Induction hob circuit

[peterf83]

Hi,

I am due to have my new kitchen fitted next month. I had a fitter come and give me a quote and I said to him that I am having a Neff Induction hob which has a rating of 10.8kw (47amp). I am also having a Siemens oven (3.65kw / 16amp) and combi microwave (3.6kw / 15amp).

When I had the house rewired 3 years ago I ask the electrician to run a suitable supply to the area where the towers housing the ovens would go as it is in a different area to where the oven is now. At the time we were going to have a gas hob.

The electrician ran a 6mm cable on a 32amp switch/breaker (?) from the consumer unit.

The kitchen fitter (who is a qualified electrician) said that the 6mm cable can go into a 60amp junction box and then be split three ways to each appliance. I was surprised by this as I was expecting him to run a new circuit for the hob, he said this wasn't required. I questioned the total power requirements of 18kw and he said that I wouldn't be using everything at full power all the time, which is true I guess. But even so this doesn't sound right to me. He said something about a diversity rule? I don't know what he means by this.

Should I be worried or not?

 

[ericmark]

My stand alone induction hob and double oven is rated at near 50A but the manufacturer says use a 32A supply which has been done and it has never opened the trip.

Although the heat areas have a boost it is only really used to boil water anything else it just burns it. Also with the auto boil then simmer I don't even use to boil water I just hold knob anti-clockwise for second then set to simmer level and forget it auto drops to simmer once boiled.

Only problem I have had with induction was my mothers which used silly touch controls so in spite of super fast reaction times for hob still had to lift pan to stop it boiling over because it took so long to use controls.

So I would say your electrician is correct no real problem. Remember although the induction hob is bigger than other electric hobs it in fact uses less power as all power goes into pan and not into room.

 

[EFLImpudence]

http://www.tlc-direct.co.uk/Book/6.5.2.htm

 

[ericmark]

http://www.tlc-direct.co.uk/Book/6.5.2.htm

Although a good guide it is a little dated now. Modern cookers have far better insulation than the older ones so the mark/space ration of the elements switching on and off is much reduced and in the main oven gets switched on before the hobs so is already at the temperature when hobs are used.

The hob ratting may be 2.8kW, 2 x 3kW and 3.4kW but in real terms one will never use all at those ratings for one only one of a pair can be used on boost at any one time so impossible to use all flat out even if you wanted to.

Again using the settings there is a mark/space ratio switching on and off and a B32 MCB will take a instant load of 160A before the magnetic part will trip and the thermal part will allow the load to increase for the few seconds when the heat areas switch on well over the 32A as long as the average is below the 32A limit.

The only real question is the microwave as it could be switched on for 15 minuets at flat out but at 5A for standard microwave not a real problem only when very large ones are used is there any real worry.

This switch may control two cookers, provided both are within 2 m of it

is interesting like to know where the rule comes from? Rotating machines need isolator within sight of the operator but cookers don't rotate no seems to define "Local" when they say local isolator

The main problem when splitting a supply to multi items is the size of the cable and getting multi cables into terminals when they all are ratted at 32A is not too much a problem but if all rated at 45A then there is an issue.

However in your case you have a tradesman working that all out for you so simple answer is yes no problem the worse case is the power trips and the cooling fans fail for oven so it could over heat the carcase but this is very unlikely and the same could happen if the mains fails so really the carcase should be able to stand the heat even without fans.

 

[JohnW2]

Using the standard diversity calculation, the OP's 18 kW (78A) of cooking appliances would, indeed, be OK on a 32A 6mm circuit. The cable would be adequately protected by the 32A MCB and, as eric has said, that MCB would easily accommodate brief excursions of load which were appreciably above 32A - and, in any event, the very worst that happens is that the MCB would trip.

However, I would imagine that the electrician would probably not be thinking of using a single isolator or switch for all three appliances (i.e. before the circuit 'splits into three'). Standard cooker switches/isolators are usually rated at 45A, and there is obviously the possibility that the total current could sometimes be appreciably above that figure.

Kind Regards, John

 

[EFLImpudence]

We've been here before but -

I would point out that the ratings for the induction hob must be inclusive of all the 'boost' functions and therefore 'normally' only use a lower current than this rating.
An element of, say, 3.4kW cannot run at that power and then be 'boosted'.

Aside from that the hob (47A) and oven (16A) can definitely have diversity applied to them jointly.
So, 63 - 10 x 0.3 + 10 = 25.9A.
(They could nearly be run on 2.5mm² and 25A MCB.)

Therefore, if you are worried that diversity cannot be applied for inclusion of the microwave then (cable installation method allowing) fit a 40A MCB.

 

[JohnW2]

We've been here before but - Therefore, if you are worried that diversity cannot be applied for inclusion of the microwave then (cable installation method allowing) fit a 40A MCB.

