Is this the norm now days

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The hob is rated at 11KW.

I dont pretend to know how the technical side of it works i am just used to seeing fat cables to ovens and hobs in the past so was concerned they were undersized at 2.5mm. Am i ok to proceed?
 
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The hob is rated at 11KW. ... I dont pretend to know how the technical side of it works i am just used to seeing fat cables to ovens and hobs in the past so was concerned they were undersized at 2.5mm. Am i ok to proceed?
To expand on winston's "Yes" answer, as you may have gathered, this is all about what is called "diversity", which is a way of taking into account the fact that, since cooking devices constantly switch their elements {separately} on/off by thermostatic control, it will very rarely be the case (except very briefly if you switch everything on from cold simultaneously) that "everything is on" simultaneously - so the average expected current drawn over any appreciable period of time is much less that the theoretical maximum.

If the 11kW is quoted at 230V (much more likely 240V, but that makes the calculation 'even better') that corresponds to a current (with everything 'on') of about 48A.

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
 
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Thanks. I think i follow this.

They plan to install 2.5mm for the 3 ovens and 10mm for the hob. So whilst the 10mm is overkill there is also no issue with it (bar a slight increase in the material cost).
 
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Thanks. I think i follow this. ... They plan to install 2.5mm for the 3 ovens and 10mm for the hob. So whilst the 10mm is overkill there is also no issue with it (bar a slight increase in the material cost).
Yes, that's all true (I presume you mean 2.5 mm^2 for each of the ovens, separately). If one considers diversity, then I would say that 10 mm^2 is pretty serious 'overkill' but, as you say, that's not really an issue other than in terms of cost (and slightly more difficult installation, with such a 'meaty'/stiff cable!).

Kind Regards, John
 
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It also means the spark does not know his stuff. One wonders what else he does not know!
 
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It also means the spark does not know his stuff. One wonders what else he does not know!
It seems that many electricians have tended to over-size cables for cooker circuits for many years - presumably either because they don't believe in diversity or (much more likely) because they are doing what they have 'learned' (from courses or other electricians).
 
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Should i suggest something smaller 6mm?
Personally I’d rather go with what my electrician recommended which will definitely be suitable, rather than go with what some stranger on the internet, who by the way is not an electrician and never has been, reckons might be suitable but doesn’t have to deal with the consequences if it isn’t, but it’s entirely up to you.
 
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Several people have said what might be suitable. Not sure if all of them are not or never have been electricians though.
 
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As has been demonstrated 2.5mmmm² is theoretically adequate for the hob but it would require a 25A MCB/RCBO
2.5mm² is totally correct for 16A supplies to <4KW ovens.

If it was my kitchen I'd install 4mm²'s for the ovens and 6mm² for the hob.
Some will say oversized/overkill but the small additional cost is insignificant in the overall cost of the kitchen refit but in a few years when refitted again you may breath a sigh of relief.

It may very well be that 3x 4mm² and 1x 6mm² is cheaper than 3x 2.5mm² and 1x 10mm²

However... I'm not on site and have no idea what derating factors need to be taken into account but your electrician is there and is in full possession of the requirements.
 
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It may very well be that 3x 4mm² and 1x 6mm² is cheaper than 3x 2.5mm² and 1x 10mm²
For the 2 options TLC prices work out at £5.89p/m and £6.14p/m so a 10m run will cost £2.50 less for my suggestion, Even the additional cost of 4mm² over 2.5mm² is only £18 for 10m. No brainer to me either way.
 
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.... Some will say oversized/overkill but the small additional cost is insignificant in the overall cost of the kitchen refit but in a few years when refitted again you may breath a sigh of relief. ....
We often hear people talking in terms of 'future proofing', and there are some situations (e.g. supplying power to an outbuilding) when it makes a lot of sense. However, in terms of cooking (and heating) appliances, I would suspect that the current (and probably 'everlasting') interest in 'energy-saving' probably means that it is very unlikely that we will see such loads getting even greater in the future.

Having said that, I agree with you that it would seem less efficient (in terms of cables etc.) to have three separate 16A oven circuits, rather than one circuit serving all three. However, the total of the OP's three ovens is only 8.9 kW, hence around 18.7A after diversity, so theoretically OK with 2.5 mm^2 cable (not necessarily Method C) on a 20A MCB/RCBO, so I'm not quite sure why you are suggesting 10 mm^2 - don't you believe in diversity?

Kind Regards, John
 
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We often hear people talking in terms of 'future proofing', and there are some situations (e.g. supplying power to an outbuilding) when it makes a lot of sense. However, in terms of cooking (and heating) appliances, I would suspect that the current (and probably 'everlasting') interest in 'energy-saving' probably means that it is very unlikely that we will see such loads getting even greater in the future.

Having said that, I agree with you that it would seem less efficient (in terms of cables etc.) to have three separate 16A oven circuits, rather than one circuit serving all three. However, the total of the OP's three ovens is only 8.9 kW, hence around 18.7A after diversity, so theoretically OK with 2.5 mm^2 cable (not necessarily Method C) on a 20A MCB/RCBO, so I'm not quite sure why you are suggesting 10 mm^2 - don't you believe in diversity?

Kind Regards, John
No I'm advocating not using 10mm², I would go for 6mm² for the hob which is good for the 48A full load.
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.

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. My view is diversity has outlived its intended design purpose, any 2 of those appliances running at full load at the same time will exceed a 20A OCPD rating and I'm quite sure there will be tripping occuring fairly quickly if they are used to any great extent. A 32A/4 or 6mm² would be different but then we run into the difficulty of unprotected cables.
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.

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, although not I've been prompted to be in a hurry to alter. If I were designing any other circuit I would design it to take the expected load and I'm sure an EICR would very quickly find it being altered if the design were wrong.
One of the things I don't understand is diversity is applied to a cooker but not a shower unit... why not? We happily design a circuit knowing it's expecting to be overloaded for some reasonable periods of time for one device but the shower has multiple heating elements and power settings just like a cooker and on top of that the shower is usually only used for a couple of minutes at a time but the circuit does not have diversity applied.
 

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