Cooker circuit, diversity, etc

[jcp]

Do I need a new cable/mcb for my new cooker? I hadn’t considered this when I bought it!

The cooker seems to be rated as 12.8 kw (at least, that number with no other explanation is on a sticky label on the back of the machine, along with a second label saying main oven alone 2.7 kw). The cooker manual gives the 4 hotplates as 1.2kw, 2.2kw, 2.2kw, 1.8kw (7.4kw in total), but oven power is only given as 0.66 and 0.74 kw/h. The manufacturer’s helpdesk is less than forthcoming about the fuse size needed, but say they used to specify 45A, but now leave it to the installer to decide.

The cooker is essentially a like for like replacement of an older cooker (4-rings, oven and grill). The cooker outlet is supplied by a 6mm cable from 32A mcb on an MEM consumer unit (house built c1990, mcb probably 321 QEB, not checked). The cable is about 6m long, and runs alone through ceiling joists and down behind dry lining (not in conduit) to the cooker unit (without socket).

So my specific questions are:

1. Assuming the second oven/grill is also 2.7 kw, the total maximum cooker load is indeed 12.8 kw (rings 7.4ks, ovens 5.4kw) – or 56A . However, as I understand it, allowing for diversity could cut this to 10 +0.3*46 = 23.8A . So would the existing 32A fuse and cable combination would be OK?

2. Surely, just doing a like for like replacement would be safe, even if the mcb were to trip, the cable would be protected.

3. Given the cable length and fixings, its allowable current could be increased to 47A, so would it be OK to change the mcb to 40 or even 45A?

4. Upgrading to a 10mm cable and 45A mcb would be quite disruptive, and presumably a new consumer unit would be recommended. Is all this really necessary just to replace my cooker?

Any advice would be much appreciated

 

[flameport]

The cooker outlet is supplied by a 6mm cable from 32A mcb

That will be suitable for more than 99% of cookers, including yours.
You don't need to change anything.

 

[PrenticeBoyofDerry]

1) If the appliance is rated at 12.8kW
By applying diversity you would have
(55.65-10)*0.3= 13.7A
13.7+10=23.7A
So even if you had a socket outlet on your cooker control unit, adding an extra 5A to the equation, you will still have less than 32Amps. So a 32A breaker will be okay.

2a) Like for Like? was your previous appliance, configured and rated the same as your new one? If not, it is not like for like.
2b) The cable would be protected, but your food wont be getting cooked, if the circuit was always tripping.

3) Your 6.00mm T&E cable has a maximum current rating of 47A, using reference method C and the route of the cable as you have described, it does not require any de-rating factors to be applied and voltage drop is within the required range.
So there is no reason why a 40A protective device can not be used.

4) It is not true that a new CU would be recommended, if you installed a new circuit. The new circuit would have to comply to the relevant standards, but that does not automatically mean an upgrade is necessary to your existing Consumer Unit.

Editted do to error in calcs.

 

[jcp]

Thanks for your comments - they have put my mind at better ease! I should perhaps have come clean that the cooker is a wedding present for my daughter, and I don't want to have landed her with a large fitting bill.

"Like for like" is an interesting concept - a more modern but relatively basic cooker for an older basic cooker is arguably like for like, but the new cooker does probably have a higher quoted load.

On the matter of replacing consumer units, I have heard of people having to do this - any change to an old system and the whole system had to be upgraded to the current regs.

Thanks again for your comments

 

[PrenticeBoyofDerry]

On the matter of replacing consumer units, I have heard of people having to do this - any change to an old system and the whole system had to be upgraded to the current regs.

You may have heard that this has been done, but that does not mean it needed to be done.
There may be cases where it was the practical and logical method to choose.
But in you instance, the only circuit that need apply to the new requirements, would be the cooker circuit, as this would be getting altered or replaced for new. Then this circuit must comply, but there are various installation methods that can be used for compliance. So the changing of the old CU would not always be needed.

"Like for like" is an interesting concept - a more modern but relatively basic cooker for an older basic cooker is arguably like for like, but the new cooker does probably have a higher quoted load.

If the output power is different to the appliance it is replacing, it can not be considered to be like for like. But again that does not always mean that it is not suitable for the circuit it is to be used on, as your situation has proved.

 

[ban-all-sheds]

On the matter of replacing consumer units, I have heard of people having to do this - any change to an old system and the whole system had to be upgraded to the current regs.

The electrical trade is no different from any other in having people in it who favour equine transportation.

You should read this: //www.diynot.com/forums/viewtopic.php?t=288450

 

[riveralt]

1) If the appliance is rated at 12.8kW
By applying diversity at 30% of FL plus 10 Amps you would have
(55.65*0.3) + 10= 26.7A, so even if you had a socket outlet on your cooker control unit, adding an extra 5A to the equation, you will still have less than 32Amps. So a 32A breaker will be okay.

Pst!! You might like to check your figures again.

12800w / 230 = 55.65 Amps
Apply diversity, which is the first 10 Amps of the rated current plus 30% of the remainder of the rated current plus 5Amps if a socket outlet is incorporated in the control unit.

55.65- 10 = 44.65 *.30 = 13.7 + 10Amps plus 5Amps (if a socket is included) = 28.7Amps

 

[PrenticeBoyofDerry]

12800/230=55.65
55.65-10=45.65
45.65*0.3=13.69
13.69+10+5=28.7A
Thank you riveralt for correcting my calculation, a little
I'll edit my post so no one gets misled by it.

 

[ajrobb]

A 32A domestic circuit is recommended to supply up to 15kW cooker appliances and a socket.

If you want to give them a useful present, get them a Period Inspection Report on the house wiring and upgrade their consumer unit to 17th edition.

 

[Tigger90]

Could someone work out what I would need please?
I am looking at getting a rangecooker, its 15kw, at the moment I have a 6mm cable going up the wall, along ceiling joists and down to the consumer unit. There is also switch on the wall to the right of the current cooker. Its all new electrics as the house was renovated 2 years ago.

Just dont fancy replacing the whole lot.... but I would like a new cooker!!

 

[EFLImpudence]

What you have will be ok for 15kw.

It would be better if there is not a socket incorporated in the cooker switch but, even with, it is just under the limit.

 

[Tigger90]

I meant to say the switch is just for the cooker, it does not have a socket for any other appliances.

 

[PrenticeBoyofDerry]

What size protective device (MCB/RCBO) have you on this circuit?
32A will be okay for a 15kW Output.
Your cable can carry 47A
So could still have a 45A breaker on it, if you wanted.