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# Oven and hob on same MCB and diversity.

Discuss Oven and hob on same MCB and diversity. in the Electrical Forum area at ElectriciansForums.net

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Arms
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I've just spent an hour getting bogged down with the maths trying to work out diversity for 2 cooking appliances on the same circuit (32A MCB) and same isolation switch,

I thought I would give my workings so others can use it as I wonder if some just assume you can still use the 15kW maximum, which you can't.

In fact the the maximum for 1 cooking appliance seems to be 19.3kW (as long as the switch doesn't incorporate a socket), however, on with 2 appliances....

Diversity is 10A plus 30% of the remaining amps (plus 5A if a socket) but this is for each appliance even if on the same circuit.

Therefore for an oven and electric hob the maximum current we have is:

[10A + ( 0.3 x Q) ] + [10A + (0.3 x Q) ] = 32A

we don't know the maximum size of the cooker and hob (that's what we're trying to find out), so I have just used the letter Q (Q is the current of either the hob or cooker minus the 10A)

Therefore

The above equation is the same as

2 x (10A + 0.3Q) = 32A

20A + 0.6Q = 32A

0.6Q = 12A

Q = 20A

We know that each appliance is allowed to draw a max current of Q +10A, which is 30A. There are 2 of them so 60A.

60A x 230w = 13.8kW (approx)

Therefore maximum combined power for 2 appliances on same circuit with same switch (without a single socket) is 13.8kW.

I think you'll all have had enough now, but the maths works out as a maximum combined power of 10.4kW allowed if the cooker switch incorporates a single socket. (Equation is 25A + 0.6Q = 32A)

If you haven't guessed. I have the day off work!

ps.. would you check the maths please @happysteve

#### Lucien Nunes

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Your numbers are correct but it is important not to overlook the significance of the 10A + 30% of remainder, otherwise you might end up following the letter of the regulations but missing their intention.

A single, self-contained cooker might offer the same functionality as an oven and a hob as two separate appliances. Assuming similar efficiency, the current consumption of any appliance to cook a given meal will be similar, i.e. it is the cooking activity that governs the load, not the layout of the rings and ovens. It would be weird then to say that 32A Ib is adequate to support the oven and hob of a cooker cooking as much as one can reasonably cook, but you need 40% more current if they are mounted separately into the kitchen units.

The glitch is that 10A. By splitting the appliance in half, a second baseline load of 10A has been allocated that is considered to be switched on all the time, that does not correspond to any extra cooking activity or elements.

Discuss.

#### Wilko

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Hi - I haven’t got my head around the calcs yet, but using OSG Table A2 line entry 3 Cooking Appliances, I get interpret the load after diversity as -
10 + 0.3*(FL Oven + FL Hob -10)

If I’ve understood correctly, this reflects LN’s point about not having a 2nd 10A allocation since it’s still one domestic kitchen.

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Esteemed
Arms
I can see what you mean Lucien. It doesn't make any sense to treat a combined oven and hob differently to a separate oven and hob as they will both be used the same (I think). However, I think you would still have treat the diversity as separate for both of them as the OSG specifically says 'individual' household installations.

The regulations can't cater for every single eventuality, so sometimes they don't quite make sense, but I don't think that is a reason to make your own judgement to change them as you do not have underlying knowledge of the calculations used to form them and you are choosing to change them based on the fact you think you are right. If you do change them, then at some point, perhaps 1 in a thousand, a problem will occur, whereas if you stuck to the regs it wouldn't. Of course, you may be right .

It's difficult to think of an example but perhaps if the combined oven and hob is changed for a standard oven and a hugely powered induction hob.

#### Lucien Nunes

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The regulations can't cater for every single eventuality, so sometimes they don't quite make sense,

Although not perfect, they usually make sense if applied appropriately, even if they don't specifically deal with the situation being considered. They are a design standard, not an instruction manual.

I don't think that is a reason to make your own judgement to change them as you do not have underlying knowledge of the calculations used to form them and you are choosing to change them based on the fact you think you are right

Would you prefer to follow a procedure that appears incorrect for a situation, or one that is more correct based on the laws of physics for which you have to accept responsibility?

As it happens, I cannot quote BS7671 verbatim on the subject of domestic cooker diversity, beyond 10A +30%. I've never installed a cooker and I don't do domestic work, so it's not high on my priorities list. But my laws of physics are the same ones that electricity follows, I know how thermostats and simmerstats and induction hobs work and the thermal characteristics of heating elements, ovens, pans of water etc, and enough about cooking to understand how the appliances are used in practice. So it wouldn't bother me very much to make my own assessment if I needed to.

perhaps if the combined oven and hob is changed for a standard oven and a hugely powered induction hob.

In that case you are changing the connected load, not the diversity calculation. Using the 10A + 30% rule, increasing the connected load will increase Ib but not in proportion, which is reasonable and that is what diversity is all about. Hence the idea of base load plus a fraction of the remainder, rather than a larger fraction of the total.

Cooker physics homework: Show that a large element working at low duty-cycle will cause a greater temperature rise in a given supply cable than a smaller element working at a higher duty-cycle delivering the same mean heat output. Assume that the voltage drop in the cable is a small fraction of the supply voltage and that the thermal time-constant of the cable is much greater than the period of the duty-cycle controller.

