Two high-power appliances on a single 40A RCD

davezawadi:

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.

Crack on in your own home if you believe that, but I would not recommend doing it in anyone else’s home.


I definitely won’t do it for a customer.


 Andy Betteridge

I really am quite disappointed Gentlemen. No one has really understood the situation properly have you?

OK you could install a new circuit, but the point is "is it necessary?". All of the above posts have failed to understand what "overload protection" really means, and seem to assume that overload should be eliminated by design, that is more final circuits! Overload already has a device provided to prevent damage to the installation and that is the CPD. Andy has tried to turn this into "catastrophic damasge to the electrical installation". This is more than foolish, it is outright dishonest. An MCB trip is catastrophic eh, that sounds like a threat to me to extract money? It is certainly very unprofessional.


There is a way to do this which is perfectly permitted. The downside is that a trip might occur if the customer uses all the appliances at once. This is his choice, it is not up to you, you should explain carefully what might happen and why. From the OP he has no intention of using the full load at once so any tripping would simply a warning that he has forgotten the choice. All the nosense that it is in some way not safe is fantastic and untrue. Some of you need to get a grip, because many people are very suspiciousof trades already and you are making this worse.


For my motor example the cables may be chosen on rating at FLC, although some extra cross section may be required by starting volt drop which make start cycles much slower and may cause motor stall. Overload protection would be by phase loss relay and suitable time delay setting and current on the MCCB. The final circuit can be overloaded but has suitable protection. Why is the phase loss relay so important? There is a time diversity on the start cycles and this will limit temperature rise to a manageable level. If starts were at a rate of one per minute with the 30 seconds start time we would need to take the average current into account in the cable rating, not the peak value because the average describes the overall heating effect. Apply that to the OP and you will get exactly my answer.


Regards

David

You still can’t beat installing a new circuit.


A bit of imagination usually allows doing so to be relatively straightforward, I installed a new cooker circuit a couple of weeks ago running externally down the side of a house, then in and along underneath the kitchen cabinets in steel wire armoured cable to an isolator then flex to the freestanding cooker. There are no marks or anything to see inside the house.


Whipping carpets and floors up to pull cables in is daily work, unless you have covered the upper floors in laminate flooring or tiles it is generally straightforward, though currently I am being paid to drive all over England and Wales to do it, but that’s more of a matter of getting someone in the right place at the right time than because it is complex work.


In amongst all discussion this no one has asked it there is an existing electric cooker circuit in the kitchen that could be utilised.


Andy Betteridge

I have not disagreed with Zoomup ?


Andy Betteridge

cfcman dont expect an answer on this forum. However look it at another way if you were to install a seperate circuit for the cooker you may well need to inform your supplier of the additional  load, but again this very really happens, an electrician will come along bang in the circuit bill you and go. What you are propossing would seem to be a good idea as there would be no additional load added to the system. Where installations have electric wet system boilers and a shower installed a shower priority switch needs to be installed.

Gosh, what a lot of postings since I looked in a few hours ago! ?


What is the design load? From the sound of it, it is not the shower plus the stove - it is the shower or the stove. The householder knows that. However, an emergency shower for when the gas is off does seem to be a great luxury.


I accept that there is a difference between intended maximum load and possible maximum load, and I also appreciate that some form of switch (change over or priority) unequivocally is compliant. However, has anybody mentioned the D word?


Put another way, you have two 40 A showers, and a 40 A cooker on a 100 A supply. Does that break the rules?


My apologies to the OP if this technical discussion is unhelpful. ?