Is this the norm now days

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No I'm advocating not using 10mm², I would go for 6mm² for the hob which is good for the 48A full load.
Fair enough. Looking back, I see that's the case, I was (unnecessarily!) confused by ...
It may very well be that 3x 4mm² and 1x 6mm² is cheaper than 3x 2.5mm² and 1x 10mm²

I agree with your comment about loads not increasing however the trend for ever increasing appliances in the kitchen seems to continue which is where I'd suggest increasing the size of the cables for the ovens.
Who knows, but, as I implied, I think there will be an "ever increasing" desire to reduce, not increase, the total energy consumption of all appliances in kitchens (and elsewhere)
Don't I believe in diversity? Oooo... tricky... yes I do but... No way on earth would I ever put those three appliances on a single 20A circuit.
... which seems to be saying that you do not believe in diversity (at least, per the current standard calculations), isn't it? It sounds as if you are probably being influenced by the fact that, 'as it so happens', we are talking about three physically separate 'appliances', and I suspect that you might think a little differently if the OP's total ('maximum') of 8.9 kW was all in 'one appliance' (quite credible for a hob, and certainly a 'cooker')?
My view is diversity has outlived its intended design purpose .... I'm not convinced diversity was originally intended to apply to multiple appliances in the way that some happily do without due consideration to their usage.
I'm not sure that is necessarily the case. Diversity calculations don't really have that much impact until one gets up to quite high total ('maximum') loads and, as above, whether the total ('maximum') load relates to 'one box' or 'several boxes' is irrelevant.
Our cooker is rated at 11KW and on a 32A RCBO, a clamp meter has read >40A for periods of 1/2 hour or so which I'm not that comfortable with ...
That ("fir 1/2 hour or so") actually rather surprises me but, even if I experienced that, it would not cause me any 'discomfort'. Don't forget that a cable suitable for protection by a 32A OPD is deemed to be able to carry 46.4A for at least an hour (and lower currents for longer periods) without coming to any harm.

Although necessarily probabilistic (hence no certainties/'guarantees'), the concept of diversity nearly always works well. Were that not the case, then, for example, DNOs would be having constant problems with their LV distribution networks, with their cables 'melting' and their substation fuses blowing all the time :)

Kind Regards, John
 
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Who knows, but, as I implied, I think there will be an "ever increasing" desire to reduce, not increase, the total energy consumption of all appliances in kitchens (and elsewhere)
I'm also sure this is the case that appliances will become more efficient, however the trend for a long while is for more appliances, how many kitchens had 2 double ovens or 2 dishwashers or 2 microwaves or 6/7 ring hobs or hot/cold/boiling taps etc 20 years ago?
... which seems to be saying that you do not believe in diversity (at least, per the current standard calculations), isn't it? It sounds as if you are probably being influenced by the fact that, 'as it so happens', we are talking about three physically separate 'appliances', and I suspect that you might think a little differently if the OP's total ('maximum') of 8.9 kW was all in 'one appliance' (quite credible for a hob, and certainly a 'cooker')?
I have no problem per se with diversity (I use it all the time with some of the temporary systems I install) but the willy nilly way some apply diversity without any thought other than the figures quoted in the big book is quite scary.

A few years back 'we' were called in to a commercial kitchen for repeated tripping, the original installers had been back several times and confirmed all installed as per regs. However commercial Chefs don't read the regs and the useage of a commercial kitchen is so far outside what the regs assume cooking appliances are used for... but the fitters/electricians had applied diversity almost to the decimal point.
Not only did we do a fair amount of rewiring we also had to add a second submain to feed it.
I'm not sure that is necessarily the case. Diversity calculations don't really have that much impact until one gets up to quite high total ('maximum') loads and, as above, whether the total ('maximum') load relates to 'one box' or 'several boxes' is irrelevant.
Actually diversity figure sometimes kick out below the rating of just one of the parts of the system. One cooker unit in another commercial kitchen consisted of a 4KW 17A oven and two rectangular 1KW 4.2A warming plates. Please tell me what the diversity gives? (I have the picture of my notebook already taken for further discussion).
That ("fir 1/2 hour or so") actually rather surprises me but, even if I experienced that, it would not cause me any 'discomfort'. Don't forget that a cable suitable for protection by a 32A OPD is deemed to be able to carry 46.4A for at least an hour (and lower currents for longer periods) without coming to any harm.
A lot depends on what's being prepared.
Although necessarily probabilistic (hence no certainties/'guarantees'), the concept of diversity nearly always works well. Were that not the case, then, for example, DNOs would be having constant problems with their LV distribution networks, with their cables 'melting' and their substation fuses blowing all the time :)

Kind Regards, John
As said I have no issue with diversity per se and fully understand this comment. One of the temporaries I've provided on an annual basis from a 32A socket usually has two urns, two 4 slice toasters and a couple of kettles. They understand the problem and try to control what's running simultaneously but dropping in some tea cakes and clicking a kettle on is such an automatic thing to do when the red light on the urn is off.
 
