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Spur off for shed

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  • 16-04-2019 6:29pm
    #1
    HappyAsLarE
    Closed Accounts Posts: 771 ✭✭✭


    Hi guys,

    I have an external heat pump that is supplied from the dist board with a 20 amp breaker. The max current required for the heat pump is 10 amps.

    Is it ok to spur off the heat pump external isolator in order to run a supply to a shed, terminating in a 10 amp breaker and then run a radial socket circuit of that?

    Shed equipment would be battery chargers, odd power tool etc.


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    #2
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,532 Mod ✭✭✭✭2011


    HappyAsLarE wrote: »
    Is it ok to spur off the heat pump external isolator in order to run a supply to a shed, terminating in a 10 amp breaker and then run a radial socket circuit of that?

    No, definitely not ok.


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    #3
    HappyAsLarE
    Closed Accounts Posts: 771 ✭✭✭HappyAsLarE


    2011 wrote: »
    No, definitely not ok.

    Thanks.

    Can I ask why? Is it a regulation or just bad practice?


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    #4
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,532 Mod ✭✭✭✭2011


    HappyAsLarE wrote: »
    Thanks.

    Can I ask why? Is it a regulation or just bad practice?

    I wouldn’t know where to start.
    Think of it more as a crime against humanity :D:pac:


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    #5
    HappyAsLarE
    Closed Accounts Posts: 771 ✭✭✭HappyAsLarE


    2011 wrote: »
    I wouldn’t know where to start.
    Think of it more as a crime against humanity :D:pac:

    Ha, we don’t want that.

    To clarify, the SWA runs out from the house into an isolator, then the heat pump is fed from this. I can’t see (with my limited knowledge) any dangers in taking another spur off the same isolator.


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    #6
    kramer1
    Closed Accounts Posts: 305 ✭✭kramer1


    Any idea what size the cable feeding the heat pump is ? You might be lucky and have a 6sq or something going out. If this is the case you may be able to get an electrician to rejig things so that you won't be causing any issues , I'd get someone who knows what they're at though . Don't want to mess up your heating system for the sake of a socket or two.


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    #7
    HappyAsLarE
    Closed Accounts Posts: 771 ✭✭✭HappyAsLarE


    It’s a 6sq cable.

    When you say rejig, do you mean run the cable to the shed and then pull back for the heating?


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    #8
    kramer1
    Closed Accounts Posts: 305 ✭✭kramer1


    Impossible to tell without seeing the set up, get a decent sparks though , should be able to sort something out for you


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    #9
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,532 Mod ✭✭✭✭2011


    HappyAsLarE wrote: »
    I have an external heat pump that is supplied from the dist board with a 20 amp breaker.

    So the MCB is not large enough to supply the shed and a heat pump.
    The max current required for the heat pump is 10 amps.

    How did you come to this conclusion, by measurement or calculation?
    Have you taken the starting current of the heat pump compressor into account?
    Is it ok to spur off the heat pump external isolator in order to run a supply to a shed, terminating in a 10 amp breaker and then run a radial socket circuit of that?

    For a start sockets should not be fed from 10A MCBs.

    Time to get a professional in.


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    #10
    HappyAsLarE
    Closed Accounts Posts: 771 ✭✭✭HappyAsLarE


    2011 wrote: »
    So the MCB is not large enough to supply the shed and a heat pump.



    How did you come to this conclusion, by measurement or calculation?
    Have you taken the starting current of the heat pump compressor into account?



    For a start sockets should not be fed from 10A MCBs.

    Time to get a professional in.

    Thanks again.

    The max current for the heat pump is stated on the model plate fixed to it.

    I am a aware I need a registered electrician either way, just trying to understand what can be done.


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    #11
    DublinDilbert
    Registered Users Posts: 5,366 ✭✭✭DublinDilbert


    It depends how far the shed is away and if the 6SQ cable is run correctly.

    Your electrican should be able to put a 32A MCB in the main fuse board (replacing the 20A that's there) and put a small sub-main board out in the shed, being fed by the 6mm sq cable. Then in the sub main board fit a 5A MCB for lights, 16A for the heat pump and 16A RCBO for the sockets.


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    #12
    HappyAsLarE
    Closed Accounts Posts: 771 ✭✭✭HappyAsLarE


    DublinDilbert wrote: »
    It depends how far the shed is away and if the 6SQ cable is run correctly.

    Your electrican should be able to put a 32A MCB in the main fuse board (replacing the 20A that's there) and put a small sub-main board out in the shed, being fed by the 6mm sq cable. Then in the sub main board fit a 5A MCB for lights, 16A for the heat pump and 16A RCBO for the sockets.

    That’s very helpful, thank you.

    The shed is only 10m from the current isolator.


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    #13
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,532 Mod ✭✭✭✭2011


    DublinDilbert wrote: »
    It depends how far the shed is away and if the 6SQ cable is run correctly.

    It also depends on how far the cable run to the heat pump is and the design load of the shed.
    Your electrican should be able to put a 32A MCB in the main fuse board (replacing the 20A that's there)

    If the earth fault loop impedance for the protected cable is sufficiently low this is possible but this is not necessarily the case. Also at 32A this may mean that the short circuit protection for the compressor motor within the heat pump is too high. Most likely the heat pump documentation stated that it should be protected by a 20A B or C type MCB. I know that when I last did a heat pump the MCB rating and characteristic was stated. You could of course add in a suitably rated MCB downstream in a sub board but this all adds to the cost that could be used to do the job properly.

    The heat pump isolator will need to be rated for the upstream protective device too.
    and put a small sub-main board out in the shed, being fed by the 6mm sq cable. Then in the sub main board fit a 5A MCB for lights, 16A for the heat pump and 16A RCBO for the sockets.

    That will only work if the 6 mm sq. cable is long enough to reach the shed which from the OP's posts I would think not. Remember the OP wanted to supply the shed from the heat pump isolator. Terminating an additional cable in the isolator is not ideal either.

    You are also assuming that the heat pump will not trip a 16A MCB which may not be the case due to the compressor starting current. With a DOL motor starting currents can be 5 times full load current. It is likely that the 20A MCB installed at present for the heat pump was correctly sized. Thsi woudl also mean increased volt drop to the heat pump which may cauuse under voltage issues.

    It would be best to do this properly and bring a fresh supply from the main DB to the shed.


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    #14
    HappyAsLarE
    Closed Accounts Posts: 771 ✭✭✭HappyAsLarE


    2011, I must say that is a very informative post, it is those types of ‘pitfalls’ i was trying to understand.

    The spec in the heat pump installation manual calls for a 16A MCB. I don’t know why they put in a 20A breaker.


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    #15
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,532 Mod ✭✭✭✭2011


    HappyAsLarE wrote: »
    2011, I must say that is a very informative post, it is those types of ‘pitfalls’ i was trying to understand.

    Potential pitfalls. It is not possible to be sure without having a proper look.
    The spec in the heat pump installation manual calls for a 16A MCB. I don’t know why they put in a 20A breaker.

    Fair enough, in that case you should get it changed.


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