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Reference Methods

Former Community Member over 4 years ago

Wondered what installation reference method people might be using for a T&E cable on a masonry wall that is then covered with plasterboard, ie a typical dot and dab affair. I think "C" is correct as all installation methods involving a cable in contact with a wall and NO insulation map to reference method "C". Wouldn't think the plasterboard or covering the cable with capping would make any difference. I raised the question due to seeing A,B, & C on various other contractors certs.

0 Chris Pearson over 4 years ago

mapj1:

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.

How the hell is a sparks supposed to know the U value of any insulation? (Methods 100 - 103, table 4A2.)
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0 mapj1 over 4 years ago

Well, that is similar to the 'what is above the ceiling ?' At some point you have to assume - and we do not care so long as the cable does not overheat, so the end of a lighting circuit supplying half an amp of load need not worry us - asking if it is a configuration that is more like 101 than it is to C is utterly moot.

The only time it matters is if the cable is near it's rated limit, and as long runs are decide by volt drop, and most short runs are to loads that are not continuous , there are not too many cases to consider.

And if we get it wrong, the cable will run hot instead of warm - PVC is rated for 70C continuous, but if you drop an off cut it into your tea or coffee (or someone else's, depending on your morality and sense of humour) it does not melt like butter, it just gets a bit more flexible. Indeed only around 120C does it start to fail in short order, so there is quite a bit of margin for the odd overload of a few hours. (As power goes as the square of current, about 150% of the rating to get double the temperature rise, and that is quite a bad misjudgement..)

It's not great, but the sky does not fall immediately

M.
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0 Former Community Member over 4 years ago

Tom, my apologies, I hijacked the thread a bit. ? Cables are found in masonry all over the place - domestics, the smarter common areas, the public side of hotels, etc.) and I have no doubt that "C" is correct and I agree with your rule of thumb.

In the example that I described, I went for 100# to be on the safe side. I don't know what was stuffed in the ceilings after I left. The only potential bugbear would be that 4 mm² would no longer do for a radial on a 32 A breaker.

I also agree concerning EIC as opposed to EICR - difficult to know exactly what is below (or above) the surface. Of course when you come to do an EICR, you will be presented with a folder containing the EIC and any previous EICRs will you not? ?

Presented with an EIC, thats happened nearly as often as i've won the Euromillions jackpot ?

The new fangled 100# are at least straightforward looking but only mention stud walls and ceilings. Pretty sure they'd all be "A" in old money.
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Reference Methods

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