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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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  • 0
    Before anything else, what does the ring final serve? i.e. the real likely demand. Also, are you going to be running any other new circuits through this soon be be insulated ceiling?

    Traditionally, first fix of cables has always been though centre of floor joists. Fine when no insulation, but later on any lateral runs are potentially to be covered by insulation. Similar in a way to lofts. Yes it is a problem. 16 amp or 20 amp ring reduction can help. With 4mm ring if I remember without calculating, a bit of a get out is a 25 amp OCD, though not all switchgear manufacturers do a 25 in some domestic CUs, though Hager has a full range that fits commercial and domestic. Not sure about other makes.
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  • 0
    Before anything else, what does the ring final serve? i.e. the real likely demand. Also, are you going to be running any other new circuits through this soon be be insulated ceiling?

    Traditionally, first fix of cables has always been though centre of floor joists. Fine when no insulation, but later on any lateral runs are potentially to be covered by insulation. Similar in a way to lofts. Yes it is a problem. 16 amp or 20 amp ring reduction can help. With 4mm ring if I remember without calculating, a bit of a get out is a 25 amp OCD, though not all switchgear manufacturers do a 25 in some domestic CUs, though Hager has a full range that fits commercial and domestic. Not sure about other makes.
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