaver74

Posts: 361
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Originally posted by: OMS

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).
concrete floor brick wall,my gut feeling is the electric fires will not be used much but I dont know. I do realise that all loads will rearly be kicking in togeather due to stats.ect

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.
this is as i intend to install


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)

I know this is in the real world going to be ott. the problem is that when i was at collage we were told how to calculate for Ref method, temperature ect but I for 1 was never told how we can manipulate the calcs the each situation. thus my need to use 21A for a calculation for the ring .I know it will not be carrying this load constantly I know in summer the heating wont be on, but I haven't experience in how to manipulate for the real situation

Regards



OMS
 30 November 2012 01:19 PM
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daveparry1

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

Dave.
 30 November 2012 01:21 PM
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AJJewsbury

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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.
 30 November 2012 01:25 PM
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AJJewsbury

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And you won't aquire that "experience"

as in 'good judgement comes from experience ... and experience comes from bad judgement'?
- Andy.
 30 November 2012 02:09 PM
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Avatar for OMS.
OMS

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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
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