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Two high-power appliances on a single 40A RCD
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?
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.
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.
