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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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    You still can’t beat installing a new circuit.


    A bit of imagination usually allows doing so to be relatively straightforward, I installed a new cooker circuit a couple of weeks ago running externally down the side of a house, then in and along underneath the kitchen cabinets in steel wire armoured cable to an isolator then flex to the freestanding cooker. There are no marks or anything to see inside the house.


    Whipping carpets and floors up to pull cables in is daily work, unless you have covered the upper floors in laminate flooring or tiles it is generally straightforward, though currently I am being paid to drive all over England and Wales to do it, but that’s more of a matter of getting someone in the right place at the right time than because it is complex work.


    In amongst all discussion this no one has asked it there is an existing electric cooker circuit in the kitchen that could be utilised.


    Andy Betteridge
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    I really am quite disappointed Gentlemen. No one has really understood the situation properly have you?

    OK you could install a new circuit, but the point is "is it necessary?". All of the above posts have failed to understand what "overload protection" really means, and seem to assume that overload should be eliminated by design, that is more final circuits! Overload already has a device provided to prevent damage to the installation and that is the CPD. Andy has tried to turn this into "catastrophic damasge to the electrical installation". This is more than foolish, it is outright dishonest. An MCB trip is catastrophic eh, that sounds like a threat to me to extract money? It is certainly very unprofessional.


    There is a way to do this which is perfectly permitted. The downside is that a trip might occur if the customer uses all the appliances at once. This is his choice, it is not up to you, you should explain carefully what might happen and why. From the OP he has no intention of using the full load at once so any tripping would simply a warning that he has forgotten the choice. All the nosense that it is in some way not safe is fantastic and untrue. Some of you need to get a grip, because many people are very suspiciousof trades already and you are making this worse.


    For my motor example the cables may be chosen on rating at FLC, although some extra cross section may be required by starting volt drop which make start cycles much slower and may cause motor stall. Overload protection would be by phase loss relay and suitable time delay setting and current on the MCCB. The final circuit can be overloaded but has suitable protection. Why is the phase loss relay so important? There is a time diversity on the start cycles and this will limit temperature rise to a manageable level. If starts were at a rate of one per minute with the 30 seconds start time we would need to take the average current into account in the cable rating, not the peak value because the average describes the overall heating effect. Apply that to the OP and you will get exactly my answer.


    Regards

    David
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    mapj1:

    It is not illegal. It may be out of his comfort zone, which is fair enough, but then he should be honest about it. Be aware that wiring in bathroom zones (i.e within 60cm of the bath or shower cubicle ) requires notification under part P, and he may not  be keen to get involved in that, or he may be worried that he gets called back if something trips.

    If it was my house, and I know who will use it, I'd parallel them up and have a variation on the single line working token system for Victorian trains, - perhaps when the bar of soap is in the kitchen it is OK to cook, when someone takes it into the shower it isn't.  Or a label on the shower for visitors - "check is anyone cooking ? Do not overload the trip !" Actually cooking is not as thirsty as folk often imagine - once initial heating is over the thermostat pings on and off and the average current is quite low. It is a cold start that is the problem.




    But you would not want a bar of the highly fragranced orange coloured Coal Tar Soap in the kitchen, it will contaminate the food and make the veg taste awful Mike.


    Z.

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    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
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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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    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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    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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    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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    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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    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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