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Discuss Max demand for double oven and hob in the Electrical Forum area at ElectriciansForums.net

R

Ray Pooley

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Just some clarification please.

Max demand for cooking devices.

OSG says 10A + 30% of "connected cooking appliances in excess of 10A + 5A" if a socket is available in the isolator. The oven and hob are two devices. Does the 15A apply to both of them or just the first one?

eg: Oven 4.8 kW Hob 5.5 kW. With a socket available.

15A + (((0.3 x 4.8) + (0.3 x 5.5))/230) = 28.4A

or

15A + ((0.3 x 4.8)/230) + 15A + ((0.3 x 5.5)/230) = 43.43A

Thanks in advance
 
Aico 3000
Pete999

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Just some clarification please.

Max demand for cooking devices.

OSG says 10A + 30% of "connected cooking appliances in excess of 10A + 5A" if a socket is available in the isolator. The oven and hob are two devices. Does the 15A apply to both of them or just the first one?

eg: Oven 4.8 kW Hob 5.5 kW. With a socket available.

15A + (((0.3 x 4.8) + (0.3 x 5.5))/230) = 28.4A

or

15A + ((0.3 x 4.8)/230) + 15A + ((0.3 x 5.5)/230) = 43.43A

Thanks in advance
Good question, not sure will ask around, Personally I would say the hob and the Oven are qne cooking device, some of the active members may be able o provide an answer.
 
Andy78

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First and foremost I'd refer to connection instructions for both appliances. I'd personally want a circuit for each of those appliances.
 
R

Ray Pooley

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First and foremost I'd refer to connection instructions for both appliances. I'd personally want a circuit for each of those appliances.
So a seperate MCB for each? Not unreasonable but I have never seen it. Friend of mine has just had an old house completely rebuilt and rewired. Including new kitchen. Oven and hob are connected to the one B32. Treating them as one device only comes to 29A with 11kW full load.
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Good question, not sure will ask around, Personally I would say the hob and the Oven are qne cooking device, some of the active members may be able o provide an answer.
Treating them as one device only comes to 29A with 11kW full load. So that woud be fine.
 
Andy78

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So a seperate MCB for each? Not unreasonable but I have never seen it. Friend of mine has just had an old house completely rebuilt and rewired. Including new kitchen. Oven and hob are connected to the one B32. Treating them as one device only comes to 29A with 11kW full load.
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Treating them as one device only comes to 29A with 11kW full load. So that woud be fine.
Yes I always try and divide the installation as much as possible. I have been known to add 6 new circuits on a kitchen fit.
It may be fine diversity wise. You can get a double outlet plate if doing this. The manufacturer's instructions for each appliance may say to connect to a dedicated 32A circuit though so worth checking them out and taking that into account in your design.
 
R

Ray Pooley

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Yes I always try and divide the installation as much as possible. I have been known to add 6 new circuits on a kitchen fit.
It may be fine diversity wise. You can get a double outlet plate if doing this. The manufacturer's instructions for each appliance may say to connect to a dedicated 32A circuit though so worth checking them out and taking that into account in your design.
A totally get that to be honest. When I first encountered the diversity rules I found the probability aspect of the diversity calcs a little uncomfortable but I understand why it's done that way. It takes into account the intermittent nature of thermostatic controls. It keeps the costs down and costs are important at the end of the day. I know of one socket circuit in a commercial kitchen that has 16 13 amp outlets on a B32.
 
Pretty Mouth

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I would add together the ratings of all cooking appliances on the circuit, and apply diversity to that. It's exactly the same as if it was just one combined oven and hob. The example you give would go comfortably on one circuit.

There's a good JW youtube on this:
View: https://www.youtube.com/watch?v=QQduU8RkjD8
 
happyhippydad

happyhippydad

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I would be happy to have both appliances on one MCB (32A) with one isolation switch (as long as both oven and hob are close enough to each other to maintain local isolation) and a dual cooker outlet plate.

As for diversity, I would treat them separately. It doesn't make sense to me to treat them as one item as they are not. If one item is on full there is no limiting device to make sure the other is not. However, if there is only one isolation switch with a socket outlet then the additional 5A (with regards diversity) is only added once.

Oven = 4.8kW = 21A.
Diversity = 10A + (0.3x11A) = 13.3A

Hob
= 5.5kW = 24A.
Diversity = 10A + (0.3 x 14A) = 14.2A

max demand = 13.3A + 14.2A + 5A(socket) = 32.5A

I realise the max demand is 0.5A over the 32A MCB but as I have rounded up the initial current for both appliances I would say it's OK. You could fit a 40A MCB assuming using 6mm cable and acceptable reference method.
 
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telectrix

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as above comments. up to 15kW cooking appliance on a 32A circuit is fine has been for donkeys years. wrt Andy78's post, putting them on separate circuits is a good idea though, purely for the fact that if 1 appliance has a fault and needs to be isolated, the other one will still be usable. depending on which one can still be used, you have a choice of oven chips or chip pan chips.
 
happyhippydad

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In addition....

Ps.. Well done with all your correct, yet perhaps OTT brackets in your OP :D

Pps.. I'm not quite sure of your maths though.

Remember the 30% is only taken from the 'excess' current.

e.g if the oven is 4.8kW = 21A, the 30% is only taken on 11A of that as you have already taken off 10A as a fixed amount. So diversity for that one appliance is the first 10A + (30% of 11A) = 13.3A
 
R

Ray Pooley

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  • Thread Starter Thread Starter
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In addition....

Ps.. Well done with all your correct, yet perhaps OTT brackets in your OP :D

Pps.. I'm not quite sure of your maths though.

Remember the 30% is only taken from the 'excess' current.

e.g if the oven is 4.8kW = 21A, the 30% is only taken on 11A of that as you have already taken off 10A as a fixed amount. So diversity for that one appliance is the first 10A + (30% of 11A) = 13.3A
Thanks for the response.

Brackets are a great way to get around BODMAS. Makes life easier. For me at least.

Ok. "in excess of". I didn't factor that in. That's true. I forgot to do that. But I think you'll find that the maths is fine in my calcs given the values I have used.

This post does seem to illustrate that there can be a variation in this calculation depending upon how you view the arrangement. My original question was about how you interpret the rule. The one appliance vs the two appliance perspective.

Anyway, whatever comes out of the calculation I think if I get up on Christmas day, put a pan of vegetables on each of the hob plates, a turkey in the oven and turn everything on the actual demand is going to be about 45A for about 5 minutes. Nevertheless the existing B32 should be ok. It's class B so it trips at a minimum of 96A.
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In case anyone is wondering, I have an electric oven and a gas hob. I am pulling the gas hob out and replacing it with an electric hob. Hence the question.
 

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