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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

    Sparkingchip:

    As I suspected the manufacturer has overstated the circuit requirement.


    That oven will be quite safe on a 2.5 mm circuit with a 20 amp MCB or RCBO. That doesn’t particularly help if you don’t have such a circuit.


    It is almost on the point of being able to be connected to a socket ring circuit with a 16 amp MCB at the point it is connected to that circuit.


    4000 Watts divided by 240 volts = 16.7 amps, if you divide be 230 it comes out at 17 .4 amps.


    If you look at a selection of double ovens you will find the manufacturers rating stated as required for the fuse or MCB doesn’t tally with the actual ratings they give for the ovens.


    To be perfectly honest in my home I would prefer to see it connected to a socket ring circuit using a 16 amp MCB than connected to the shower circuit, after all the existing oven has been wired that way for many years.


    Though I would still recommend a new circuit is installed.

     




    That's interesting. The sockets in the kitchen are on a 32A circuit breaker in the consumer unit. A new circuit is really not feasible for me, mainly on grounds of cost. So are you saying that a separate 16 or 20 amp MCB could be installed in the kitchen - e.g. where the socket for the existing oven is located? (it's a double socket which is also used for a toaster).

  • 0
    The calculation based on the requirements of BS7671


    Rating on plate 4.0 kW


    So 17.4 amps at 230 volts.


    Therefore 10 amps plus 30% of 7.4 amps gives an assumed current demand of 12.3 amps


    So is everyone satisfied with having a double pole B16 MCB in a metal enclosure connecting the oven to a 32 amp socket ring circuit, rather than increasing the potential load on the shower circuit by over 12 amps?


     Andy Betteridge 


  • 0

    4000 Watts divided by 240 volts = 16.7 amps, if you divide be 230 it comes out at 17 .4 amps.




    It doesn't work like that.

    If you are going to use 230V for design then it will be 16A - surprise.

    At 250V, it will be 17.4A so perhaps one should allow for that just in case.




    If you look at a selection of double ovens you will find the manufacturers rating stated as required for the fuse or MCB doesn’t tally with the actual ratings they give for the ovens.



    There aren't that many fuse/MCB ratings to choose from and no reason why a manufacturer should specify one anyway.

    Most (single) oven instructions state that a 16A supply is required (even if the actual amperage is lower) because that is the rating of circuits they have in Europe.


    Presumably a double oven states 32A for the same reason - i.e. two circuits except in Britain.

    Even that is only 16.6A after applying diversity.

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    Wow, this discussion is getting a bit heated isnt it?

    well my tenpernorth.

    our traditional diversity for a "cooker" has prettymuch stood the test of time.

    sensible use of the standard clac is allowed providing that we consider if there are any underlying reasons not to use it.

    i have, over time, considered a few where oven and hob are on different circuits but together present no more diversified load than a full blown traditional cooker.if someone wants to put those two units on ond ciruit i would not usuallyhave a problem with that unless cir umstances dictate otherwise.

    however this combinstion we are discussing is different, two possibly unrelated activities.that is the problem.

    the OP suggests that their particular usage pattern would not present a problem and that could be correct in intention and indeed in pratice.

    however, the stiuation does leave to open to abuse, either by the OP or others.

    I have no problem with a sensible change over/power sharing arrangement that prevents both appliances running at the same time if separate circuits would be too inconvenient to install for some reason. Not the most elegant of circuits but hey ho doable.

    what i do feel uncomfortable about is a circuit that invites overloads big or small.

    yes we do allow overloads up to 13% as normal and a circuit must not trip. Cable calcs etc give us that headroom.

    at 45% overload a circuit must trip , altjough it could take hours to trip. 

    a masive overload would (should) trip quite quickly.

    i would not like to run a circuit that encourages a breaker to run on  its curve for any meaningfull duration.

    i feel that this circuit encourages the possibility of such overloads even rarely and is best avoided.

    so my vote is either two circuits or a switchover such as a shower priority type set up.

    by the same token i would not like a ring (or radial) with a 16A mcb in circuit running an oven in most instances, again separate circuits or a low wattage plug in typd if you must.
  • 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
  • 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 discussion from twelve years ago


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


    Andy Betteridge
  • 0
    Hmmmh. It caused a bit of a debate even back in that age then!
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    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
    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.  
                          

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