isolators and 20A protection

[johnsville]

Hi, a built-in oven, consumes 3.6 kW. It needs 20A protection. It is small enough to be taken off the ring main in the kitchen. Running a cable for it back to the CU is impractical, a massive job, so off the ring it has to go. there is enough capacity on the ring.

At a push a small 2 way Wylex CU taken off the ring, with a 20A mcb, in an adjacent cupboard can be fitted. I would rather not though. There are 40 amp DP isolators available that can go over the worktop in standard metal boxes, which is what I would rather have. But how do I get the 20A protection. Fuses only go up to 13A, a so a fused spur switch is out.
My prime question: are there any dedicated isolators with 20A fused protected available that fit into a normal recessed metal box?

Another point. This metal Wylex 2Way CU. Can it be flush mounted with the front plate flush to the wall?
http://www.screwfix.com/p/wylex-2-way-metal-main-switch-consumer-unit/61591
Screwfix Product Code: 61591

Thanks in advance

 

[bernardgreen]

I do not know if your idea is permitted by BS7671 guidelines.

But operationally it is a bad idea.

A correctly designed and installed ring circuit has a maximum design load of 7.36 Kwatt ( 230 volts and 32amp MCB)

The oven and a 3 Kwatt kettle on at the same time will overload the ring ciruit. The MCB won't trip immediately as that is only a small overload.

If you have fridges or freezers with motors on the same ring circuit the in rush current as the motor starts with oven and kettle on is very likely to trip the MCB

 

[ban-all-sheds]

Hi, a built-in oven, consumes 3.6 kW. It needs 20A protection. It is small enough to be taken off the ring main in the kitchen.

No.

Running a cable for it back to the CU is impractical, a massive job, so off the ring it has to go.

It may not.

It either has it's own circuit or where it goes is back to the shop.

there is enough capacity on the ring.

I'm afraid that doesn't matter - it's too large a single load. Let me guess - it's made by a company based in mainland Europe.

At a push a small 2 way Wylex CU taken off the ring, with a 20A mcb, in an adjacent cupboard can be fitted.

You may not have a CU on a ring final circuit.

I would rather not though. There are 40 amp DP isolators available that can go over the worktop in standard metal boxes, which is what I would rather have. But how do I get the 20A protection.

You can't, on a ring final.

You need a new circuit for it.

My prime question: are there any dedicated isolators with 20A fused protected available that fit into a normal recessed metal box?

No, and even if they were you would not be allowed to have one on a ring final circuit.

Another point. This metal Wylex 2Way CU. Can it be flush mounted with the front plate flush to the wall?
http://www.screwfix.com/p/wylex-2-way-metal-main-switch-consumer-unit/61591[/QUOTE]
There are flush-mount CUs available - I don't know if there's such an option for the 2-way Wylex, or indeed any 2-way ones. Probably not.

Don't forget that new circuits and CUs and a lot of work in kitchens are all notifiable.

 

[johnsville]

Thanks. My calcs ware that a 3kW kettle and the oven will draw just under 29A.

I see many rings with 40A mcbs at the CU. And some with 32A. I assume a ring can up uprated to 40A using 2.5mm cable. Is that so?

thanks

 

[ban-all-sheds]

I see many rings with 40A mcbs at the CU.

No you don't.

I assume a ring can up uprated to 40A using 2.5mm cable. Is that so?

No it is not.

 

[johnsville]

thanks sheds, yes, it is a Miele.

You may not have a CU on a ring final circuit.

I find that odd, as a 2 way CU on a ring is just an isolator with an mcb, no more.

A pro will sign all this off. I want to know the most efficient and cheapest way of doing it. I really do not fancy ripping the house apart, which will be a "massive" job.

 

[ban-all-sheds]

thanks sheds, yes, it is a Miele.

Not designed for the UK market, with it's ring final oddity.

I find that odd, as a 2 way CU on a ring is just an isolator with an mcb, no more.

