Two high-power appliances on a single 40A RCD

Hmmm....yes I can see what you're saying. I guess it's the distinction between "sensible" and "legal" that should have been made clear to me. I have no problem with a tradesperson telling me that something is not advisable, or something that they are not comfortable with. I do have a problem if they have incorrectly advised me that it is "illegal".

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.

45A splitter  look at the pic of one wired up. Designed to  fit on top of  standard or deep single socket box sunk into the wall. One in through the back box , two out on the surface.

Most commonly seen behind cookers to feed hob and oven  from common cable. Do not put it in the bath /shower room !!

If you do find yourself tripping the breaker more than very occasionally then you will need to re-think and invest in 2 cable runs all the way to the board.

On reflection he was pretty forthright and inflexible regarding other aspects of the work (i.e. the kitchen fitting stuff) - opting only for the priciest options, and insisting that other options were not viable. My instinct is that he was perhaps not the best person to have been advising me on this. Disappointing, as I found him via the British Gas "Local Heroes" site.

Ah OK so when he said he wasn't "legally" able to do it what he maybe should have mentioned that this was due to a limitation of his professional certification, rather than being an absolute legal constraint. So basically I was talking to the wrong type of tradesperson?

It is not illegal. It may be out of his comfort zone, which is fair enough, but then he should be honest about it. Be aware that wiring in bathroom zones (i.e within 60cm of the bath or shower cubicle ) requires notification under part P, and he may not  be keen to get involved in that, or he may be worried that he gets called back if something trips.

If it was my house, and I know who will use it, I'd parallel them up and have a variation on the single line working token system for Victorian trains, - perhaps when the bar of soap is in the kitchen it is OK to cook, when someone takes it into the shower it isn't.  Or a label on the shower for visitors - "check is anyone cooking ? Do not overload the trip !" Actually cooking is not as thirsty as folk often imagine - once initial heating is over the thermostat pings on and off and the average current is quite low. It is a cold start that is the problem.

TBH I would be quite happy with simultaneous use, as the shower is not used from one year to the next (and only when our boiler breaks down), but the kitchen fitter told me that connecting the oven and the shower on the same circuit would be illegal for him to do.

???

What is wrong with simultaneous use, it will trip the MCB at some point, strangely that is what the MCB is for!!!! Belt and braces efforts at design are not required. Kitchen fitter does not have a clue! I wonder which regulation that is? Answers on a postcard to the Screwfix forum.

Yes it's an MK Electric 7937S which is an MCB/Residual Current Device I believe? OK so I would have the MCB changeover switch installed in the kitchen as a replacement for the existing on/off wall switch for the supply to the electric shower?

I was told by a kitchen fitter that he was not legally able to put two high power devices on the same circuit, so presumably this would address that issue?