Ring main behind insulation

Jan 14, 2014 11 Replies
R

Hi Folks and belated new year greetings to all. Must be some time since I looked in as there were 18871 headers to download. ;-(



I was under the impression that if wiring is covered by insulation, specifically a ring main under 2" of Celotex or Kingspan plus plasterboard as dry lining on an outside wall, the cable has to be derated but I was chatting to a building acquaintance recently and he said his electrician was using normal 2.5mm wire. Is that according to the regulations? I know there are derating tables but typically have been unable to track anything down on the Internet.


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J

we will have to stop talking about you now ;-)

A cable in insulation does have its current carrying capacity derated. However that does not automatically mean that you need to increase the size of the cable for a given application, since there is a certain amount of spare capacity with most of the "standard" circuits.

The actual detail of how its insulated also matters. So a cable clipped to a masonry wall, or laying on plasterboard before being covered with insulation is much better off than one that is surrounded on all sides by insulation.

Have a look at :

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Then look at the Installation Methods section that follows.

If you take the case of a normal ring circuit. The cable must have at least 21A of capacity in each leg of the circuit. So installed as per method 100 it would still meet the spec in spite of the insulation.

If you want to get more deeply into specific cases there there is also:

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B

Would not this depend on the length of wire though?

Brian

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A

Yes, it should be derated. If touching the brick/block wall, then the current carrying capacity of

2.5mm cable will be 21 Amps. If not touching the wall, then it will be 13.5 Amps. However, in your case, he may well have used a design current of 20Amps, which is regarded as the CCC of half a 2.5mm ring final circuit [1] leg, so if touching the wall, it will comply, even if derated from its surface mounted / masonry embedded rating of 27 Amps.

So basically, if it is touching the inner wall, it complies. If free-floating in insulation, it will not.

[1] A ring main is what the distributor uses for high voltage, you use a ring final circuit, or ring circuit in your house.
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A

Modern Trollop terminology - they were ring mains when they were introduced in the early 1940's using TRS 7/046 cable and as far I I am concerned they are still ring mains whatever your modern regs book tells you :)

Andrew

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R

So that is why my ears tingle from time to time. :-)

Many thanks John. Glad to hear I was wrong and that the electrician was compliant.

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J

Not really no...

The length in insulation sometimes comes into play, but not the total length. (total length affects other parameters like earth loop impedance)

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M

I'm guessing you mean 7/.029

NT

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C

7/0.044 was the next size up.
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A

Correct ! Old age not only makes you grumpy, you forget things :)

Andrew

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