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2 single ovens and an induction hob?

acalmenvoy

Will the existing wiring ( Cooker switch to 32amp fuse in fuse box) be enough or we talking major rewiring, both ovens are 3.5KW each and the hob is 7.4KW total connected load?

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2011

That is too much for one circuit. The hob alone is about right for a 32A MCB.

acalmenvoy

2011 wrote: »

That is too much for one circuit. The hob alone is about right for a 32A MCB.

Thanks for the reply, would the diversity factor be applicable here?

2011

acalmenvoy wrote: »

Thanks for the reply, would the diversity factor be applicable here?

It would but you are still looking for too much.

Risteard81

It's actually likely fine. 15kW of appliances can go on a 32A circuit owing to diversity. I have an oven and 7kW induction hob on a 32A circuit with 6mm^2 cable here. It is fully functional, safe and compliant. I probably wouldn't add a second oven though personally.

3d4life

Expect cooking complexites on Christmas day if everything off one breaker.

Risteard81

3d4life wrote: »

Expect cooking complexites on Christmas day if everything off one breaker.

That's an imagined problem which simply doesn't happen in the real world. Diversity works.

Lenar3556

Risteard81 wrote: »

That's an imagined problem which simply doesn't happen in the real world. Diversity works.

It does when you are dealing with a diverse enough loading profile. But a bit of a stretch in this case. He is talking about over 60 amps of cooking there. Is it a house? If there is need for two ovens it follows that they are both expected to be on at the same time. And it’s not unreasonable to expect to have some hob going along side them.

YellowBucket

With cooking appliances you'll definitely get scenarios where most of the load could be drawn simultaneously, even if it's only christmas day or some dinner party, you'll find someone won't be too happy if their MCB trips in the middle of some complicated dish.

We have ours on 2 x 32A circuits.

GreeBo

Once its an induction hob I would always give it its own circuit.
When you can join zones for larger pots etc its just not worth it.

Maybe my house is different but it would be rare that the oven and hob wouldnt be on at the same time.

Cerco

GreeBo wrote: »

Once its an induction hob I would always give it its own circuit.
When you can join zones for larger pots etc its just not worth it.

Maybe my house is different but it would be rare that the oven and hob wouldnt be on at the same time.

Maybe, but I can imagine a regular scenario of the vegetables on the hob with the roast in the oven. Probably two rings in use. But then many roast their veg so it depends. Personally I would go for the two circuit option. It’s a once off cost for lasting peace of mind.

Risteard81

YellowBucket wrote: »

With cooking appliances you'll definitely get scenarios where most of the load could be drawn simultaneously, even if it's only christmas day or some dinner party, you'll find someone won't be too happy if their MCB trips in the middle of some complicated dish.

We have ours on 2 x 32A circuits.

It won't be drawn simultaneously as the thermostats will cycle the elements on and off

2011

Risteard81 wrote: »

It's actually likely fine. 15kW of appliances can go on a 32A circuit owing to diversity. I have an oven and 7kW induction hob on a 32A circuit with 6mm^2 cable here. It is fully functional, safe and compliant. I probably wouldn't add a second oven though personally.

Just because you have not had an issue with your setup does not mean it is acceptable.
As already stated I accept that diversity can be applied to some extent but this is a bridge too far. Let’s look at the maths as it is quiet simple:

2 ovens switched on from cold = 6 kW is 26 amps at unruly power factor before a stat even kicks in. If I switch on just one ring of the job (assuming 4 rings of equal rating) that is another 8 amps at unity power factor. This is a total of 34 amps which exceeds the rating of the MCB which is against the regulations.

How likely is it that someone would switch on 2 ovens and one ring from cold? Very likely in my opinion. Would you get away with it? Most likely yes but it is against best practice and I would strongly advise against it. It is best to comply fully with the National Wiring Rules.

Naturally if the power factor is not perfect (it generally isn’t) or more rings are switched on with the 2 ovens the current could be far higher.

2011

Risteard81 wrote: »

It won't be drawn simultaneously as the thermostats will cycle the elements on and off

This does not apply when started cold. Stats only switch off when the set point is achieved.

Risteard81

2011 wrote: »

Just because you have not had an issue with your setup does not mean it is acceptable.
As already stated I accept that diversity can be applied to some extent but this is a bridge too far. Let’s look at the maths as it is quiet simple:

2 ovens switched on from cold = 6 kW is 26 amps at unruly power factor before a stat even kicks in. If I switch on just one ring of the job (assuming 4 rings of equal rating) that is another 8 amps at unity power factor. This is a total of 34 amps which exceeds the rating of the MCB which is against the regulations.

How likely is it that someone would switch on 2 ovens and one ring from cold? Very likely in my opinion. Would you get away with it? Most likely yes but it is against best practice and I would strongly advise against it. It is best to comply fully with the National Wiring Rules.

Not only is diversity acceptable - it is essential for economic design. It is fully compliant with the Wiring Rules. Why would you think otherwise?

