Loft Insulation - Electrical Cables and Kingspan Therma

[JohnW2]

Appendix 4 is informative - nowhere in the regulations are cable sizes wrt load explicitly prescribed in a way which would require them to be strictly interpreted according to any wording.

I suppose that's strictly true, but I think it could be an uphill struggle to invoke and support that argument if one were ever challenged.

I do sympathise and empathise with your approach, which corresponds with common sense and pragmatism. Would you similarly argue that (all other things being appropriate), say, a 4mm² unfused spur from a ring final supplying multiple sockets was compliant, or that a 1.5mm² cable from a ring final to an FCU was compliant?

Kind Regards, John

 

[JohnW2]

As I said you could put plasterboard between the cable and the insulation - but that seems a bit OTT.

An interesting suggestion - but what effect do you believe that would have on 'installation Method', hence de-rating factors? To me, it seems doubtful that it would make much material difference - existing guidance (Appendix 4) regarding cables "in contact with plasterboard" surely assume that there is air, not thermal insulation, on the other side of the plasterboard, don't they?

Kind Regards, John

 

[ban-all-sheds]

I suppose that's strictly true, but I think it could be an uphill struggle to invoke and support that argument if one were ever challenged.

I don't. It would be hard for anyone to argue that fitting the "103 curve" to other installation methods where reason says that the same principle of short-length considerations must apply was actually unreasonably careless and lacking in skill. And therefore it would be hard for anyone to argue that if I had done that that I was wrong to say that to the best of my knowledge and belief, i.e. that to my mind it complied with BS 7671. Remember - the declaration is not an assertion that what you have done does comply in an absolute fashion, it is a declaration that to the best of your knowledge and belief it complies.

And anyway - the only circumstances in which a challenge of any significance would arise would involve considerations of reasonable doubt.

I do sympathise and empathise with your approach, which corresponds with common sense and pragmatism.

It corresponds with what the regulations say, with what the responsibilities of the designer are, and what he may say about the way in which he has discharged those responsibilities.

Would you similarly argue that (all other things being appropriate), say, a 4mm² unfused spur from a ring final supplying multiple sockets was compliant,

You'd have to show that you had exercised reasonable skill and care in ensuring that what, from the POV of the ring final, was a load at a single point, was not likely to result in the ring final cables being overloaded for long periods. I've not given any thought as to how easy that might be, but there is nothing else which prevents it. 433.1.204 requires a minimum of 2.5mm² and a minimum Iz of 20A. If (all other things being appropriate) a 4mm² spur doesn't need an FCU then it doesn't need one, and no regulations are contravened.

or that a 1.5mm² cable from a ring final to an FCU was compliant?

If it's 2-core MICC it is. If it is any other sort of cable it is not - 433.1.204 is explicit about that.

 

[JohnW2]

Would you similarly argue that (all other things being appropriate), say, a 4mm² unfused spur from a ring final supplying multiple sockets was compliant,

You'd have to show that you had exercised reasonable skill and care in ensuring that what, from the POV of the ring final, was a load at a single point, was not likely to result in the ring final cables being overloaded for long periods.

You would, but that's a side issue, since exactly the same would be true if you were originating multiple 2.5mm² spurs from (separate) points on a ring which were very close together.

I've not given any thought as to how easy that might be, but there is nothing else which prevents it. 433.1.204 requires a minimum of 2.5mm² and a minimum Iz of 20A. If (all other things being appropriate) a 4mm² spur doesn't need an FCU then it doesn't need one, and no regulations are contravened.

Exactly - but, because of the 'informative guidance' of Appendix 15, I think that many would be nervous about doing that.

Kind Regards, John

 

[ban-all-sheds]

Then they should learn to exercise informed, competent, independent thought, shouldn't they.