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Discuss 32A MCB feeding 1.5mm & 2.5mm T&E cables via 30A terminal block in the Electrical Forum area at ElectrciansForums.co.uk.

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  1. Monchichak
    Offline

    Monchichak EF Member

    Location:
    Berkshire
    Quickie - Would you say this is ok?

    32A MCB feeding 6mm T&E which is then connected to a 2.5mm T&E & 1.5mm T&E cable via a 30A terminal block in plastic terminal block housing buried in stud wall.
    The 2.5mm feeds a 3kW immersion heater with 13A SFU at the heater end.
    The 1.5mm feeds gas boiler with 13A SFU at the boiler end.

    Initially I thought the 1.5mm cable should not be fed with a 32A fuse but thinking about it - fault protection is provided by the 32A MCB & overload protection is provided by the 13A SFU at the end of the circuit rather than the beginning. Just doesn't seem good practice to feed a 1.5 & 2.5mm cable from a single 6mm cable protected by 32A but is it ok?

    Not ideal to have boiler & water heater fed off same supply.

    Fuse in switched fused spur to water heater is on the limiter - non fused switched unit would be better?

    Connection should be maintenance free.

    Should the connection be rated for design current or be equal to or greater than the fuse (in this case 32A)?

    Many Thanks

    Chris
     
  2. Pete999
    Offline

    Pete999 Forum Mentor

    Location:
    Northampton
    Business Name:
    None
    In a few words, no this is not an ideal situation. In my opinion.
     
    • Agree Agree x 1
  3. Murdoch
    Offline

    Murdoch Electrician's Arms

    Location:
    Woking
    What is your involvement? EICR?
     
  4. Wilko
    Offline

    Wilko Electrician's Arms

    Location:
    Berkshire
    Business Name:
    Wilko Electrics
    Hi - I think Regs 433.2.1 and 433.2.2 may apply to this. Hope that helps :) .
     
  5. westward10
    Offline

    westward10 In echoed steps I walk across an empty dream. Staff Member Moderator

    Location:
    Northamptonshire
    The fcu does not provide fault protection to the cable but it does limit current flow. The cpc of the cable will need to carry fault current required for the 32A device to trip without suffering thermal damage.
     
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    • Agree Agree x 1
  6. Monchichak
    Offline

    Monchichak EF Member

    Location:
    Berkshire
    Noticed this whilst carrying out other work in the dwelling, reduced the mcb to 6A & removed heater from circuit as temporary, writing customer a report so he can call back plumber/electrician to rectify new install.

    Thanks for all your replies.
     
  7. Wilko
    Offline

    Wilko Electrician's Arms

    Location:
    Berkshire
    Business Name:
    Wilko Electrics
    For brain exercise -
    Using eqn in 543.1.3 with k=115, I get that 1 mm will support approx 360A for 100ms without damage. So if Zs is more than approx 0.54 Ohms but less than 1.1 Ohms it should work to disconnect from a B32. If circuit Zs is lower, then the 1.5mm cable with 1mm CPC will be heat damaged to clear a fault.
    Appreciate it if anyone would like to check this?
     
    • Useful Useful x 1
  8. Vortigern
    Offline

    Vortigern Electrician's Arms

    Location:
    England
    Business Name:
    F.H. Electrical
    Sorry can't check it because I am only allowed to use adiabatic for main earth. But sounds about right anyway.
     
  9. Pete999
    Offline

    Pete999 Forum Mentor

    Location:
    Northampton
    Business Name:
    None
    So it was a Plumber that butchered the wiring in the first place was it?
     
  10. littlespark
    Offline

    littlespark Electrician's Arms

    Location:
    Scottish Borders
    Sounds like the 6mm supply cable for a cooker has been reutilised for other things.
    I wonder what the cooker is connected to now? Nearest light switch?

    No, Pete.... it was a plumber slash electrician. One of those highly double trained tradesmen we should be envious of
     
    • Funny Funny x 1
  11. Pete999
    Offline

    Pete999 Forum Mentor

    Location:
    Northampton
    Business Name:
    None
    I see now it all make sense, bit like connecting the gas cooker to the 15 mm water main, done by a Plumber/Electrician/Gas Safe Engineer.
     
  12. Monchichak
    Offline

    Monchichak EF Member

    Location:
    Berkshire

    Plumber fitted gas boiler & water tank with immersion heater in loft & took the power from an old shower - lots of other issues too.

    Thanks for your replies
     
  13. johnduffell
    Offline

    johnduffell Regular EF Member

    Location:
    uk
    Very helpful! My comment would be that the MCB would likely be specified to trip quicker than 100ms, although good luck reading that off the graph!
    My other comment is that Zs at the end of the 1.5mm is not the right thing to measure, it's the Zs at the point where the csa reduces to 1.5mm.
     
  14. westward10
    Offline

    westward10 In echoed steps I walk across an empty dream. Staff Member Moderator

    Location:
    Northamptonshire
    Zs should be measured at the furthest point, the supply terminals of the fcus.
     
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  15. johnduffell
    Offline

    johnduffell Regular EF Member

    Location:
    uk
    Agree but for adiabatic the relevant impedence is the lowest case not the highest as it would be for disconnection time. Because there could be a fault anywhere on the cable and it would be worse case closer to the origin.
    So closest. Not sure what it's called, not Zdb in this case as it's half way! Zr for reduction in csa maybe.
    The only exception would be where the cable in question didn't need fault protection along its entire length, which i think is unlikely in this case.
     
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