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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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  • Former Community Member
    0 Former Community Member
    Alcomax:

    Assuming you may need two circuits, if you did use the existing 2.5 ring, what would that serve at the end of the day?

    Edit: looks like a kitchen!

    A 2.5 ring on a 20 will work, but not on a large kitchen. Wood supports between joist to have it " clipped" is interesting, but it would still be surrounded by insulation anyhow.  If the existing 2.5 was just two beds and a living room 20 amp would work. With 4mm the trade off 25 amps or 32 amps may not be a big deal. only 7 amps; think the maximum demand of the whole house and the number of circuits,  many rings are not going to be a problem with a 25 amp circuit breaker.


    I think I am already at the stage of starting again with 4mm2. There is so much of the existing circuit that would need to be modified with the added complication of junction boxes that I can do without.


    There is a natural split down the middle of the kitchen, effectively dividing it into two halves. From the point of view of balancing out the two ring finals better, supplying 1 side of the kitchen, 1 side of the living area (kitchen and living area are open plan 50 sq metre, square dimensions) and 2 bedrooms on one ring final, and the other half on the other seems like the best option. Like you say demand from living area and bedrooms will be relatively low, and the kitchen demand substantially higher.


    Looking more carefully I think that reference method 100, if I can clip to wood consistently (it mentions joist but if the noggin is the same size as the joist I cannot see there being any difference with heat transfer):


     



    Reference Method 100



    Flat Twin and Earth cables - See Table 4D5 for current-carrying capacities:



    Installation methods for flat twin and earth cable clipped direct to a wooden joist, or touching the plasterboard ceiling surface, above a plasterboard ceiling with thermal insulation not exceeding 100mm in thickness having a minimum U value of 0.1 W/m2K.


    If the insulation is >100mm then its method 101 however this is still >20 Amps per leg if I use 4mm2.


    It looks like the solution is my mitre saw, a couple of lengths of 2x6, a reel of 4mm2 T&E  and some graft..........


Reply
  • Former Community Member
    0 Former Community Member
    Alcomax:

    Assuming you may need two circuits, if you did use the existing 2.5 ring, what would that serve at the end of the day?

    Edit: looks like a kitchen!

    A 2.5 ring on a 20 will work, but not on a large kitchen. Wood supports between joist to have it " clipped" is interesting, but it would still be surrounded by insulation anyhow.  If the existing 2.5 was just two beds and a living room 20 amp would work. With 4mm the trade off 25 amps or 32 amps may not be a big deal. only 7 amps; think the maximum demand of the whole house and the number of circuits,  many rings are not going to be a problem with a 25 amp circuit breaker.


    I think I am already at the stage of starting again with 4mm2. There is so much of the existing circuit that would need to be modified with the added complication of junction boxes that I can do without.


    There is a natural split down the middle of the kitchen, effectively dividing it into two halves. From the point of view of balancing out the two ring finals better, supplying 1 side of the kitchen, 1 side of the living area (kitchen and living area are open plan 50 sq metre, square dimensions) and 2 bedrooms on one ring final, and the other half on the other seems like the best option. Like you say demand from living area and bedrooms will be relatively low, and the kitchen demand substantially higher.


    Looking more carefully I think that reference method 100, if I can clip to wood consistently (it mentions joist but if the noggin is the same size as the joist I cannot see there being any difference with heat transfer):


     



    Reference Method 100



    Flat Twin and Earth cables - See Table 4D5 for current-carrying capacities:



    Installation methods for flat twin and earth cable clipped direct to a wooden joist, or touching the plasterboard ceiling surface, above a plasterboard ceiling with thermal insulation not exceeding 100mm in thickness having a minimum U value of 0.1 W/m2K.


    If the insulation is >100mm then its method 101 however this is still >20 Amps per leg if I use 4mm2.


    It looks like the solution is my mitre saw, a couple of lengths of 2x6, a reel of 4mm2 T&E  and some graft..........


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