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Two high-power appliances on a single 40A RCD

Former Community Member
Former Community Member
I have an electric shower installed on a 40A RCD, in a room adjacent to my kitchen. The shower is only used in an emergency - i.e. when our gas boiler is unable to provide hot water to our main bathroom. I would like to take a spur from this 40A connection to use for a new double oven, which is rated at 32A. Can anyone advise on a safe and legal way to do this, ensuring that only one of the two appliances can be connected at any one time?
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    Perfectly correct Chris, I would not consider this a long duration. I hinted about current ratings and time earlier and this 25 minutes is not a long duration.


    I also do not believe that a 30A switchfuse could "blow off the wall". From experience this takes quite a large supply of 100s of kVA and a direct short with PSSC in the many tens of kA.
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    Sparkingchip:
    433 Protection against overload current 


    433.1 Co-ordination between conductor and overload protective device 


    Every circuit shall be designed so that a small overload of long duration is unlikely to occur.


    I replaced a fused switch with a 30-amp cartridge fuse in it after the whole thing blew off the wall, the customers teenage daughter was spending around twenty five minutes in a shower with a 8.5 kW electric shower, her dad kept packs of fuses ready to replace the fuses as they blew, then one day he went into the garage and there was just a big soot mark on the wall where the fused switch used to be.




    8.5 kW @ 230 V = 37 A, so almost a 25% overload. Is that small? Is 25 min a long duration? I personally would view a "small" overload as being one which lies outside the trip curve so the overload could continue indefinitely.


    In any event, a 30 A BS 1361 fuse should have been capable of handling 37 A for half an hour, so perhaps something else was going on?


     

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    That does run into part P Geoff.
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     It has a 40A CPD and if installed correctly a 10mm2 cable. But lets assume it has a 6mm cable buried in a wall or somewhere clipped direct. The shower is obviously satisfactory and probably takes 37A. The oven could potentially take another 16A or so and you are screaming (or actually Alcomax) that the cable could melt. Could it? A 40 A breaker actually trips quickly at at least 52A so our customer should never get a trip.



    Why would "installed correctly" require 10sq.mm.?

    Assuming, as you say, method C and 37A, then 4sq.mm. would be adequate.


    On the other hand if it does have 10sq.mm. cable and method C, then change the CPD to 63A and all is compliant.
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    If I did a site  survey I would assess:
    • The size of the shower cable.

    • The installation method of the shower cable.

    • The kW rating of the shower.

    • The kW rating of the double oven.

    • If the manufacturers instructions state that it needs a 32 amp MCB when the oven cannot draw more than 20 amps.


    Then I would determine if the largest possible current that can be drawn on the circuit is 57 amps and go from there.


    Andy Betteridge 


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    davezawadi:

    Andy

    Consider carefully the OP and the real circuit. It has a 40A CPD and if installed correctly a 10mm2 cable. But lets assume it has a 6mm cable buried in a wall or somewhere clipped direct. The shower is obviously satisfactory and probably takes 37A. The oven could potentially take another 16A or so and you are screaming (or actually Alcomax) that the cable could melt. Could it? A 40 A breaker actually trips quickly at at least 52A so our customer should never get a trip.


    Regards

    David




    Exactly what BS 7671 Regulation 413.1 tells you not to do.


    Andy Betteridge

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    Sparkingchip:
    433 Protection against overload current 


    433.1 Co-ordination between conductor and overload protective device 


    Every circuit shall be designed so that a small overload of long duration is unlikely to occur.


    Andy Betteridge 




     

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    Sparkingchip:




    davezawadi:

    What is wrong with simultaneous use, it will trip the MCB at some point, strangely that is what the MCB is for!!!! Belt and braces efforts at design are not required. Kitchen fitter does not have a clue! I wonder which regulation that is? Answers on a postcard to the Screwfix forum.




     

    Crack on in your own home if you believe that, but I would not recommend doing it in anyone else’s home.


    I definitely won’t do it for a customer.


     Andy Betteridge 

     




    Just wondering, as per the earlier question, what eicr code would it attract (and why)?


    F

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    Andy

    As you can find nothing wrong with my simple suggestion you try and play the H&S card! Perhaps you do this with your customers, I hope not because it is simply fraudulent. You appear to think that rings and radial final circuits should be limited to less than three 13A sockets on the basis that otherwise they are dangerous, using your own logic and comments. Perhaps you should explain why, and quote the accident rate due to the practice of unlimited numbers on either type of circuit. A little homework on this should be a very necessary lesson on truth. I thought I had heard everything electrical but this "takes the biscuit", using a colloquialism because the proper words would probably be moderated out!
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    And remember your duty of care to other members of your family, visitors to your home and neighbours.


    Andy Betteridge

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