Reference Methods

Hi Geez


It may be helpful if you would include the options for your reference to A B and C too to help community members to offer an opinion. ?

Hi Lisa


I am referring to the installation methods in table 4A2 of BS7671 and the reference methods A,B,C to which they apply.

I agree with C - see installation methods 57 and 58 in Table 4A2.


Now how about T&E which has been clipped to the underside of the floor above, or web joists with 4" of sound-proofing laid on the ceiling plasterboard? (Wet underfloor heating so at least 50 mm from both surfaces.)

I presume the cables go through an insulated ceiling after being clipped up the wall if we are talking about a standard domestic installation ?



Gary

I would say that the reference method would be C as the other reference methods imply a greater liklihood of heat generation due to space restrictions caused by poor or reduced thermal transfer.

Legh

I suggest to use the cable ratings from case  "C" is a  pretty good  catch all for the common none-insulated wall constructions. Realise that these are all approximations - the annex is informative,  not normative - so you should look at it as a guide and use your judgement, you do not have to follow it if you think it is not appropriate, or to interpolate between similar cases , when you think cables are more or less cooled than the reference case - it is cooling that  is the crux of the issue..


To that end wires that pass though a few inches of insulation can lose some heat out of the ends a bit - or at least the temperature along the wire does not step instantly from the cold to the hot case as you enter the insulation, rather there is a gradual rise temperature over the first  inch or two going into the lagging or foam  - and things like how tightly the cable fits the hole in insulation have an effect too, and are hard to judge in advance.

You may find that attaching the cable to a wooden 'heat spreader' means that side is better cooled - wood is not a good thermal conductor, but it is more conductive than the average block of celotex or what have you. A 3 or 4 inch dive in celotex would not worry me on 2.5mm cable or larger.  By the time we are much thicker on the insulation I might be derating or slipping a metal plate or metal capping in there to diffuse the heat out a bit.

On some insane things I have seen, there have laid pumped liquid cooling pipes intertwined with cables, but it is not funny and is really only  a solution on lab demonstrators and specialist kit where the aggro is justified. (mind you 1mm2 copper is suddenly good for over 50A continuous in a tube of  running water or the right sort of oil...)

Mike.

Gary,

These are not actually domestics, the walls form the common stairwell in a block of flats so the end of the circuit is just the top light fitting.


I personally think reference methods are an easy source of confusion, there are common installation methods that are not specifically mentioned in the BS. Looking at all the methods I can deduce the following:


Covered in insulation, whether in containment or not = "A"

Cable inside containment, no insulation = "B"

Cable touching, or very close to a wall = "C"

Cable on cable tray = "E or F"


They do deviate but that seems to cover 90%


Also easy for an EIC, you know exactly what you've installed but far more tricky on an EICR.


Tom

 

Covered in insulation, whether in containment or not = "A"

Excepting that for T&E you'd probably be looking at methods 100 - 103.

  - Andy.