E2A my first sentence was badly phrased and I have changed it. I did not mean to imply that you personally might not intelligently apply the regs, rather the opposite.

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Arms
Would you prefer to follow a procedure that appears incorrect for a situation, or one that is more correct based on the laws of physics for which you have to accept responsibility?
If a procedure was clearly wrong then no, you wouldn't follow it, but in this case I don't think it's clearly wrong. I think it may be right, although I'm not sure. I'll expand on my example......
In that case you are changing the connected load, not the diversity calculation. Using the 10A + 30% rule, increasing the connected load will increase Ib but not in proportion, which is reasonable and that is what diversity is all about. Hence the idea of base load plus a fraction of the remainder, rather than a larger fraction of the total.
example expanded.....

The diversity applies to 'individual' cooking appliances. I'm not sure if there are single cooking appliances that contain a fairly low powered oven with a huge high powered induction hob. Therefore that type of cooking appliance would not have been catered for when working out the diversity calculation.

Also, would it make a difference how far apart they were (and still on the same circuit). If one was in a different room would you still feel they should be classed as one appliance? My point is that we'll have to make a judgement, and I wonder if the OSG diversity for cooking appliances is trying to avoid you making this judgement and just give a blanket calculation to avoid, or minimise the chance of overload.

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#### LastManOnline

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Although not perfect, they usually make sense if applied appropriately, even if they don't specifically deal with the situation being considered. They are a design standard, not an instruction manual.

Would you prefer to follow a procedure that appears incorrect for a situation, or one that is more correct based on the laws of physics for which you have to accept responsibility?

As it happens, I cannot quote BS7671 verbatim on the subject of domestic cooker diversity, beyond 10A +30%. I've never installed a cooker and I don't do domestic work, so it's not high on my priorities list. But my laws of physics are the same ones that electricity follows, I know how thermostats and simmerstats and induction hobs work and the thermal characteristics of heating elements, ovens, pans of water etc, and enough about cooking to understand how the appliances are used in practice. So it wouldn't bother me very much to make my own assessment if I needed to.

In that case you are changing the connected load, not the diversity calculation. Using the 10A + 30% rule, increasing the connected load will increase Ib but not in proportion, which is reasonable and that is what diversity is all about. Hence the idea of base load plus a fraction of the remainder, rather than a larger fraction of the total.

Cooker physics homework: Show that a large element working at low duty-cycle will cause a greater temperature rise in a given supply cable than a smaller element working at a higher duty-cycle delivering the same mean heat output. Assume that the voltage drop in the cable is a small fraction of the supply voltage and that the thermal time-constant of the cable is much greater than the period of the duty-cycle controller.

E2A my first sentence was badly phrased and I have changed it. I did not mean to imply that you personally might not intelligently apply the regs, rather the opposite.
Would I be correct in concluding you are in agreement with using the principle of "Common sense" though the regs may not refer to it specifically?

#### Lucien Nunes

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Reductio ad absurdum: If a 4-ring hob were divided into single hotplates each connected to an FCU and you treated them as separate appliances, you would have to allow over 40A for the hob alone, whereas 32A is sufficient for an entire cooker including the oven and grill.

Notwithstanding the issue dealt with in my homework question, a high-powered hob won't use much more power to cook the same meal as a low-powered one. Yes, you can cook more, not least because you can bring things to the boil faster etc, so there is a likelihood that more current will be used and this is reflected in the diversity equation.

A hob of 9.2kW connected load comes to 19A with diversity, one of 13.8kW comes to 25A. More current, because the hob can heat stuff more quickly, but not 50% more because it is very unlikely for all the extra available heat to be utilised by normal cookery.

If one was in a different room would you still feel they should be classed as one appliance?

No, because that would suggest that the manner of usage might be different. I am not arguing that cooking appliances in different rooms would count as one appliance simply because they share a circuit. Clearly, one person might be cooking a full meal on one and another person doing the same in the other room. I am arguing that the same elements fulfilling the same purposes as those of a single, conventional cooker in a domestic kitchen, will take approximately the same current even if they are physically arranged as two separate boxes, and therefore it is rather arbitrary to insist on using a different method to calculate their total design load.

Amongst the possible incidental relationships between appliance configuration and current that I considered, was that a user with more advanced cookery skills might both specify 'better' appliances and also use more current by being a more streamlined cook working with a greater number of dishes at once. But BS7671 does not rate appliances by 'goodness', only by loading, and if they specify a larger appliance then the design current will indeed be higher after diversity. FWIW a 'better' oven might have better thermal insulation, in which case it will use less current to cook the same meal, or might be bigger without using more current.

#### buzzlightyear

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Arms
most of the time not all elements will be on unless the chef is in a hurry.
With the new A rated appliance now in.
EU gistappo made up a rule trying to dicarage us from using so much power.
So us brits can go to work on a egg. Lol

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#### pc1966

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So long as the MCB provides overload protection for the cable, I would side with Lucian and say use the 10A + 30% of remainder of the combined appliances load as it is essentially one cooking station.

Even with a single cooker you can turn everything to max and draw the rated power and so exceed the diversity calcs, it is just no sane chef would be doing that...

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