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No one seems bothered about the waste of copper. I wonder how much it would be if everyone in the world used more than twice as much as was actually needed.
Many/most electrical installations probably already are!

I'm not sure whether I am one of those "no-one"s but that's primarily the issue which I was referring to when I talked about the (in)efficiency of multiple separate circuits for separate appliances 9ovens in this case), just as I feel similarly about multiple small-capacity radials (which may well involve more copper than a smaller number of larger-capacity circuits).

In some senses, Amd3 might be expected to improve that situation somewhat, since it opens up the possibility of 1.0 mm^21 16A radials- but I do wonder whether (for irrational 'historical'reasons) a fair number of people will be nervous/uncomfortable about 1.0 mm^2 "power circuits" for a long time to come?

Kind Regards, John
 
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I'm also sure this is the case that appliances will become more efficient, however the trend for a long while is for more appliances, how many kitchens had 2 double ovens or 2 dishwashers or 2 microwaves or 6/7 ring hobs or hot/cold/boiling taps etc 20 years ago?
Yes, but the other question to ask is how much more 'cooking' are people doing than they did 20 years ago, despite the plethora (and, in some cases, larger size) of the cooking appliances they now have. Unless people are being exceedingly 'inefficient', they should really be doing much more 'cooking' (using whatever appliances) than they did back then - and, quite apart from 'saving the planet', the last few weeks has probably made people even less inclined to use more 'cooking energy' than necessary!
I have no problem per se with diversity (I use it all the time with some of the temporary systems I install) but the willy nilly way some apply diversity without any thought other than the figures quoted in the big book is quite scary.
One either 'believes in' (are comfortable with) diversity (as we currently apply it) or not. Some may feel that "as currently applied' is not but, again, if they do they are not happy with the concept of diversity 'as it is currently applied'.
. Actually diversity figure sometimes kick out below the rating of just one of the parts of the system. One cooker unit in another commercial kitchen consisted of a 4KW 17A oven and two rectangular 1KW 4.2A warming plates. Please tell me what the diversity gives?
You seem to be overlooking the fact that we are talking primarily about domestic installations, in a DIY foruym.

Recommended diversity calculations are (rightly) very different in commercial kitchens, since the pattern of usage is very different and much more 'demanding' of energy. Per the OSG, for cooking appliances in a commercial kitchen, the diversity calculation is ...
OSG said:
100% f.l. of largest appliance +80% .... f.l. of second largest appliance +60% ... f.l. of remaining appliances
... so, for what you describe, would be 22.88 A (17A + {4.2A * 0.8} + {4.2A * 0.6}). not much less to the 'without diversity' total of 25.4A.

A lot depends on what's being prepared.
It does and, obviously 'how much is being prepared' (which is why, as above the diversity calculations are so different for commercial kitchens).

However, with any sort of cooking, after the initial brief period of 'getting up to temp', each individual heating element/whatever will, under thermostatic control, only be 'on' for fairly brief and fairly infrequent periods. To illustrate this as best as I can, it may be worthwhile wheeling out something I posted here a year or three ago, when I wrote ...

.... "I have no electric cookers, ovens or hobs, so, as I have reported before, I have undertaken experiments using a 1,700 W deep fat fryer, which I switched on from cold and ran for an hour or so at ‘chip frying temperature’. which is probably not a bad model of the likely behaviour of an oven. What I found is illustrated in the graph below."

[ Before someone asks, I should explain that the data plotted are averages over one minute, so the plotted points which are less that 1,700W are due to the element only being 'on' for a portion of the minute in question ]

As you can see, although the frying was 'on' continuously, and keeping the oil at the desired temp (similar to an oven maintaining its cavity ) temp), it was only consuming energy for brief (generally 1-2 minute) periods not all that frequently (roughly once every 10 minutes) - hence a pretty low 'duty ratio' (probably less than 20%).

xxx19may22.jpg

.
As said I have no issue with diversity per se and fully understand this comment. One of the temporaries I've provided on an annual basis from a 32A socket usually has two urns, two 4 slice toasters and a couple of kettles. They understand the problem and try to control what's running simultaneously but dropping in some tea cakes and clicking a kettle on is such an automatic thing to do when the red light on the urn is off.
Yes - but, as I've been trying to explain, an additional 'overload' of 13A or whatever for the couple of minutes it takes to boil a kettle or toast a tea cake is not actually going to do any harm to cables.