Risteard81

2011 wrote: »

This does not apply when started cold. Stats only switch off when the set point is achieved.

For a few minutes only. Time is an important factor when designing.

It is fine and completely compliant to have an oven and induction hob sharing a 32A cooker circuit. That is an irrefutable fact. 15kW of cooking load is the maximum for a 32A cooker circuit owing to diversity. Fact.

2011

Risteard81 wrote: »

Not only is diversity acceptable

Agreed and already stated. However it can only be applied to a point. I have shown why I think the rating of the MCB would be exceeded under quite unremarkable circumstances. How about you show how it won’t be?

It is fully compliant with the Wiring Rules. Why would you think otherwise?

Simple, the design current under very normal conditions exceeds the rating of the protective device.

I only applied 25% loading for the hob which is very generous in terms of diversity and it was still an issue.

Risteard81

2011 wrote: »

Agreed and already stated. However it can only be applied to a point. I have shown why I think the rating of the MCB would be exceeded under quite unremarkable circumstances. How about you show how it won’t be?

Simple, the design current under very normal conditions exceeds the rating of the protective device.

I only applied 25% loading for the hob which is very generous in terms of diversity and it was still an issue.

The design current doesn't exceed the rating of the protective device. It is the total connected load which does. Incidentally these are often quoted for elements which cannot even be simultaneously energised. I'm at work so will prove you wrong with quotes from published books on the matter later.

GreeBo

Risteard81 wrote: »

The design current doesn't exceed the rating of the protective device. It is the total connected load which does. Incidentally these are often quoted for elements which cannot even be simultaneously energised. I'm at work so will prove you wrong with quotes from published books on the matter later.

Modern Induction hobs can have multiple if not all zones on at once.
I have seen hobs rated at over 7kw.

3d4life

YellowBucket wrote: »

With cooking appliances you'll definitely get scenarios where most of the load could be drawn simultaneously, even if it's only christmas day or some dinner party, you'll find someone won't be too happy if their MCB trips in the middle of some complicated dish...

Exactly.

A single circuit design is not doing any favours to anyone. So why do it ?

( of course its fine if your work phone is turned off on Christmas day

)

2011

acalmenvoy wrote: »

Will the existing wiring ( Cooker switch to 32amp fuse in fuse box) be enough or we talking major rewiring, both ovens are 3.5KW each and the hob is 7.4KW total connected load?

3.5 + 3.5 + 7.4 = 14.4 kW

Risteard81 wrote: »

Not only is diversity acceptable - it is essential for economic design. It is fully compliant with the Wiring Rules. Why would you think otherwise?

I agree 100% which is why I stated this before you posted on this thread.
So let's apply a very generous 0.6 diversity factor.

14.4 x 0.6 = 8.64 kW = 8640 W

Therefore at unity power factor (best case scenario) 8640/230 = 37.6 amps. If we are to take this as the design current it can be seen that it exceeds the rating of the MCB (32A) as such it fails to comply with the National Wiring Rules.

If you plan on demonstrating that both ovens ad hob can be fed from a single 32A MCB you will have to apply a much lower diversity factor which in my opinion is inadvisable. However anyone doing this would need to be able to justify it with numbers rather than simply stating it is ok as a "fact".

Risteard81

From the "Electrical Installation Design Guide (3rd Edition) - Calculations for Electricians and Designers", published by the Institution of Engineering and Technology:

"Example 2: cooker circuit
See row 5 of Table 3.1

Consider an electric cooker with:

* hob comprising 4 off 3kW elements
* main oven 2kW
* grill/top oven 2kW

Therefore the total installed capacity = 16kW at 240V.

As in example 1, I=(1000P/Uo cos Theta) and cos Theta = 1 for a resistive load

Hence, I = ((1000 * 16) / (240 * 1)) = 67A

From Table 3.1, row 5 circuit load is:

'The first 10A of the rated current plus 30% of the remainder of the rated current plus 5A if a socket-outlet is incorporated in the control unit.'

Therefore I = 10 + (30 (67-10)) /100 = 27.1A

Hence, a 30 or 32A circuit is selected."

The reality is that this works and has for decades - even on Christmas Day, and even with an induction hob.

2011

Risteard81 wrote: »

From the "Electrical Installation Design Guide (3rd Edition) - Calculations for Electricians and Designers", published by the Institution of Engineering and Technology:

"Example 2: cooker circuit
See row 5 of Table 3.1

Consider an electric cooker with:

* hob comprising 4 off 3kW elements
* main oven 2kW
* grill/top oven 2kW

See the part in bold? That diversity is applied to a single unit.
The OP has 3 units. So it would be 10 amps per unit + 30% of the remainder.

Besides, the above diversity equation was in an earlier version of ET101 and the ETCI decided to remove it well over 10 years ago (with good reason). As you know we do some things differently in the ROI than they do in UK.
Now that we have powerful induction hobs applying these levels of diversity would not be a good idea.

