Two high-power appliances on a single 40A RCD

David. 


When connecting a shower does the circuit protective device have to provide overload protection for the cable?


If only the shower is connected is it not possible that the circuit protective device could be rated at 40, 45 or 50 amps with a 6.0 mm twin and earth cable?


Andy Betteridge

Thank you Gentlemen, several here are in the area where I would take away any qualifications you have, or perhaps don't! Which regulation says that you may not connect more loads than the CPD rating? Actually none as you well know from ring and radial final circuits. If too much load is presented to the CPD it will trip in all cases. So which part of this is in any way a "DANGER", it is a potential inconvenience at worst. When will people realise that they don't understand what they are doing and go and be dustmen or something. You are giving FALSE advice to a member of the public who has come to the IET for proper information. There is nothing wrong with his idea, of all the loads in parallel and if he uses too many the CPD will operate SAFELY. Please list the regulation number you are citing immediately for the basis of your advice.


David Stone CEng MIET (and 2391 etc!).

The kitchen fitter didn't inspect the cable or existing switch (or even ask to see the consumer unit).

That sounds like an extremely high estimate of the cost of installing a new cooker circuit.


However it’s not actually the kitchen fitters fault or problem, it is more than reasonable for the kitchen fitter to say no leaving you arrange for a new connection to be provided for the appliance.


As of yet you have not actually said what size the shower cable is, it could easily be 6.0 mm twin and earth with a 8.5 kW shower connected to it.


So actually loading the cable with 40 amps could easily overload it and potentially the cable insulation could fail.


It is quite permissible to have a shower that cannot possibly overload a cable protected by a circuit protective device that is rated at more than the design current of the cable.


Just because it has a 40 amp MCB protecting the cable doesn’t mean that you can rely on the circuit breaker to stop the cable from melting, particularly with over sixty amps on it.


Get a new circuit installed or installed a changeover switch at the load end.


 Andy Betteridge

A little context....I live in a townhouse, The consumer unit is on the ground floor, at the front of the house. The kitchen and shower room are on the 1st floor, at the back of the house. Running a new circuit would involve pulling up floorboards and almost certainly some cutting into walls and subsequent redecoration. Hard to see it costing less than £1000. The oven that we want to install cost £400 (which is the whole reason for the job).

Look at it the other way around, if asked to connect a new 8.5 kW shower to a cooking appliance circuit what would you say?


I would say no.


Just pay to have a new circuit installed for the cooking appliances and do a decent job.


Andy Betteridge

I would refuse to do the job without additional equipment installed at the cooker and shower end of the circuit.


The kitchen fitter is being quite reasonable and the sensible thing to do is to install an additional complete new circuit for the cooking appliances.


Andy Betteridge.

I did actually suggest that some sort of 3-way switch (oven/shower/off) might be considered, but was told that this would not be "legal". 

Thanks for all the feedback - it's given me a much clearer idea of what I should be considering.

Kitchen fitter does not have a clue! I wonder which regulation that is? Answers on a postcard to the Screwfix forum.

That'll be regulation A.26 "A final sub-circuit having a rating exceeding 15 amperes shall not supply more than one point...."  - although you have go back to the 14th Ed for that - so not quite a current requirement!

  - Andy.