The system we use is really fairly 'fail safe' as far as cables are concerned - if the circuit is designed correctly, then the OPD will operate if the (magnitude and duration) current goes beyond what the cable is deemed to be able to carry without coming to harm.

Kind Regards, John
 
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Many/most electrical installations probably already are!

I'm not sure whether I am one of those "no-one"s but that's primarily the issue which I was referring to when I talked about the (in)efficiency of multiple separate circuits for separate appliances (ovens in this case), just as I feel similarly about multiple small-capacity radials (which may well involve more copper than a smaller number of larger-capacity circuits).
There are so many options for the OP's appliances from what the 'electrician' has suggested; i.e. all on separate oversized circuits to all (just about) on a standard cooker circuit - definitely 40A/45A and 6mm^2 - method C of course - would be alright . This would probably be inconvenient with all lost with one circuit fault.

The two smaller ovens could be plugged into the socket circuit and the other oven and hob on standard cooker circuit 32A 4mm^2 (method C of course).

The 'electrician' suggested two 2.5mm^2 radials for the smaller ovens - which is effectively another ring.

In some senses, Amd3 might be expected to improve that situation somewhat, since it opens up the possibility of 1.0 mm^21 16A radials- but I do wonder whether (for irrational 'historical'reasons) a fair number of people will be nervous/uncomfortable about 1.0 mm^2 "power circuits" for a long time to come?
I doubt that will happen any time soon.
 
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There are so many options for the OP's appliances from what the 'electrician' has suggested; i.e. all on separate oversized circuits to all (just about) on a standard cooker circuit - definitely 40A/45A and 6mm^2 - method C of course - would be alright . This would probably be inconvenient with all lost with one circuit fault. ... The two smaller ovens could be plugged into the socket circuit and the other oven and hob on standard cooker circuit 32A 4mm^2 (method C of course).
All agreed.
The 'electrician' suggested two 2.5mm^2 radials for the smaller ovens - which is effectively another ring.
Maybe - although I suppose that depends to some extent on the relative locations of the ovens.
I doubt that will happen any time soon.
As I implied, so do I.

Kind Regards, John
 
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Yes, but the other question to ask is how much more 'cooking' are people doing than they did 20 years ago, despite the plethora (and, in some cases, larger size) of the cooking appliances they now have. Unless people are being exceedingly 'inefficient', they should really be doing much more 'cooking' (using whatever appliances) than they did back then - and, quite apart from 'saving the planet', the last few weeks has probably made people even less inclined to use more 'cooking energy' than necessary!
That I don't know but seeing how some have multiple appliances running simultaneously where those appliances didn't exist before I can only hint at a guess.
One either 'believes in' (are comfortable with) diversity (as we currently apply it) or not. Some may feel that "as currently applied' is not but, again, if they do they are not happy with the concept of diversity 'as it is currently applied'.
I think the figures for cooking diversity were probably appropriate when formulated but I believe we've moved up a bit and feel the formula is overdue for amendment. I am concerned that the following circuit is designed to potentially overload the cable by nearly double (and ironically well over double when the man comes round to add the government grant insulation) and the OCPD by 50% for unpredictable extended periods (thermostatic activity appreciated and understood) whereas the shower I mentioned before no diversity is permitted despite the fact it only runs for a few minutes.
For cooking appliances, one calculates the "after diversity" current as the first 10A plus 0.3 times the remainder of the theoretical maximum - hence, for the 11 kW hob (48A), that becomes 10A plus (0.3 times 38A), which is 10A + 11.4A, i.e. 21.4A. Using the most common method of installing 2.5mm cable, it can carry up to 27A, so fine in relation to the 'after diversity' demand of your 11 kW hob.

Kind Regards, John
You seem to be overlooking the fact that we are talking primarily about domestic installations, in a DIY foruym.

Recommended diversity calculations are (rightly) very different in commercial kitchens, since the pattern of usage is very different and much more 'demanding' of energy. Per the OSG, for cooking appliances in a commercial kitchen, the diversity calculation is ...

... so, for what you describe, would be 22.88 A (17A + {4.2A * 0.8} + {4.2A * 0.6}). not much less to the 'without diversity' total of 25.4A.
I offer my apologies, this information is some which has slipped my mind through lack of use, however it is also something which seems to escape other electricians too. That oven was running on a 15A type 1 MCB which regularly tripped. I was asked to look as it was reported the FCU had failed (it was actually a 20A DP switch) I only started looking at the current as the MCB tripped within maybe 1 minute of switching on from cold.
1652999832877.png

It does and, obviously 'how much is being prepared' (which is why, as above the diversity calculations are so different for commercial kitchens).

