Is a 45A circuit suitable for a 17.1kw electric cooker?

[xdave]

Hello all. I am in the process of having the prep work done for a new kitchen to be fitted next month. This is being carried out alongside an extension and a garage conversion under a building notice, and due to relocation of things like the cooker and sink everything is being ripped out and replaced. I am using an electrician and a plumber to carry out the appropriate work, I'm fitting the units, and then fitters will do the worktops. The spark has already done work in the extension and I am happy that he knows what he is doing, but I would like a second opinion on something he mentioned today.

Due to circumstances preventing the use of a gas hob, I am having an electric induction range cooker fitted. The complete cooker (hob + ovens) is rated at 17.1kw, which by my understanding means it could draw up to 74A assuming a 230V supply (or 70A given the 245V actually present here). The electrician is aware of the cooker I have chosen and has contacted the technical helpline to confirm that a 45A supply is sufficient. It is indicated as such in the installation instructions. Therefore he will be installing a cooker connection point with a separate 45A isolator switch, and the cooker supplier will make the final connections on delivery.

He mentioned that this was acceptable due to diversity applied to cooking equipment, and from his explanation of that it makes perfect sense to me. I thought nothing more of this until I began searching about induction hobs on this forum where the consensus among people who look to be experienced sparks is that you cannot apply diversity to them because of the way they work. If this is true then installing to a 45A supply strikes me as a potential problem.

I have no reason to question this guy's work; as I said I have been very impressed with what he has done for me so far and the level of knowledge he has expressed in explaining everything to me in great detail, so I don't want to question him on this unecessarily. When I asked he provided a logical answer so my question to you guys is should I bring this up again or is it all ok?

 

[Spark123]

You can apply diversity to a cooker, the On Site Guide gives this as 10A + full load 30% of connected cooking appliances in excess of 10A. Add 5A if the CCU has a socket.
I tend to take these figures with a pinch of salt - if it doesn't have a socket then you might get away with a 30A supply however you might end up with cold turkey at christmas if you put everything on full belt!
Your thinking that the current goes down with the voltage increasing is wrong - the resistance of the element is aproximately constant so the current will increase as the voltage does.

 

[kevindgas]

I see where your coming from, but don't know the answer I will watch with interest the answers

 

[ColJack]

what are the odds that you will be using every single "ring" on full power at the same time, along with using every oven at full temp?

and for how long?

 

[xdave]

I'm not surprised I got the numbers wrong, I just googled for a unit calculator and typed the numbers in and so don't really understand what is going on behind them.

So using 10A + (30% over 10A) with no socket that I am aware of, that gives (assuming my 74A figure is correct?):

10A + 19.2A = 29.2A. That leaves 15.8A spare 'capacity' for christmas dinner? I have no idea what the actually relates to in terms of cooking capacity, but it only sounds like a couple of hob rings and the second oven.

In answer to how often would everything be on full at once - I probably only cook that amount two or three times a year such as christmas, easter, etc, and even then it wont be for a long period of time (about an hour max). One of the reasons I went for a range cooker though was for the extra hob rings when cooking veg. I am unlikely to ever use the second oven for anything but keeping plates warm before serving but I would likely make use of all the available rings most Sunday lunch times. The second oven only uses 3kw of power; most is used by the hob.

 

[Lectrician]

45amps will be fine, and as it is stated in the instructions you should be doubly satisfied......actually, trippaly, as you also spoke to the manufacturer.

If you did have to go for the full load you would need a very large cable, an ugly isolator in the kitchen (you wouldn't get a posh crome flush one), and you would likely also need the supply entering your property increased! (many supplies are only 60 or 80 amps, 100amp is rare). I doubt the inlet terminals onthe cooker would take the size of the cable you would require.

 

[electronicsuk]

The answer is simple - if the manufacturers instructions state a 45A supply is required, then a 45A supply is required. As you've rightly said yourself, applying the diversity rule may give you a figure of even less than this, but the mfrs instructions must take precedence in this case.

 

[ericmark]

As electronicsuk has said you must follow manufactures instructions. The difference between some induction hobs and the other types is the induction hob often has a boost so that when you are only using two rings it can be taking near to the same draw as with four rings so diversity if any is much reduced.

You can apply diversity to the whole cooker so if full load is x but two rings are y then supply would need to be oven + y or x which ever is larger but in real terms this will not work as most manufactures will stipulate not only smallest but also largest supply and will require it to be supplied through an overload device too small to supply both oven and hob.

So in real terms you need separate supplies for both. You could in theory supply a mini consumer unit in the kitchen then split from that there as some flush units available but most likely is two supplies.

If one reads section 314 of the regulations it refers to every installation being divided into circuits. The way it is worded is not plain but I would question if one is allowed to give one supply to two appliances which are so big. Indeed it says in Appendix 15 (Click here to read in full) referring to ring mains that the regulations can be meet by (iii) connecting cookers, ovens and hobs with a rated power exceeding 2 kW on their own dedicated radial circuit.

It is not cut and dried and one could argue the pros and cons for many pages but is all comes back to manufactures instructions and often these only arrive when it is too late so I would always give separate supplies to oven and hob.

 

[LiamPope]

Eric - it's a single appliance. Full electric range cooker with dual ovens and induction hob!

 

[ericmark]

Eric - it's a single appliance. Full electric range cooker with dual ovens and induction hob!

Had not realised that in which case answer very simple if manufacture says 45A then that's OK.

 

[xdave]

Thanks guys, you have set my mind at ease.

 

[17thman]

how is or how will the cable be run from the cu to the cooker?

 

[securespark]

And what size cable is your spark going to use?

 

[xdave]

I will see him again on Monday and will ask what size cable will be used.

The route runs up into the ceiling from the top of the consumer unit, along a couple of rafters and then back down directly into the top of the cooker isolator switch. (I live in a bungalow so there is no upstairs.) The existing cooker circuit takes a similar route. The cable will be in a round metal conduit from the consumer unit to the switch and from the switch to the connection point behind the cooker.

The consumer unit was new about a year ago. There is a B40 MCB* marked 'Cooker' that is currently off and unconnected so it will be connected from there.

* Is a 40 amp MCB enough? Also, with the exception of the security and smoke alarms, all the other circuits are on RBCOs with their own trip buttons. This was all fitted by a different electrician last year, and although he knew what I wanted done with the kitchen I hadn't chosen the cooker yet so does that need changing now? I hope not because (aside from costing me a small fortune) everything is marked Contactum and apparently they have gone bust...

 

[xdave]

Hi guys, I have the answer to your questions (below). Please let me know if anything is bad practise or wrong.

The cable will be 6mm T&E. It will run a total of 8m in metal conduit along the rafters above the insulation and then chased into the kitchen wall to a 45A DP isolator. The cable will then run in conduit to the cooker connection unit. The conduit will be connected to the metal consumer unit and earthed. A 40A MCB is required to protect the cable as it needs to be less than the cable rating. No RCD is needed because the cable is protected by the conduit.