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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
  • 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.
  • 0
    In practice I suspect most folk do not worry so much and there are no issues, but if you do want to do so, some options.


    Clip cables to joists - I know it seems odd, but in rockwool the wood is the better heatspreader than 'free' cable. Dress cables either above or below insulation as far as possible, so insulated on one side only.

    Battens and plasterboard for an empty  suspended ceiling a few 10s of mm under the sound proof one ?

    Run round the walls in the 6 inch zone below the ceiling.

    Mike,
  • 0
    Make it two 16 amp radials with 2.5 twin and earth cables.
  • 0
    Ah the building is going to be flats, is that why you have the acoustic insulation? You are making an assumption about the acoustic insulation, using standard Rockwool insulation does not give much soundproofing. The acoustic product comes in panels about 50mm thick, which have considerable density. This is necessary to absorb low frequencies which are acoustically much more troubling, but they offer good high-frequency attenuation too. Commonly such is carried out with the slabs fixed to the ceiling with very tight joins, and then covered with 2 layers of high-density plasterboard, joins all staggered. All this has quite a high acoustic attenuation, and also the necessary level of fire control. The slabs just between the joist are difficult to fit and lose a considerable proportion of the sound attenuation. All this should be in the hands of an acoustics designer, and if done in this manner you still have an insulation-free ceiling void. It would stop any fire damage above unless there is a very considerable fire, burning for a long period, hours, as the acoustic insulation is very effective against heat transfer too.
  • 0
    Which RM/IM are you using? 2.5 mm² should be good for a 32 A ring, but 4 mm² may not be for a radial. I have considered boxing in the wiring to get round this problem.


    (AFAIK all new builds require sound insulation.)
  • 0
    100mm of insulation is to be installed throughout the new ceiling

    Provided you can position the cable next to a joist or plasterboard - it sounds like method 100 to me - which table 4D5 gives 2.5mm² T&E a rating of 21A.

       - Andy.
  • 0
    OK.  The OP has quite correctly come to the conclusion that some of the existing RFC 2.5mm will be sandwiched between 2  lots of 100mm insulation type material. So imagine 2.5 mm twin/earth in pipe lagging, but the lagging is 100mm rockwool or similar.

    Yes, some will be resting on the plasterboard, some may be clipped to side of joists, but  likely a significant amount is routed through joist holes for some distance between joists. This is all existing. Suddenly this cable is derated. I know many wish that away, but it derates all the same. If you are adding to this circuit or altering it, you need to take this into account. After all you are aware that insulation is going to be installed? 

    If this was new work you could use all sorts of exotic and time consuming construction methods, or you could install as normal , holes through joist but wire the ring final in 4mm with a 25 amp OCD . 2 x 4mm twins in a socket is no big deal; many contractors routinely use 4mm for rings now.  25 amp covers all bases, 32 amp is fine with careful routing of cable.
  • Former Community Member
    0 Former Community Member
    Alcomax:

    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.




    Thanks for the reply:


    The ring final currently supplied a living room and two bedrooms. I didnt want to overcomplicate the already slightly complicated first post but can see more clarity would be useful.


    The groundfloor is being reconfigured and the living room becomes a kitchen, the extension the new living room. First floor is also reconfigured from 2 bedrooms to 3 bedrooms and an ensuite.


    Loads are likely to be relatively high so ring reduction below 32 Amps probably isnt going to be sufficient, certainly not for the kitchen component.

    Total floor area served will be 90 square metres though I am mindful that two ring finals would probably be a better solution given the appliances that are  likely to all be in use at the same time (washing machine, dishwasher, heat pump tumble drier, coffee machine, toaster, hairdryer etc). Although the figures suggest I can just sneak in under the 32 Amps I would much prefer a belt and braces approach.


    In the new area (extension) section I can run above the insulation clipped to joists as the joists are 225mm but to get to there I have to route along 170mm joists.


    As you say the first fix is indeed through the 'centre' of floor joists.


    Would a solution be to install substantial noggins between joists adjacent to the hole in joists and therefore be able to have the cable clipped and in contact with the noggins or joist throughout its path, provided I run 4mm2?


    If I understand this correctly I would then apply reference method A, which for 4mm2 in T&E gives me 25 Amps per leg.

    If I do this on alternate sides of the noggin this would give me 2 x 32 Amp ring finals 


    Any flaws to my logic here?


  • 0
    By my reckoning, you have 70 mm to play with above the wadding. If the floorboards are 20 mm, that gives you a zone of 40 mm in which you can clip the cables. It will be a fiddle, but feasible.


    I have faced this challenge in a house with wet underfloor heating, so leaving aside the thickness of the floor, there is (should be) no risk of penetration by nails from above. With web joists, it was possible to clip everything up out of harms way.


    The only misgiving that I have is that I have stated IM 100# on the EIC, but the wadding was put in after I left so I don't know what is actually in there now.
  • 0
    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.

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