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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
    Chris Pearson:
    jimmyhorns:

    Any flaws so far?


    Yes, if you plugged everything in and switched on, how much current would it add up to?



    32 A is a lot for a garage unless you have an EVCP. Could be 1 x 3 kW heater, but then what?

     




    I thought that was the purpose of diversity - to correct for the fact that everything is unlikely to be on at the same time.


    To answer your question, with everything plugged in and at full capacity, a lot of Amps, but in practice I cant see that happening.


    For example, the immersion heater 'never' gets used etc.


    In reality the cooker, washing machine, tumble dryer, hairdryer, full garage load and the lights are all that would be on simultaneously which would give:


    21+13+8+32+2 = 76 Amps


    The garage is more of a workshop and garage. 3kw heater + 4kw compressor + Mill + lathe + Plasma cutter + (insert any of the other toys here). Average demand is actually very low, but if its cold, the compressor kicks in while I am using the plasma cutter you are looking at 13 + 20 + 13 amps so already over spec - I'm careful with what runs simultaneously of course limiting it to one machine if the heater is running or two machines if it isnt.


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  • Former Community Member
    0 Former Community Member
    Chris Pearson:
    jimmyhorns:

    Any flaws so far?


    Yes, if you plugged everything in and switched on, how much current would it add up to?



    32 A is a lot for a garage unless you have an EVCP. Could be 1 x 3 kW heater, but then what?

     




    I thought that was the purpose of diversity - to correct for the fact that everything is unlikely to be on at the same time.


    To answer your question, with everything plugged in and at full capacity, a lot of Amps, but in practice I cant see that happening.


    For example, the immersion heater 'never' gets used etc.


    In reality the cooker, washing machine, tumble dryer, hairdryer, full garage load and the lights are all that would be on simultaneously which would give:


    21+13+8+32+2 = 76 Amps


    The garage is more of a workshop and garage. 3kw heater + 4kw compressor + Mill + lathe + Plasma cutter + (insert any of the other toys here). Average demand is actually very low, but if its cold, the compressor kicks in while I am using the plasma cutter you are looking at 13 + 20 + 13 amps so already over spec - I'm careful with what runs simultaneously of course limiting it to one machine if the heater is running or two machines if it isnt.


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