First reaction to your question was to say no you need two separate circuits. I then wanted to follow up and say why. 433.1 does talk about a small overload of long duration and would stop the use of Shower units slightly bigger than the MCB protecting it. But I was unable to find a regulation that said you can’t do what you suggest.
Maybe I have just missed it the book is very big.
To allow the system to overload on a regular basis would of course be wrong as it would damage the MCB and there could be an issue if it overloads the RCD here we are looking at manufactures instructions as to the current which it can switch and the use of RCBO would be out as a result.
But if we look at the radial socket circuit then we can plug many 13 amp rated units into a 20 amp supply the only difference is we can only jump in increments of 13 amp so first overload stage would be 33 amp and it would trip before we could plug in any more devices, where with a 9.5Kw shower one would need a 50A MCB so adding a 8.5Kw shower at same time would give a 156% load in one foul swoop, but with radial example it would be max of 165% so if we can’t use two showers from one supply one must also consider the whole principle of multiple 13A sockets on a 20A supply!
I would prefer to see a change over switch or relay and of course and star/delta contactor is designed so only one contactor can be energised at one time but with the consideration that using two showers together would draw 78 amps and the whole supply to many houses is only 80 amp running two showers off a single 50A MCB may be better option than risking blowing the main incoming fuse?
If once in a blue moon one forgets and you run two together then the 50A MCB will protect even without a change over contactor or switch and since it is hoped this would be rare may be it is a reasonable option?
And it would need 16mm cable which has in the past been used for whole supply to house. At 57 amp rating it would be 137% loaded and at clipped direct it would not be overloaded.
But you may have problems getting 16mm cable into an 8.5Kw shower! And you will need to convince who ever is signing paper work that this is permitted.
I think if you asked me to do it without a change over relay I would refuse.
I will be interested to see if anyone can find a regulation to say you can’t do it.
As I said to start with goes against the grain but I can’t find a reason why if doing a PIR on a house with this done I could fail it?
I am sure I will be shot down in flames so I would wait and see the flack I get.
Sorry Eric, I can't tell you that you are wrong. Everything about this question indicates that there should be two circuits. However, I do not know of any regulation in BS7671 which prevents two items of fixed equipment being wired to the same circuit. Perhaps this would prove impractical. Your idea of change-over contactors is innovative but you would probably agree that it would be cheaper to install another consumer unit (I am guessing that the lack of a spare 'way' is the reason why this question was posed in the first place). Practical considerations would also rule this out: you mention the size of cable which could easily be 25mm sq.
Thank you glen miller I was wondering if I was missing the point. Glen Miller is missing was a good song from Rock follies! Although change over switches are made these do tend to be commercial in design and I would not want one in my house. Mind you neither would I want an electric shower as even the best only dribble compared with the one I use from the 27Kw hot water boiler. Gas of course. Had I still retained my cistern I could have installed a power shower even better hardly a need to use soap then the dirt is blasted off.
I have been surprised on how rare it is to blow the incoming fuse with induction hobs, electric showers and double ovens etc.
I even cook my Welsh rarebit in the micro wave using grill bit of course but I can't warm up oven grill and do a single slice in 6 min 3 mins per side that micro wave does it in!
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!