Two high-power appliances on a single 40A RCD

Oh dear we are all becoming far too agitated. The Ib is not greater than In for this circuit is it, there is no intention to use both cooker and shower at the same time. I have pointed out that this is no different to any domestic final power circuit. The cable is protected fully against overload. Nothing bad is going to happen anyway, and the circuit is fine as far as BS7671 is concerned.


Then we get to the bit where the moral attitudes of "I know best" come in. The MCB may wear out if the customer is careless etc. It may but would then fail to contact rather than weld up (as you should know). This criticism may be applied to any circuit breaker, presumably in your installations of those on ring or radial power circuits, or lighting with filament lamps. Then we get to the "we must prevent both being used at once even by accident". Why is this, everything is under control and the only outcome is a possible MCB trip, so what? Then I am attacked for "unprofessional" something or the other just because you have not quite understood what BS7671 says. I give examples and no one likes them. This circuit is designed to power either the shower or the cooker, not both! I do know BS7671 rather well, I teach electricians to pass the wee 18th test very successfully. I am however often surprised by the lack of knowledge of what it really says, rather than even looking up the correct answers. The regulations operate in the same way as UK law, the law saying what you cannot do rather than what you can, just like BS7671. It may give some hints of what you can do but these are in the appendixes at the back or in the notes to various regulations.


I pointed out that changing a CPD to a different greater value is notifiable under part P, You are changing the circuit to a new one by definition, you have changed its design and rating. You ask what code this should be given, and I would say none, in that it is compliant with BS7671 as you have all found out by now, although it may not be something you have seen before. Saying that is dangerous or likely to fail in some way fails to understand its usage, and is not accurate or true as I have pointed out. Long term slight overloads are a risk for circuit designs but you need to understand exactly what that means, and the interpretation being given by me for this circuit explains what you need to examine. These are the circuit loss, thermal time constant, installation method and real terms loading etc. so not just a yes / no question. The slight overload is not 1% overcurrent, it is enough to cause significant extra cable heating, and needs to take account of the fusing factor, so perhaps 30 - 40% or more. The diversity on a circuit includes all loads, and the times they are likely to be operational, which were explained in the OP.


I think that everything has been explained several times now by me, and useful comments from some others. Use this as a thinking exercise, it is useful because you have probably never considered this kind of thing before, and it is the process necessary to design a more complex installation.

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. ?

 

• 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....

But that is kind of the point, is it not?  


Is that a deflection or a cloud (sic)?

Its a female aardvark

Again, what diversity can you apply to a shower?


Regards


BOD

You can apply it to a final circuit with more than one load, if appropriate. The shower is the only thing on the circuit, so that is the final circuit current demand. So my banana....diversity is not a consideration. The design current Ib is the current demand of the only point of use. But before it is suggested that by adding another point of use, diversity could then be applied to this shower and the connected load and, by engineers magic, Ib remains the same, that can only be so in a controlled environment. A domestic house is not that, as, no disrespect to the OP, that environment can be subjected to misuse. So a changeover system is required

Is the existing oven gas or electric?


Is the existing oven supplied by a plug from socket circuit or goes it have its own circuit, if it foes have its own circuit what size is the cable and the protective device got it in the consumer unit?


Exactly what oven have you bought?


 Andy Betteridge 

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.


Andy Betteridge