Electric Oven Install

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Richard Moth

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I have just ripped out my old kitchen and in the process off installing a new one. We have chosen to install an electric built under doulbe oven. The oven says it requires 20 amps, in the process of installing the new electrical cables the electrican installed 6mm cable for the oven on it own mcb in the consumer unit. This mcb is rated at 40 amps. So my question is do I need to get the mcb changed to a lower rated mcb or can I leave it connected to the 40 amp mcb?
 
The oven will operate quite happily as it is. The MCB is there to protect the cable ,are you sure its 6 mm not 10 mm ? It would be a bit unusual for the electrician to choose a 40 amp MCB ,when the load on the circuit is just your oven ( drawing less than 20 amps),what else is on the circuit ? A 32 amp MCB to protect the 6 mm cable would be more common ,but depending on the installation method and cable length a 6 mm cable could safely carry more than 40 amps.
 
Hi Terry

Thank you for your reply, the cable is 6mm. We re-wired all but the kitchen about 10 years ago, at the time the electrican connected the old cooker circuit to the 40amp mcb, as the old cooker connection also had a plug socket, with the kitchen re-fit we have re-wired the kitchen as well now so the electrics are safe. The electrican that worked in the kitchen last week just re connected the circuits to the mcb's that was already used. At the time I did say to the electrican the oven only needs 20 amp's. So for the case of about 3 quid for a new mcb I would prefer to be safe then sorry. That's why I was asking about changing the mcb, the cable run is about 10 meters at max to the dp cooker isolator which is rated to 45amps and then down to the cooker outlet which is also rated to 45amps.
 
There is no reason why you can't have a 32 amp MCB fitted to the cooker circuit ,if it will put your mind at rest. The fact that your previous cooker circuit also had a socket outlet still wouldn't require a 40 amp MCB.
 
Once you change the MCB the circuit will need a complete test and an EIC
 
terryplumb said:
The MCB is there to protect the cable ,are you sure its 6 mm not 10 mm ? It would be a bit unusual for the electrician to choose a 40 amp MCB ,when the load on the circuit is just your oven ( drawing less than 20 amps),what else is on the circuit ? A 32 amp MCB to protect the 6 mm cable would be more common
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That's just the legacy of BS3036 fuses.

There is no reason why a 40A MCB should not be on 6mm² - method C of course.
 
terryplumb said:
It would be a bit unusual for the electrician to choose a 40 amp MCB
Click to expand...

The electrician might have reasoned that the homeowner might at any time decide to install an electric cooker, or even an electric range, and that this is much easier if it just needs to be wired into the existing wall outlet.
 
Hi All

Thank you all for the advice, I thought the reason the mcb should be sized correctly is to protect the cable to over current if the oven become faulty. If I leave the oven on the 40 amp mcb the oven could draw to much and cause a fire? I might be wrong here.
 
You are wrong.

The MCB is sized correctly. That is - less than the 47A the cable can safely handle (method C, clipped to surface or buried in masonry).
The 40A MCB protects the cable.

What you put on the end of it will have its own internal protection if it needs it - boilers, TVs etc.

Ovens do not as they cannot draw more current than they normally need for their normal operating.
 
EFLImpudence said:
That's just the legacy of BS3036 fuses.

There is no reason why a 40A MCB should not be on 6mm² - method C of course.
Click to expand...
Quite so. We have no idea what installation method has been used, there is no reason why a 32A MCB should not be protecting it .
 
JohnD said:
The electrician might have reasoned that the homeowner might at any time decide to install an electric cooker, or even an electric range, and that this is much easier if it just needs to be wired into the existing wall outlet.
Click to expand...
Maybe John ,if he was that forward thinking . more likely the 40A MCB was already in the CU ,he has installed the new cable and there is no issue.
 
Hi All

Thank you all for the advice. I have just got home and checked the cu and they are type b mcb's, the cu is a Wylex and the mcb's are NSBxx. Not sure what the original electrican used the 40 amp as there is 1 32amp and 1 16amp mcb's spare.

So going back to my original question do I get the 40amp mcb replaced with a 20 amp mcb or just changed over to the spare 32amp? Or am I ok to leave it as it is?

The installation instructions just says 4mm cable from the connector box, 4400 - 4800 watts.
 
Richard Moth said:
So going back to my original question do I get the 40amp mcb replaced with a 20 amp mcb or just changed over to the spare 32amp? Or am I ok to leave it as it is?
Click to expand...
It is fine as it is.

The installation instructions just says 4mm cable from the connector box, 4400 - 4800 watts.
Click to expand...
Actually for 20A, 2.5mm² cable and 20A MCB would have been adequate - but then you would be complaining if you wanted to have a bigger oven or to add a hob.

You have a circuit that is good for any cooking appliances you could possible buy.
 
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