Ring final circuits are very restricted on what they may serve.

A pro will sign all this off.

No he won't.

i.e. no reputable electrician will come along and "sign off" work done by someone else, even if it doesn't contain blatant errors like CUs on ring finals.

I want to know the most efficient and cheapest way of doing it. I really do not fancy ripping the house apart, which will be a "massive" job.

Then you need to return the oven and get one which is a smaller electrical load.

 

[johnsville]

A pro will sign all this off.

No he won't.

Sheds, he will. I haven't finalized what is to be done yet, hence asking here for views.

A 3 kilowatt kettle can be off a ring, and this oven is only 600W over that.

 

[bernardgreen]

Thanks. My calcs ware that a 3kW kettle and the oven will draw just under 29A.

Correct for equipment rated at 230 volts. But if the supply voltage goes up to the maximum permitted 253 volts ( 230 +10% ) then the current into the resistive elements will increase to 31.5 amps worst case.

The current at which an MCB will operate varies on time and ambient temperature as well as manufacturing tolerances and it may operate at a current 10% below its rated current flows for a few minutes. Therefore it is not sensible to run a circuit at the full rated current of the MCB protecting it if relaibility of supply is necessary.

I see many rings with 40A mcbs at the CU. And some with 32A. I assume a ring can up uprated to 40A using 2.5mm cable. Is that so?

You may have seen them. Like I have seen cars being driven without MOT's and/or dangerous modifications.

But I suspect you have seen 40A MCBs feeding circuits that use cable larger than 2.5 mm CSA ( cross sectional area )

 

[ban-all-sheds]

A pro will sign all this off.

No he won't.

Sheds, he will.

Well - if you have found a dangerous criminal idiot prepared to falsify electrical installation certificates then why not just go ahead and do WTF you like instead of wasting everybody's time here.

You could usefully use the time you save to think about what you're going to do if that cowboy gets caught and they start going back over his history of fraudulent certifications.

A 3 kilowatt kettle can be off a ring, and this oven is only 600W over that.

Over is over.

It's too large for a ring final.

 

[johnsville]

A pro will sign all this off.

No he won't.

Sheds, he will.

Well - if you have found a dangerous criminal idiot prepared to falsify electrical installation certificates then why not just go ahead and do WTF you like instead of wasting everybody's time here.

You have an attitude problem. It is obvious what I am asking and doing. It will be done right.

 

[johnsville]

I am curious. If BS states that a CU cannot be off a ring, then how come I have seen garage CUs off a ring and run to the garage? The new house I once lived in had that. But they took it off a 13A fused spur and then had a 16A fuse in a garage CU. Dumb I know. It led me to believe they did it wrong.

 

[holmslaw]

'BAS', you have an attitude problem.

No No, he has an altitude problem, ie the rest of us are mere underlings.

You've been told by at least eight people that you're cooker/circuit proposal is unacceptable and potentially dangerous. Unfortunately, this good and expert advice does not suit you're incorrect assumptions based on your obvious lack of electrical knowledge.

Mind you don't get burnt, you're (literally) playing with fire.

I'm out.

 

[ban-all-sheds]

It is obvious what I am asking and doing.

Indeed it is.

It will be done right.

If you install a lash-up and get someone to fraudulently sign it off then that is not doing it right.

A CU on a ring final is not doing it right.

Doing it right is using an oven which is an OK load for a ring final, or using an electrician to install a new circuit.

I am curious. If BS states that a CU cannot be off a ring, then how come I have seen garage CUs off a ring and run to the garage? The new house I once lived in had that. But they took it off a 13A fused spur and then had a 16A fuse in a garage CU. Dumb I know. It led me to believe they did it wrong.

They did, insofar as the CU in the garage was utterly pointless.

You're not talking about putting a mini-CU for this oven onto a fuse spur though, are you.

 

[holmslaw]

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