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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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     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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    That does run into part P Geoff.
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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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    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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    If the cable is 10 mm AND clipped direct it could potentially run 64 amps, so it could become a distribution circuit to a second consumer unit. 


    It all depends on the installation method and real maximum current draw of the double oven. 


    Highly unlikely, as it seems the first and final parts of the circuit are embedded in walls, but not impossible. 


    Andy Betteridge 


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    davezawadi:

    That does run into part P Geoff.




    What does that mean?


    Everything in dwellings comes under Part P.

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    Chris Pearson:




    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?


     


     




    As a rule of thumb, if you repeatedly replace fuses or have to keep resetting a MCB something is wrong and needs sorting out.


    It is not because of user error due to them not realising they cannot use the shower whilst the oven is on.


    This discussion is going no where, I have quoted the regulation 433.1 from the Wiring Regulations word for word, the refusal to accept that twenty five minutes can be considered a relevant duration is merely trying to justify the unjustifiable.


    The comments about motor circuits is also irrelevant as many motors have restricted duty cycles, for example if you repeatedly start a borehole water pump in a short period of time the motor itself will overheat and the thermal cut out within will operate, hence you need to set the water storage tank up so that the pump will start, run for a few minutes, then shut down and not restart again to soon to avoid it over heating, which also reduces the stress on the cable.


    As I said, this discussion is going no where.


    Andy Betteridge 

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    Okay. Bravado grandstanding and hysterical outbursts seem to be the order of the times; a deflection away from an initial outrageous statement, taking the narrative to a wholly unrelated tangent with equally bombastic claims. I suppose it clouds the issues sufficiently for the message spouted by the most loud and most brash to be the headline for the casual observer.


    The simple issue was an existing circuit. A shower circuit. It had already been installed, presumably fit for purpose. The Ib was assessed when it was installed diversity applied, that formed the type/rating of OCPD and the cable. So In>=Ib.

    Someone wants to use that circuit for something else on the basis that the shower is unlikely to be used.


    Salami diversity should not be applied to a circuit that has already had diversity applied to it.
    has anybody mentioned the D word?


    Put another way, you have two 40 A showers, and a 40 A cooker on a 100 A supply. Does that break the rules?

     



    The above is diversity applied to the whole installation and is perfectly normal. It is a whole different thing to again apply diversity  to one of the above shower circuits, again, in order to justify adding another load to it. That is Salami Style.


    We are in the realms of BS7671 but some are using methods outside this remit, specifically ESCQR, in an attempt to underpin some dubious benefit of Badger work in a BS7671 context.

     
    By example take the cables in your street. They are probably protected by a 600A fuse and feed many houses. You may have a 10mm2 service if you have an old house (I used to some years back). A short on the service cable can be quite spectacular because the short circuit rating is quite suspect at a 5kA fault. If each house takes power and it exceeds about 1kA for a reasonable period the 600A fuse will blow, but it takes time. The street cable will not really be warm. If they all have off peak heating and 800A flows all night (not unusal) the cable will be pretty hot by morning. Is our little used shower a problem, very unlikely.

     





    The point is the vast majority of this DNO equipment is not inside the house. Differing application, differing risk and very much different methods to control the risk.


    Do not confuse risk with only  being something that results in danger. With respect to efficacy, that needs to be also evaluated. Again the circuit was originally installed to have a realistic lifespan surely? Exceeding the In of an MCB multiple times is eventually going to result in its failure and it needs to be replaced. Fit for its intended purpose for a reasonable period of time is the usual expectation.


    Conflating the objections to simultaneous usage as charges it is " dangerous" practice is misleading. I expect one form of extremism generates the equal an opposite extreme response.


    The OP said


    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?



     


     




    Some simple solutions wrt BS7671 were given.


    Then someone who should know better, actually states

     
    What is wrong with simultaneous use, it will trip the MCB at some point, strangely that is what the MCB is for!!!!

     





    It is a circuit breaker, not a load limiter.

    If, purely just for example, the shower was 11kW with a 32 amp OCPD, then Ib >In. Would that be a fit for purpose arrangement? Would it be fit and proper to use the MCB as a load limiter, restricting shower use to a few minutes before it trips?


    The some undeserving electrician has the temerity to say, without genuflecting first

    The screwfix forum says
    433.1.1 [i]   the rated current of the protective device is not less than the design current.


    It is one thing to say something is not going to be used. It is still there and could be used. The kitchen fitter is being responsible; why should they take on additional risk?

    The answer by Broadgage concerning "Shower priority unit" is a legitimate way to go. For legitimate, read sensible.

    Circuit breaker as a "load limiter" is a bit rough; for the cavalier it may be a badge of honour , but really should have no place for installation work done for reward and done to some kind of standard.





    Some want a "trial by EICR code".  If Ib is demonstrably > In,  it is then recommend improvement.  You start with a shower circuit where In is equal to or just below Ib,  then you add a double oven, then suddenly Ib > In.


    You are going to engage in installation works; adding to a circuit. The expectation is that, at a minimum, you are not going to leave things  worse than when you stated. Creating a "recommend improvement" out of a situation that was originally compliant with BS7671 really is egg on the face territory.

     

    Say I have a large motor circuit say 100 kW which is designed to start a few times per hour. I use a type D (MCCB but this is to help you) CPD. Do the cables need to be rated at 6 times the running current? If so which regulation are you quoting? Just some background this motor may take a minute (60 seconds) to reach FLC due to the load inertia. What size cables should I use? How do I control the number of starts per hour? How is this different to the example I gave or the OP?




    "How is this different to the example I gave or the OP?"  Answer:  You are not going to add a double oven to it later on.


    David, your discussions on diversity and how it should be applied by Application have merit, but not in context of this OP. Some right words, but the wrong discussion.  That said though, the general thrust seems to be a attempt at besmirching the integrity of professional electricians. That is not welcome and hints at an ivory tower mentality.

     

     


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    No it is not what you say, it is the fact that this is perfectly permitted, and as I commented the same as any domestic  power final circuit where one has no idea what loads may be presented. The customer has told you the conditions of use which are clear and the chance of a trip very small. No problem. The problem is the attitude which you present, "I know this is not ideal so I will prevent it without any valid reason". That is the problem which you do not see. If you examined many other installations you would know that this kind of circuit is common (not domestic I grant you) and is not in any way risky or anything else. The question is as I first posed, what is wrong with it?  The regulations answer "nothing". So why the fuss. You say I besmirch the integrity of professional electricians, well if you are wrong that is your problem, you are wrong!!! I would happily wire another 100kW motor in parallel with the one I discussed, but if you try to use it you will trip the CPD. Your problem. Obviously I would not do so if they are both required together, but that is up to the user to define as in the OP. As long as my OPD was set correctly the design is perfectly fine.
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    The circuit protective device to a 100 kW motor circuit does not have to provide overload protection to the circuit cable, just fault protection.


    The overload protection is provided by the motor starter.


    The arrangement you describe seems perfectly reasonable in a situation where there are two motors that one will not be run simultaneously with the other, for example one is a backup water pump that can be used whilst the other is out of action.


    So long as there is a method of working that stops both motors being run at the same time, which could be as simple as having a lock off isolator which is moved from one motor isolator to the other as they are swapped over in and out of service, there’s absolutely no problem at all and as such equipment would be supervised by competent people there’s no problem at all with the arrangement.


    If I turned up and wired a new shower up in a garage conversion by piggybacking it off a cooker switch on the other side of the wall in the kitchen you are saying that is a perfectly acceptable design and installation, so long as the customer knows they cannot use them both at the same time.


    There is not really much to say, is there?


    Andy Betteridge 




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