However, with any sort of cooking, after the initial brief period of 'getting up to temp', each individual heating element/whatever will, under thermostatic control, only be 'on' for fairly brief and fairly infrequent periods.
Depending on what's being cooked that brief period may not be as brief as you seem to imply.
Yes - but, as I've been trying to explain, an additional 'overload' of 13A or whatever for the couple of minutes it takes to boil a kettle or toast a tea cake is not actually going to do any harm to cables.

The system we use is really fairly 'fail safe' as far as cables are concerned - if the circuit is designed correctly, then the OPD will operate if the (magnitude and duration) current goes beyond what the cable is deemed to be able to carry without coming to harm.

Kind Regards, John
That particular tea tent regularly tripped the breaker to the point the B32A RCBO in the dist board was replaced with a D40 MCB and resited to the outdoor enclosure housing the socket. I oversized my parts to accomodate the long run and expected high currents.
 
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That I don't know but seeing how some have multiple appliances running simultaneously where those appliances didn't exist before I can only hint at a guess.
Maybe, in some cases, but I would have thought that, over the decades, domestic cooking has probably become 'lazier', rather than the opposite, so I would not expect most people to be cooking more things at once (e.g. having more pans on hobs) than they did in the past.
I think the figures for cooking diversity were probably appropriate when formulated but I believe we've moved up a bit and feel the formula is overdue for amendment.
You're certainly not alone in suggesting that, but I wonder what you (and the others) really think has changed. The formula is primarily based on the average proportion of time for which each individual hob/oven element/whatever is electrically energised (under thermostatic control) and I don't see why that should have changed appreciably over time. The absolute numbers may have gone up, but I don't really see why the 'proportions of time' (which is what diversity is primarily all about) should have changed.
I am concerned that the following circuit is designed to potentially overload the cable by nearly double (and ironically well over double when the man comes round to add the government grant insulation) and the OCPD by 50% for unpredictable extended periods (thermostatic activity appreciated and understood)
As I said, (a) the design process is such that the OPD should operate if a high enough current flows for a sufficient period of time for the cable to be deemed to be at risk of harm, and (b) although probabilistic (i.e. 'no guarantees'), the diversity concepts and calculations we work with are such that it is 'very unlikely' that any overloads potentially harmful to cables will persist "for extended periods" - and, if/when trhat 'very unlikley' happens, (a) is there to protect the cable.
whereas the shower I mentioned before no diversity is permitted despite the fact it only runs for a few minutes.
I think you are probably muddling diversity with other considerations. The concept of diversity cannot be applied to most showers since (a) there is 'only one of it' and (b) the heating element usually runs continuously, rather than cycling on/off (since the latter would result in wild variations in the temp of water delivered) - so there is neither the possibility of 'diversity across elements' nor 'diversity across time'.

You seem to be talking about the fact that cables can tolerate considerable 'overloads' for short periods of time without coming to harm. Other than as a part of the diversity concept, standard design principles (e.g. per BS7671) do not generally allow one to consider that, presumably not the least because one can rarely guarantee that a high load will not persist for longer than 'expected' (particularly if one is talking about showers and teenage daughters :) ). However, at the extreme, the existence and use of C-Curve and D-Curve MCBs/RCBOs acknowledges the fact that cables may sometimes carry (for very short periods, such as due to 'inrush currents') currents which are more than 5 or 10 times their 'current-carrying-capacity'.
I offer my apologies, this information is some which has slipped my mind through lack of use, however it is also something which seems to escape other electricians too.
That wouldn't surprise me but, as I said, this is a DIY forum where we are (should be!) really only concerned with domestic installations (so similar 'slipping of the mind' might occur if a 'domestic electrician' found him/herself working on the installation of a commercial kitchen).

As I said and illustrated, because of the very different patters of use of the appliances, there is a dramatic difference between the diversity calculations for a domestic kitchen and a commercial one. The allowance for diversity in a commercial kitchen is very modest - as I illustrated, with your figures merely reducing a 'without diversity' load of 25.4 A to a 'with diversity' one of 22.88 A.
Depending on what's being cooked that brief period may not be as brief as you seem to imply.
Again, we are (should be!) talking about domestic kitchens/cooking, not commercial kitchens with many gallons of liquid being boiled in giant pots! Most people are (like me) not very patient, and would be moaning all over the place if it took more than a fairly small number of minutes for a pan 'to come to the boil' or for an oven to 'get up to temperature'! I would again refer you to my graph for the fat fryer to see how long it took to 'get up to temp'.

Kind Regards, John
 

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