As the OP has decided to have 2 ovens we have to assume that they will be used simultaneously from cold unless we have a very good reason to think otherwise.
When switched on together from cold it is a 6 kW load straight away (over 26 amps). Your assertion that the rating of the MCB will only be exceeded for “a few minutes only” (which you seem to have abandoned) is incorrect as ovens can take 25 minutes to preheat.

Fact: Your proposal is to apply a calculation intended for one unit to three units, that is from another jurisdiction, which was very deliberately removed from our own national rules a very long time ago to support your position.

Bruthal

Risteard81 wrote: »

this works and has for decades.

So had twin and earth with smaller cpc, no earthing on light fittings, 3 or 4 fuses for a house, no rcds on sockets, and the list goes on.

acalmenvoy

I should give more information, fascinating discussion by the way. This is really a vanity project. The wife wants her dream kitchen. It will probably be our last. All but 1 of the kids have moved away.
The top oven is a glorified microwave. I can see the oven part never being used. Grill and microwave will be used occasionally.
Bottom oven will be used as conventional oven, grill part will never be used.
I could never see more than 3 zones on the hob being used.
Yes, Christmas day would be interesting.
That being said, from the advice on this thread, I have a guy calling next week to price a new 32a circuit.
Peace of mind is priceless.

Risteard81

You are very wrong @2011 - the IET have explicitly stated that a separate oven and hob can be treated the same as a single cooker. Just admit you made a mistake.

Millions of installations have this setup without adverse effect - even on Christmas Day. You're not infallible.

YellowBucket

Decades ago most cookers were also single appliances and had their loads fairly tightly calculated to be within the range of a single classic “cooker circuit”.

Very large cookers like the wider “stove” type devices often sat on a 45amp circuit

They were designed as single units though and the manufacturers managed the overall max load.

Some even had interlocks to prevent certain very high draw combinations being selected. Actually, most hobs will prevent you exceeding the maximum ratings as they will turn down the output on other zones if you apply boost etc

These days cooking appliances are much more diverse and the vast majority of kitchens, certainly since the 1990s use separate ovens and hobs.

Ovens now often have self clean options and quite high consumption rapid heat modes and induction hobs have very high consumption boost modes.

Over all these appliances are more efficient - ovens are much better thermally insulated for example and induction hobs manage to get more heat into the pots and less into the air / wasted through thermal radiation, but they are all far more likely to have big initial power needs during rapid heating cycles.

There also needs to be consideration given for the fact that kitchens may see new appliances go in with some of those functions in the future.

To me running one circuit to a kitchen for those is just asking for inconvenience for the sake of saving a relatively minor bit of work. The wiring will in all likelihood follow the same route back to the distribution board and it’s also practical to be able to isolate the ovens and hob separately.

I know we had a situation where someone applied that diversity concept to our old utility room, which contained a dryer, washing machine and dishwasher.

We replaced a washing machine with a Miele which must have had a bigger element and it resulted in the breaker tripping if the washing machine and dishwasher both hit heating cycles at the same time.

Overall both newer appliances were much more efficient than their predecessors - less water and power used but they both were capable of pulling a lot more energy for brief periods as they ran through their programmes.

The other aspect is these are all continental designs and they will be assumed to be installed on separate radials to the manufacturer spec. Ireland and the U.K. are both outliers on this by not doing so.

2011

acalmenvoy wrote: »

I should give more information, fascinating discussion by the way. This is really a vanity project. The wife wants her dream kitchen. It will probably be our last. All but 1 of the kids have moved away.
The top oven is a glorified microwave. I can see the oven part never being used. Grill and microwave will be used occasionally.
Bottom oven will be used as conventional oven, grill part will never be used.
I could never see more than 3 zones on the hob being used.
Yes, Christmas day would be interesting.
That being said, from the advice on this thread, I have a guy calling next week to price a new 32a circuit.
Peace of mind is priceless.

The part in bold is the part of your post you should focus on.

Risteard81 wrote: »

the IET have explicitly stated that a separate oven and hob can be treated the same as a single cooker.

You have ignored the following facts:
- The IET are a foreign organisation. In Italy they install sockets in bathrooms, should we also be guided by their regulations?
- The ETCI saw fit to remove this calculation from our rules a very long time ago. Should we ignore this? Was that a mistake?
- Ovens can take 25 minutes to preheat

Millions of installations have this setup without adverse effect - even on Christmas Day.

Already addressed here.

2011

Just to add I think the IET are a great bunch of lads, they presented me with a cash prize and certificate as a design award many years ago

3d4life

2011 wrote: »

.....The IET are a foreign organisation. In Italy they install sockets in bathrooms....

Good for them ... the natural home of the lavatrice is in the bathroom ( in apartments anyway )

( Probably star wired dedicated protection anyway)

Terminator.

With 2 ovens and a hob

I'd be inclined to treat the 2nd oven as a separate appliance needing a separate connection