Yes, we've been there before (in detail) - which is why (despite some of my feellings) I didn't question (essentially even 'condoned' and 'defended'!) the concept of applying diversity across all three appliances

However, as I said, the maximum current rating of the switch/isolator is a somewhat different issue, and I don't really think it's appropriate to apply diversity for that purpose, do you? In any event, as I said, I rather doubt that the electrician would be intending to have a common isolator for all three appliances.

Kind Regards, John

 

[EFLImpudence]

I'm not sure.

Larger than 45A cooker switches are not pretty.

However, would a 45A one not be satisfactory regardless with a 32A or 40A OPD?

 

[peterf83]

Hi,

Thanks for the replies, it sounds like I can put my mind to rest! Just to confirm, the installer did say that there would be a switch (I think he referred to it as a cooker panel) inside one of the cupboards for the hob and then there would be a double one in the oven housing for the oven and microwave.

Please bare in mind that the microwave is a combination oven microwave so it can be used as an oven as well. Not sure if this makes any difference?

Thanks again.

 

[JohnW2]

I'm not sure. Larger than 45A cooker switches are not pretty.

Indeed, but I wasn't suggesting that - rather that it might be better to have two or three isolators. However, we've also been through that one before, and you felt that a single isolator would be 'safer' (less chance of confusion in an 'emergency').

However, would a 45A one not be satisfactory regardless with a 32A or 40A OPD?

I doubt that cooker switches were designed with a large allowance for diversity - i.e. I would assume that "45A maximum" probably means exactly what it says (rather than 126A maximum)! Don't forget that even a B32 would allow the theoretical maximum load of the OP's appliances (78A) to flow for an appreciable period of time. In reality, I realise that there is probably never going to be anything like 78A, but we generally design for the 'possible'. Would you be happy with, say, an "8kW cooker" (about 35A, 17.5A after diversity) to have a 20A switch/isolator (even if the circuit was protected by a 20A MCB)?

Kind Regards, John

 

[JohnW2]

Just to confirm, the installer did say that there would be a switch (I think he referred to it as a cooker panel) inside one of the cupboards for the hob and then there would be a double one in the oven housing for the oven and microwave.

That's essentially what I was suggesting. It sounds fine, and addresses the (very slight) concern I raised.

Kind Regards, John

 

[EFLImpudence]

Would you be happy with, say, an "8kW cooker" (about 35A, 17.5A after diversity) to have a 20A switch/isolator (even if the circuit was protected by a 20A MCB)?

Mmmm. I don't really know.

It does get more risky the smaller the loads.

Surely a 45A switch is satisfactory with a 45A MCB on a no-diversity circuit and able to cope with any overload.
Not the same as a cooker, I know.


Wilst discussing switches, I am often puzzled by the fact that, whatever the rating of a switch, the actual contacts are frequently of very small area - being made only by two hemispheres which 'touch'.

 

[JohnW2]

Would you be happy with, say, an "8kW cooker" (about 35A, 17.5A after diversity) to have a 20A switch/isolator (even if the circuit was protected by a 20A MCB)?

Mmmm. I don't really know. It does get more risky the smaller the loads.

Quite

Surely a 45A switch is satisfactory with a 45A MCB on a no-diversity circuit and able to cope with any overload.

Well, I don't know, and that's what I've been discussing. What, I wonder, does a 45A 'maximum current of rating' for a switch actually mean. Does it mean what it says on the tin (namely that the maximum current it should carry and/or be asked to break is 45A) or does it mean that the maximum permissible current is the maximum current that a B45 MCB will allow to flow for some certain period of time? I don't know - do you?

Wilst discussing switches, I am often puzzled by the fact that, whatever the rating of a switch, the actual contacts are frequently of very small area - being made only by two hemispheres which 'touch'.

That's true, but those 'two hemispheres' do generally get larger as the rating increases. However, I have to agree that they are usually surprisingly small, in all standard accessories - particularly given that 'hemispheres' only allow 'point contact'. Of course, things change with much meatier switchgear - but even a 50A MCB or 80/100A RCD or main switch has surprisingly modest 'switch' contacts.

Kind Regards, John

 

[bernardgreen]

Contact pressure is as important as contact area,

Pressure being the force pressing the contacts together divided by contact area.

Generally given the same force on the contacts a large area of contact will have a higher contact resistance than a smaller area of contact.

The carrying current of a pair of contacts is not the same as the switching current. Carrying current is generally higher than the current the contact can switch.

Isolators and switches are not the same thing. Some isolators have high current carrying capacity but can only switch that current a few times before excessive contact damage occurs. Shower isolators are not designed to be ON -OFF switches for the shower and if used as such with the shower itself switched ON then the isolator will often fail after a short life time.