Induction Hob again

Playing a bit of Devil's advocate, if you rely upon diversity, why do you need such powerful appliances?


To my mind, one hob + one oven = one cooker.


Paragraph 311 of BS 7671 permits diversity to be taken into consideration. Table A1 of the On-Site Guide allows 10 A of the rated current + 30% of the reminder = 10 + 15 = 25 A.


If the cable runs in an insulated stud wall (insulation may be there for sound attenuation) the current carrying capacity of 6 mm² T&E is 35 A provided that the cable is in contact with the inner wall surface.


One of the problems here is that a large proportion (the majority?) of the population have no idea about cookery. The big range on Christmas Day is the usual example. In engineering terms, if you turned everything on at the same time, it would briefly pull the full rated current, but as the thermostats click in and out, diversity comes into play. In culinary terms, you do not cook your vegetables when the turkey (yuk!) goes in, and you do not put your peas on at the same time as your potatoes.


Don't even think of putting ovens on the ring.


IMHO, there is no problem with the installation as described.

Am I simply wrong?

I don't think so either. 30% after the first 10A (plus 5A if there's a socket in the cooker control unit) is a well known rule going back generations. Provided the manufacturers instructions don't say otherwise I too reckon a 25A or higher common circuit should be fine.

   - Andy.

https://www.youtube.com/watch?v=QQduU8RkjD8

is also helpful in thinking about this

If your appliances really did together consume 60A for an appreciable period of time, the worst that should happen is that after about half an hour the circuit breaker will trip, providing you with a useful reminder that using every ring and the oven simultaneously to bring large saucepans of cold water to the boil to fill up a broken hot tub perhaps wasn't the best idea! All that will happen to the cable during this time is that it will get quite warm.

The problem with kichen fitter electricians is they tend not be be experienced in the real world but go by the book.

I had the same problem was you when we had our kitchen refitted. The sparks was dead set on rewiring the cooker circuit. I simply asked him to explain how a stand alone cooker with a double oven and ceramic hob worked well with the existing circuit but suddenly a similar unit needed a new cable. 'The book says so and I always do it that way' canme the answer - bit like the swich for the cooker suddenly got moved to the back of the cupboard at high level  -' thats what people want came the excuse.

It got better as I found the earthing tail from the metal conduit not even connected in the light switch. Thats the standard of NICEIC domestic installers for you!!


Ask hime to justify the new cable.

A chunkier cable might be nice, but it sounds like there is no real risk, unless the 6mm cable has some other issue, like it changes colour from one end to the other - implying some unknown hidden joint, or the wall is full of insulation or worse some nasty plastic that reacts with PVC.

handy cable rating tables

(table-4d2a is a good starting point to see the dramatic effects of insulated walls versus free air..)

So long as the breaker is such that  it protects the cable, nothing terrible will happen - as others have noted, you will fire the trip first, which may be a real pain in the wotsits in mid soufflé or whatever, but not actually dangerous is it.

Do check the wall filling though as a 40A breaker would allow a 6mm cable in an insulated wall to be over-run quite a bit, which is not so good.

Thinking about your sparks' advice, you may wonder if he has considered your company fuse rating  ? It may say  "100A" on the holder next to the meter, but a 60A or an 80A fuse in a 100A holder is quite common, and on a housing estate then there may be 20 or more similar houses on each 400A fuse at the substation end (or more like 50 per phase on a larger transformer) so if you took the sparks' advice at face value, you'd probably have to disable the shower and the sockets while cooking and liaise with the neighbours to wash on alternate days. In reality, such measures are not needed and do not happen.


There are alternate hacks sometimes seen when it gets more desperate, like cables that pop outside and run along the wall for a bit, or multiple parallel runs, but in this case it seems a bit OTT.

M.

Half way between Keighley and Skipton?

Thank you BPD. I had watched that before I saw the kitchen fitter electrician. I had even sent him the same link before his visit. To no avail. The other challenge that I have come across is that induction hobs are in some way different and that diversity shouldn't be considered. Although I can see that there is power management in the hob circuitry it doesn't seem to make it more likely that prolonged high power would be called for in routine domestic cookery.