Two high-power appliances on a single 40A RCD

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

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

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!).

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

Shower circuits may not be protected against overload.


If you overload a shower circuit by connecting a cooking appliance to it, the cable may well overheat and the insulation may fail.


Andy Betteridge 

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.

Not relevant. The topic is fixed appliance on a final circuit , not a ring or radial socket circuit.


Which regulation says you can connect more loads than the CPD rating?   I cannot find one, even on Mumsnet.

 

Please list the regulation number you are citing immediately for the basis of your advice.

Should have gone to specsavers


 

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

 

The  "idea" seems to be to have Ib>In

Is that proper information to give ?


Remind me again of this Forum catagory.................................................or has this thread been moved to Savoy Place Virtual Club?

Andy

Consider carefully the OP and the real circuit. It has a 40A CPD and if installed correctly a 10mm2 cable. But lets assume it has a 6mm cable buried in a wall or somewhere clipped direct. The shower is obviously satisfactory and probably takes 37A. The oven could potentially take another 16A or so and you are screaming (or actually Alcomax) that the cable could melt. Could it? A 40 A breaker actually trips quickly at at least 52A so our customer should never get a trip. The circuit has 3 loads with diversity, the shower probably 15 mins max then nothing (it is used occasionally as a gas standby) and 2 ovens of perhaps 4kW total  maximum. The ovens heat up then are on and off for a few minutes at a time. The cable will never reach 70C conductor temperature, in fact have you ever seen a melted PVC cable in the fixed installation? So you have a simple diversity problem and there is no particular danger. The risk of real cable melting needs overcurrent operation at 2x rating for a long time, and that is hours. I do understand that diversity is poorly understood but giving advice is tricky and there are reasonable limits to what one should say. "Its not allowed by the regs" requires solid proof and understanding of the whole BYB, in many ways it is not absolute in the way that many think. This is particularly true of intermittent loads and diversity, and also CPD ratings and circuit configurations. The danger is almost all from long term fairly small overloads, which is not true in this instance.


By example take the cables in your street. They are probably protected by a 600A fuse and feed many houses. You may have a 10mm2 service if you have an old house (I used to some years back). A short on the service cable can be quite spectacular because the short circuit rating is quite suspect at a 5kA fault. If each house takes power and it exceeds about 1kA for a reasonable period the 600A fuse will blow, but it takes time. The street cable will not really be warm. If they all have off peak heating and 800A flows all night (not unusal) the cable will be pretty hot by morning. Is our little used shower a problem, very unlikely.


I'm afraid things are not quite as simple as the OSG.

Regards

David

Alcomax


See the post to Andy.

That Ib is potentially larger than In is not in itself a problem but perhaps slightly unusual. In this case I suspect that it is entirely satisfactory for the reasons in the OP. The regulations are all limitations to what you can do. This circuit is exactly the same as any other final circuit, it is subject to protection against overload for long periods. There is no regulation simply because it is not prohibited. Surely you have been taught that at some point. Perhaps this is the reason why electrical designers and installation assessors need to be somewhat skilled?

David.


So if the oven is connected to a 32-amp socket ring circuit every will be okay, so long as everyone knows they cannot was use any portable appliances supplied by sockets on the same circuit at the same time?


Are you actually saying it is okay to connect the oven to any circuit that has a circuit protective device rated at 32-amps or more?


I ask because I have seen some catastrophic failures of cables and equipment due to people having done things like that.


 Andy Betteridge

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.


On carrying out an EICR and finding this, how would you code it everyone?


Regards


BOD