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Two high-power appliances on a single 40A RCD

Former Community Member over 5 years ago

I have an electric shower installed on a 40A RCD, in a room adjacent to my kitchen. The shower is only used in an emergency - i.e. when our gas boiler is unable to provide hot water to our main bathroom. I would like to take a spur from this 40A connection to use for a new double oven, which is rated at 32A. Can anyone advise on a safe and legal way to do this, ensuring that only one of the two appliances can be connected at any one time?

0 Alcomax over 5 years ago

davezawadi:

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?

The OP specifically said

Can anyone advise on a safe and legal way to do this, ensuring that only one of the two appliances can be connected at any one time?

Having Ib>In is not proper design. It seems the OP suspects this. David, you are making a lot of assumptions about this existing circuit . Giving advice that it is perfectly okay to increase Ib of an existing circuit is dancing on the cutting edge of worst practice. As I have stated in the past, the regs requirements have a bit of headroom built in for a reason. This over-engineering allows some margin for error for the the whole life service of the circuit. This is not a temporary arrangement that can take a bit of abuse for a few weeks, then on to the next venue. This is in someones home or business.
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0 Former Community Member over 5 years ago

Interesting debate! To further clarify, in case of any doubt, the circuit in question has no sockets. It is a hard-wired connection which was installed specifically for the electric shower, 10 years ago. The proposed solution would add a 2nd hard-wired connection, for the oven (with a switch to ensure that only one can use the circuit at any one time).
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0 David Stone over 5 years ago

This is interesting.

None of you have yet found a regulation number which is in opposition to my suggestion. BOD is being BAD because he doesn't have one either. However I will continue the discussion. Say I have a large motor circuit say 100 kW which is designed to start a few times per hour. I use a type D (MCCB but this is to help you) CPD. Do the cables need to be rated at 6 times the running current? If so which regulation are you quoting? Just some background this motor may take a minute (60 seconds) to reach FLC due to the load inertia. What size cables should I use? How do I control the number of starts per hour? How is this different to the example I gave or the OP?

And this may be a valuable training exercise. I will discuss further tomorrow.

Regards

David Stone CEng MIEE.
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0 Sparkingchip over 5 years ago

perspicacious:
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

As I have already pointed out the existing circuit may not have overload protection.

How can you rely on non-existent overload protection?

There are an awful lot of assumptions being made to save a few quid.

Andy Betteridge
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0 Sparkingchip over 5 years ago

David.

Regards the motor, will the circuit protective device will provide the fault protection and the motor starter will provide the overload protection?

Andy Betteridge
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0 Former Community Member over 5 years ago

None of you have yet found a regulation number which is in opposition to my suggestion. BOD is being BAD because he doesn't have one either.

Crikey, that was a very quick swerve there, to quick to even use indicators when being presumptive...............

314 gives a broad hint...............

Regards

BED
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0 Former Community Member over 5 years ago

None of you have yet found a regulation number which is in opposition to my suggestion. BOD is being BAD

If I were to be BAD, I would have asked if none is singular or plural, and use of "have".........

Sweet dreams

Regards

BED properly now!
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0 Sparkingchip over 5 years ago

I'm not typing it out at this time on a Friday night.

Andy Betteridge
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0 Sparkingchip over 5 years ago

perspicacious:
None of you have yet found a regulation number which is in opposition to my suggestion. BOD is being BAD because he doesn't have one either.

Crikey, that was a very quick swerve there, to quick to even use indicators when being presumptive...............

314 gives a broad hint...............

Regards

BED

314.3 tells you where to start looking.

Andy Betteridge
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0 Alcomax over 5 years ago

cfcman:

Interesting debate! To further clarify, in case of any doubt, the circuit in question has no sockets. It is a hard-wired connection which was installed specifically for the electric shower, 10 years ago. The proposed solution would add a 2nd hard-wired connection, for the oven (with a switch to ensure that only one can use the circuit at any one time).

..something to ensure that they cannot be used simultaneously is fine.

To David, the reg number is a few pages back.

This is an existing circuit. A shower circuit. Not relevant how often it is used. I It was designed " as is". So Ib has been decided before any inkling of adding to it. Your motor example is a different application and environment.

To add another load increases Ib by dint.

It does,however raise another issues of the bar for safe for continued service as opposed to compliance with BS 7671.

You could, for example, install to your own judgement, ignore BS7671 and bang test it. It works (up to a point). It could then be subjected to trial by EICR code , have a load of C3's and be found "satisfactory"......essentially avoiding BS7671.

Do you think that is a good thing? Your margin for error is gone or reduced. How does this not adversely affect the process as being fit for purpose? It is worth repeating; this is someone's house.
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Two high-power appliances on a single 40A RCD

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