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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?
  • Former Community Member
    0 Former Community Member

    Chris Pearson:



    The point about the shower is that whilst you may not intend to use it routinely, a future occupier might wish to do so.


    There are plenty of appliances in that kitchen - IMHO wiring in a more powerful oven to the socket circuit is asking for trouble.


    So back to the shower. The surface-mounted switch is easily accessible in the kitchen. It really wouldn't be difficult to replace it with a changeover switch. Job done!


     




    Yes I can see the important point being raised by you and others regarding future behaviours/future occupiers (or "abuse" as someone else referred to it). Your point regarding the other appliances on the kitchen circuit was something I have also been mulling over - it doesn't feel like the best option for the new oven to share a 32A circuit with a range of other appliances in the kitchen (everything from a washing machine and dishwasher to a toaster and bread machine), particularly when there is a separate 40A supply available within a couple of metres which is unused for 99.9% of the time.

    So yes, from what I've read so far, the changeover switch remains the solution which represents the best balance between cost and safety. 

    So what might I expect to pay to have the existing switch replaced with a changeover, and to have the circuit extended 2-3 metres to the oven area?

  • 0
    What ever you do swapping from a single to a double oven will add stress any of the existing circuits, unless you can ensure that other loads are reduced or removed. But the oven without an electric hob isn’t going to draw 32 amps of current.


    Now the shower circuit needs reassessing as a cooker circuit, but it is highly unlikely that the oven going to overload a shower circuit and as the current it can draw is restricted again the circuit MCB or RCBO does not actually need to give overload protection to the circuit cable, only fault protection.


    Whatever you do now will be in place for many years, so it’s worth doing the best possible job, are you really sure that someone with a set of cable rods cannot pull a new cable in through the floor voids?


    Andy Betteridge 


  • Former Community Member
    0 Former Community Member

    Sparkingchip:

    Whatever you do now will be in place for many years, so it’s worth doing the best possible job, are you really sure that someone with a set of cable rods cannot pull a new cable in through the floor voids?




    Technically anything is possible, and if cost were not a factor then it might be something I would consider. But, as for many people, cost is a major consideration for me, so I am looking for the best balance of cost and safety. Oh and a solution that doesn't involve pulling up the floor covering and boards in the kitchen has some obvious advantages ;0)

  • 0
    After a very long day of de-badgering, my reasoning on why ,"Simultaneous use of a Shower and Cooker on the same 40 amp circuit, in a domestic environment", is not compliant with BS7671 :

    Unusually it is claimed the above is not a breach of BS7671, but is clearly contrary to 433.1.1 [iii] and would attract a C3 after the event. It is a house, an uncontrolled environment. An obvious way to comply with BS7671 in this instance is some kind of changeover switch preventing simultaneous use. It is clear many commentators agree with this, including the OP, but there is still the claim that simultaneous use is compliant.
    I think the sticking point is the interpretation of what is “normal service”.
    BS7671 definition of Design Current [Ib] of a circuit:  The magnitude of the current to be carried by the circuit in normal service.

    The design method applied will give the resultant Ib, in normal service.  Some will use a variety of diversity techniques to demonstrate that Ib is < In in normal service for economic purposes. This environment will be a controlled one, a business or a factory for example. Controlled methods will be in place to ensure there is no or very little, simultaneous use. There may be no actual physical impediment to simultaneous use; it could just be signage and instruction.

    433 is in part 4 of BS7671. It is protection against overload.
    Though the text includes “protecting a conductor against overload”, do not let that distract from the thrust of 433.1.1 [i] that says “….rated current of the protective device [In] is not less than the design current [Ib] of the circuit….”

    The regs has a Hierarchy as we all know. The assessment of characteristics for a design that is fit for purpose begins with 120.1 “This standard contains the rules for the design, erection and verification of electrical installations as to provide for safety and proper functioning for the intended use”. [My emphasis]

    It continues with 301.1, in particular an assessment shall be made for:
    “[i] The purpose[s] for which the installation is intended to be used……[ chpt 31]”
    “[iv] Its maintainability [chpt 34]”
    [Again my emphasis]

    Where there is control, you have normality; normal service.

    In a domestic environment there is little or no control. You obviously still require normal service, however that can only be reliably achieved by designing the circuit so that it is not likely to have Ib>In.

    If that is not enough, chapter 34 [Maintainability] is quite clear.

    “341.1  An assessment shall be made of the frequency and quality of maintenance the installation can reasonably be expected to receive during its intended life………………Those characteristics are to be taken into account in applying the requirements of parts 4 to 7  [ My note , part 4 includes 433.1.1[iii] ], so that …………………..”

    “341.1 [ii] the effectiveness of the protective measures for safety during the intended life shall not diminish and
    341.1 [iii] the reliability of equipment for proper functioning of the installation is appropriate to the intended life”
    [Again my emphasis]

    There is quite a clear distinction in the design process as to what can reasonably be controlled and what cannot. Simply this can be the choice between the work or non-work environment. The designer of the circuit has to make this choice before applying requirements of Part 4 and this determines how you treat Ib in reg 433.1.1 [iii], what magnitude ,if any, of diversity is applied.

    Therefore, in the home environment, repeated simultaneous use would result in premature failure of the 40 amp circuit breaker. Simply Ib >In, reg 433.1.1 [iii]. It is not fit for its intended purpose, not proper functioning and inappropriate to the intended life.

  • 0

    Alcomax:
    After a very long day of de-badgering, my reasoning on why ,"Simultaneous use of a Shower and Cooker on the same 40 amp circuit, in a domestic environment", is not compliant with BS7671 :




    I had a long day of de-badgering too, although I get no impression that it was (at any point) DIY.


    Where (the hell) does this cable come from, and where does it go to; what is it's purpose; is it live or dead? (Safe isolation practiced.)


    I cannot disagree with Alcomax's well-considered analysis. ?

  • 0
    But normal service in this case is that cooker and shower won't be used together. It's no different than saying that in normal service, one won't put a large pan of cold water on every hob ring and simultaneously turn them and both ovens on. In either case, if the combined overload lasts too long then the cable won't reach an excessive temperature before the OCPD trips. After which, the unskilled householder in their uncontrolled house will deduce that you can't use all those things at the same time and avoid doing so again.
  • 0
    Back in the days when all loop testing was carried out at high currents before there were RCDs in the circuits and installation some specifications said that fuses had to be replaced after the loop testing had been completed, because the stress put on them by the loop testing could lead to early failure.


    So at one time some fuses such as 5 amp fuses in lighting circuits were considered scrap after having 20 amps whacked across them got a very short period during a loop test.


    Circuit protective devices are not indestructible.


    Andy Betteridge 


  • 0
    I think there's a big difference between e.g. an under-specced 16A kitchen circuit tripping every other day because whenever you turn on more than one or two of kettle, washing machine, toaster etc it exceeds the design current; and a shower+cooker circuit tripping once per year.


    If we are to follow the literal letter of the "law", then we shouldn't install both a cooker and a shower in the same house which is protected by only a 60A cut-out fuse.  We should become po-faced and tell the customer that they must contact the DNO to get their supply upgraded to 80A plus before we install the shower.
  • 0
    I always advise against having a 10.8 kW shower, generally there won’t be enough water.


    Andy Betteridge
  • 0

    You are not wrong on that one




     

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