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Insulated ceiling ring final nightmare.

Former Community Member
Former Community Member
I am having a bit of a nightmare trying to adequately size cable for a ring final. The existing circuit is 2.5mm2 T&E which runs in the ceiling void between ground floor and first floor (this is a residential property). Currently there is no ceiling insulation installed. An extension means a new ceiling and significant modification to the ring final, and also (to meet regs) sound proofing insulation installed in the ceiling. This has a similar U value to thermal insulation in most cases.


100mm of insulation is to be installed throughout the new ceiling - which is 50% old circuit.


My first thought were to re-wire the whole circuit in 4mm T&E as with 2.5mm2 and the 18th ed tables its method of installation in all places wont give me 20A on each leg of the ring.


Great I thought, problem solved with plenty of overhead. More of a pain at 2nd fix, but not the end of the world.


Except now when I am looking through it seems like 4mm2 might not be enough if its going to run the risk of being covered with insulation. Joists are 170mm, so while the cable will not be covered completely the insulation will tend to balloon around it, so it looks like I will have to de-rate by a factor of 0.5 which would mean needing 6mm2 cable which seems crazy.


Any thoughts? Where am I going wrong in my assumptions / calcs?


Thanks in advance


James
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    davezawadi (David Stone):

    Ask the Building Inspector about this inter-floor insulation. I have never seen it. Keep asking until he shows you the regulation which requires it.


    David, it is all in Part E. Although it seems rather onanistic, there are good reasons for it. For example, at one stage Daughter and her beau had a semi-basement flat in Town. Unfortunately, every little tinkle, splash, and flush from the loo in the landlady's flat above was audible. ?


    I note what you say above about acoustic insulation. I have not yet found a value for its U value, but presumably it is a little less than that of thermal insulation. In the meantime, provided that cables are kept to the edges, I think that IM 100# is the best that we can do.


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  • 0
    davezawadi (David Stone):

    Ask the Building Inspector about this inter-floor insulation. I have never seen it. Keep asking until he shows you the regulation which requires it.


    David, it is all in Part E. Although it seems rather onanistic, there are good reasons for it. For example, at one stage Daughter and her beau had a semi-basement flat in Town. Unfortunately, every little tinkle, splash, and flush from the loo in the landlady's flat above was audible. ?


    I note what you say above about acoustic insulation. I have not yet found a value for its U value, but presumably it is a little less than that of thermal insulation. In the meantime, provided that cables are kept to the edges, I think that IM 100# is the best that we can do.


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