Cooker supply

[whistler]

currently have electric oven and gas hob but looking to replace with new electric oven and hob.

Existing oven is 4.6kw fed by 6mm cable with 20amp mcb.

The cable is 2metres in plaster and approx 15metres loose laid in loft over insulation.

The new appliances will be rated 3.8kw and 7kw, so at max could take 45amps.

As I understand 6mm cable is rated 46amps, would it be ok to just upgrade mcb to 50amp?

I guess the alternative solution would be to use the existing supply for the 7kw hob and install a second 20a supply for the oven.

 

[JohnW2]

The new appliances will be rated 3.8kw and 7kw, so at max could take 45amps.

The concept of diversity (which takes into account the fact that not all bits of a cooker will be drawing power simultaneously for significant periods) allows that cooking appliances with a maximum total load of 45A can be considered as 20.5A.

As I understand 6mm cable is rated 46amps, would it be ok to just upgrade mcb to 50amp?

Because of the above, a 32A MCB is normal for cooker circuits - all the way up to a total maximum load of about 19 kW!

6mm² cable protected by a 32A MCB would be fine for your oven and hob.

Kind Regards, John

 

[Taylortwocities]

As I understand 6mm cable is rated 46amps, would it be ok to just upgrade mcb to 50amp?

The answer to that one is no. There are several considerations
1. 50 amps is more than 46 amps!
2. An assumed 46amps for a 6mm² cable is in ideal conditions. Your cable run may need it to be derated.
3. The use of a higher value MCB also depends on the earth fault impedence test value. If the numbers dont add up then the MCB wont trip, but the cable could overheat.
Any change to the value of an MCB needs doing by someone who is qualified to make the decisions, but item 1. above is the party-pooper.

But as John says,above, it'll be fine as it is.

 

[whistler]

Thanks for the replies. The 50 amp was a typo, I had actually meant to write 40amp, but based on your answers a 32 amp would perhaps be more appropriate?

Just have to tell the missus to go easy with the Xmas lunch!

 

[JohnW2]

Thanks for the replies. The 50 amp was a typo, I had actually meant to write 40amp, but based on your answers a 32 amp would perhaps be more appropriate?

As a said, 32A would certainly be adequate, and 'normal'.

Kind Regards, John

 

[JohnW2]

But as John says,above, it'll be fine as it is.

Not quite "as it is" (with a 20A MCB), since even after diversity the load is probably (fractionally) more than 20A. However, it would be fine with a ('standard') 32A MCB, subject to your points (2) and (3) (which will almost certainly be satisfied with 6mm² cable, installed as the OP has described).

Kind Regards, John

 

[Taylortwocities]

Not quite "as it is" (with a 20A MCB),

Oh yes, apologies. Really should be a 32A MCB, just to cover the turkey + pudding scenario....

 

[Iggifer]

Assuming it is 6mm² and not 4mm². I'm aware 4mm² is good for 32A clipped direct, but any derating factors and that brings you back to 6mm². There may be a good reason it's on a 20A MCB

 

[EFLImpudence]

I'm aware 4mm² is good for 32A clipped direct,

37A

but any derating factors and that brings you back to 6mm². There may be a good reason it's on a 20A MCB

I had that thought but - OP says not.

 

[Iggifer]

37A


I had that thought but - OP says not.

I know it's more than 32A! I was just talking in terms of MCB sizes

 

[JohnW2]

There may be a good reason it's on a 20A MCB

There may be a good reason, but there may also be a fairly 'bad reason' - in the form of the oven manufacturer's instructions! At least whoever installed it apparently had the sense to put in 6mm² cable, with the future in mind!

Kind Regards, John

 

[whistler]

Just to clarify, there used to be an electric oven and hob supplied from an old fuse box and I think it was fused at 30amp. I had the hob changed to gas and a couple of years ago had a new consumer unit fitted. The electrician checked the oven rating 4.6kW and put it on a 20a mcb. His report confirms that it's 6mm cable.

Currently 4.6kW, 6mm cable, 20a mcb
Proposed 11kW, 6mm cable, 32a mcb.

 

[JohnW2]

Just to clarify, there used to be an electric oven and hob supplied from an old fuse box and I think it was fused at 30amp. I had the hob changed to gas and a couple of years ago had a new consumer unit fitted. The electrician checked the oven rating 4.6kW and put it on a 20a mcb. His report confirms that it's 6mm cable.

Fair enough

Currently 4.6kW, 6mm cable, 20a mcb ... Proposed 11kW, 6mm cable, 32a mcb.

Indeed.

Kind Regards, John

 

[securespark]

Just have to tell the missus to go easy with the Xmas lunch!

Tell her to sit back and you'll take the strain this year.

Then nip out and get a Chinese...