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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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    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.

  • 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
    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

    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
    Some observations.


    From what is being described, it would seem wise to me to assume that the ownership of the property or the way in which it is currently used could be changed, with many more occupants for example. Therefore, something that relies on existing patterns of use, or a “procedure” by a person with some knowledge would potentially concern me. A judgement has to be made about risks that could reasonably be foreseen, these might include the risks of having to reset a protective device, certainly not recommended having emerged from a shower.  


    When I was originally trained on the regulations (14th), it was emphasised that the regulations did not preclude alternative methods as designed by a suitably qualified person.  I won’t get sucked into the technical argument here, but what it illustrates is that there are a variety of different approaches, seeking to balance cost with risk. Ultimately the client has to pay for it and it may seem unreasonable to place upon them the cost of what might or might not occur in future, if the risks are relatively low.  If it were my home, then I would make my own judgement and be very wary of anyone who insisted that expensive additional work was required. The problem might arise if an inspection is needed, because the design isn’t documented, so the inspector is potentially left “holding the baby” if something goes wrong.  This might involve someone falling down stairs or having some other mishap, because of the design of the installation.    


    The additional issue was raised of the difference between a “trade” and a “profession”.  Perhaps the answer lies here   https://www.youtube.com/watch?v=9tXBC-71aZs  as much as anywhere.  This is an IET forum, so members of the IET sign up to a code of conduct and those who register with Engineering Council have a further additional obligation to maintain a standard of competence appropriate to their post-nominal and to act within it.  There is certainly no reason why someone who meets the Eng Tech standard and is engaged on electrical installation work should not be a highly professional expert.  


    I won’t rehearse here what the differences might be between the three “types” codified by Engineering Council. However for simplicity in these type of circumstances, I would expect a professional Technician to rely more on compliance with regulations, whereas an engineer should evaluate and provide justification ,with reference to regulations as necessary.  Each needs to accept responsibility for their actions.


    The extent that having engaged in higher education makes a difference versus practical experience, it is probably most evident in formal design and “reporting”.  So for example, on a recent extension home extension project, I hired an architect, a structural engineer, builders, a plumber, and an electrician.  The first two gave me documentary evidence and satisfied public bodies (eg planning and building control), the first builder improved on the plans in practice, although sadly eventually lost productivity, the plumber has left me with an impressive and quite complex mini plant room to play with. The electrical work was probably the most straightforward, but it was quite a faff getting it certificated for Building Control , after I had to finish off the job myself.  Scale this up to a major project and we have a “professional team” responsible for perhaps spending hundreds of millions of the client’s money.  
                          

  • 0

    cfcman:




    Chris Pearson:



    Thank you for the photograph. I feel obliged to make a couple of observations (with all due respect):


    (1) It is not a large property, so a shower for emergency use seems a little lacking in credibility;


    (2) There isn't a lot of room for a double oven. I also note what appears to be a stack of at least three microwaves. ?



    Oh dear it seems that a picture is not always worth a thousand words :0(

    I'm not entirely sure how your inferences were derived regarding the size of the property, or the "credibility" of a shower for emergency use - I'm not entirely sure what was meant by that?....

    But to clarify:


    • The property is a 3-storey townhouse. I'm not sure what your definition of "large" is, but my guess is that the average UK homeowner would not describe it as "small"

    • The photo shows a small corner of a reasonably-sized kitchen (approximately 12 square metres)

    • There is plenty of space for a double oven to replace the existing single oven (I don't see much value in another photo to illustrate this particular point)

    • The property has three bathrooms - one on each floor. The photo shows the kitchen on the middle floor. The adjoining bathroom is used as a WC, but also has the electric shower in question, which was installed 10 years ago, specifically as a backup in the event of a boiler failure. The associated power supply was installed at the same time.

    • Those are indeed three microwave ovens - well spotted.


    I'm not entirely sure how/whether any of this is relevant, but I guess more information is usually better than less....


    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!

  • 0
    Hmmmh. It caused a bit of a debate even back in that age then!
  • 0
    A discussion from twelve years ago


    Note it it was a 6.0 mm cable and a 30 amp fuse.


    Andy Betteridge
  • 0
    In the past if someone said that they were having a double oven most electrical practitioners and kitchen fitters assumed it would be 4.8 kW and draw a maximum of 20 amps and go from there, installing it on a 20 amp MCB and a suitably rated circuit, which can be 2.5 mm though generally it would end up as 6.0 mm, though 2.5 mm or 4.0 mm can be okay depending on the installation method.


    The last few years many of the manufacturers have started telling people they need 6.0 mm circuits with a 32 amp MCB for ovens rated below 4.8 kW, so like for like swaps don’t comply with the manufacturers instructions in many instances.


    I double checked the rating of your oven On another website  and that says 4.0 kW as well on a 32 amp MCB 


    And you will have to forgive me for doing a simple Ohms Law calculation to calculate the amperage, despite it not being a D.C. current, you can have a go at the calculation Here on Rapidtables


  • 0
    A new circuit rated to supply both the ovens and a electric hob would be ideal to future proof the installation, if a cable is being pulled in you might as well do a really solid job.


    There has not been any mention of the hob, presumably that’s going to be replaced with another gas hob.


    So a 32 amp socket circuit that is 50% loaded or an overloaded shower circuit, what other appliances are on the socket circuits? Is there a washing machine and tumble dryer on it as well?


     Andy Betteridge

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