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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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    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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  • 0

    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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