Adding Induction Hob to Oven circuit

[JamesK]

Hi,

We're about to replace our kitchen and would like to go over from a gas hob to an induction hob.

We currently have an AEG single oven which can draw 3.5kW (due to the pyrolytic cleaning function). The cabling is 10mm and is connected to a 32A breaker in the consumer unit.

The majority of induction hobs we've looked at are rated at 7.4kW.

So am I ok to get an additional connection added to the existing oven circuit? Is the cabling ok? Is the 32A breaker sufficient or does it need to be upgraded? Or do I need to get a whole new circuit installed?

I'll be getting an electrician to do the work for me, I just wanted to have a bit of knowledge up front ahead of getting quotes.

Many thanks in advance...

 

[PrenticeBoyofDerry]

32A on full-load will allow for 7.3kW, diversity can be offered to the circuit as it will be very unlikely that you have full-load being demanded.

The rule of thumb would allow 15kW of output on a 32A circuit.

So you should be fine, and the 10mm2 is more than adequate, providing there are no extreme de-rating factors to apply to it.

 

[JamesK]

32A on full-load will allow for 7.3kW, diversity can be offered to the circuit as it will be very unlikely that you have full-load being demanded.

The rule of thumb would allow 15kW of output on a 32A circuit.

So you should be fine, and the 10mm2 is more than adequate, providing there are no extreme de-rating factors to apply to it.

Many thanks for this. An electrician came round to look at it today and he said he would need to run a separate circuit. His reasoning was because the the 3.5kW oven and 7.4kW hob added up to nearly 46A. Unfortunately I wasn't home when he came round so couldn't question it.

Out of interest I did some research about the diversity calculation and I'd read that it was 10A + 30% of the remainder + 5A for the socket attached to the isolation switch on the wall. My calculation works out at just over 25A. I got my info from here: https://www.tlc-direct.co.uk/Book/6.5.2.htm

The cooker and hob are both less than a metre away from the isolation switch. The consumer unit is probably 12ft away from the isolation switch.

Is the electrician trying to generate additional unnecessary work here?
I'm going to get someone else round to look at it.

 

[PrenticeBoyofDerry]

Is the electrician trying to generate additional unnecessary work here?
I'm going to get someone else round to look at it.

What credentials did they come with? Can they be found on the registered competent persons website?

If they are not try to generate extra/unnecessary work, they probably don't know what they are doing.

Diversity would come in at 26.2A. Cable can handle that and the breaker can deliver it.
It's a dual outlet plate jobby, if both appliance are together.

 

[JamesK]

What credentials did they come with? Can they be found on the registered competent persons website?

I found him on Yellow pages and I know other people that have used him for work and been happy. So I'm actually a bit taken aback at what he's said needs doing.

I've found another electrician to come and quote so I'll see what he says.

Thanks very much for giving me this information. I had one of those dual outlet plates at my last house, so I'll make sure it gets used here as the hob will be directly above the oven.

 

[xr4x4]

If you have a 10mm cable you could upgrade the mcb.

 

[PrenticeBoyofDerry]

If you have a 10mm cable you could upgrade the mcb.

You could if you believed the circuit may overload, but I doubt it will.

 

[ban-all-sheds]

He could, but why?

James - invite the electrician back to discuss the job, and when he arrives (don't tell him this over the phone) ask him to go over the diversity calculations with you, to show why you need a new circuit.

FYI, the rule of thumb used for cooking appliances is to calculate the load at 10A + 30% of the remainder over 10A (+5A if the cooker control switch has a socket). The principle is that cookers do not run flat out all the time - once the oven is up to temperature it turns on and off via the thermostat to maintain the temperature set. Induction zones modulate their output down to keep the pan simmering or gently frying or whatever. Plus it's relatively rare for people to be using all of the hob at once anyway.

You have a 3.5kW oven and a 7.4kW hob = 10.9kW = 45A (assuming the manufacturer ratings are at 240V). But let's be pessimistic and say that they are at 230V, and that your CCU has a socket.

10.9kW = 47A.

10 + 37x0.3 + 5 = 26A. Not even 32A, let alone "nearly 46A". He's not done any diversity calculations at all - either he is trying to rip you off or he is incompetent.

You are going to have to kick him into touch either way, so you might as well waste his time and make him squirm a bit first.

He might try to say that it's two separate appliances, and therefore the normal diversity rules don't apply. If he does, you can turn the screws a bit more by inviting him to consider a 10.9kW freestanding cooker with a 3.5kW oven and a 7.4kW hob, and ask him to explain how the two parts of it suddenly start to use more electricity when they are sold as two physically separate units.

 

[JamesK]

If you have a 10mm cable you could upgrade the mcb.

You could if you believed the circuit may overload, but I doubt it will.

I'm not exactly Jamie Oliver, there's little chance of 2 burners being used on full blast simultaneously, let alone all 4 and the oven being used at the same time!

 

[ban-all-sheds]

And even if you did stick 4 large cast-iron Le Creusets full of ice-cold water on the hob and switch all 4 zones on, it would, until they boiled, draw 32A - well within the capability of 6mm² cable to carry.

PS - apologies for my FYI in the post above - somehow I completely missed the fact that you said you'd researched that and knew it.

 

[JamesK]

PS - apologies for my FYI in the post above - somehow I completely missed the fact that you said you'd researched that and knew it.

No need to apologise, I did my research using Google, so wasn't sure it was correct. I appreciate people taking the time to respond and explain.

It's just disappointing that there are tradesmen out there who are giving incorrect advice & quotations.

 

[PrenticeBoyofDerry]

It's just disappointing that there are tradesmen out there who are giving incorrect advice & quotations.

It is very disappointing, but don't tar us all with the same brush!

I suggest that you look hear for your electrician, http://www.electricalcompetentperson.co.uk/

They are all registered, that does not guarantee some wont try it on, but most schemes have a complaints procedure.

 

[JamesK]

So I had another electrician visit. He is convinced from what he can see of the cable in the kitchen isolation switch that the cable is 6mm and not 10mm as I'd previously been advised.

However, he took the same final stance as the previous electrician. He said it would be possible to connect the oven and hob to the same supply, but I would be restricted to using only one appliance OR the other, not both at the same time.

I explained I'd received advice on-line and he started rambling about diversity and actually couldn't understand what he was mumbling about.

Ultimately he said I would need a new circuit. His 'compromise' position was he could take the garage 16A supply (adjacent to kitchen) and put a cable through the wall and power the oven from that. This is fine because I rarely use the garage power sockets for anything.

If the cable from consumer unit to isolation is 6mm does it change what you guys have previously advised? I've read 6mm is perfectly adequate for a 32A circuit.

If not, I'm worried that 2 electricians have said similar things to me. Most important is that the electrics are safe. However, I'm not paying out for works that don't need doing.

 

[EFLImpudence]

He is wrong.

If the cable from consumer unit to isolation is 6mm does it change what you guys have previously advised? I've read 6mm is perfectly adequate for a 32A circuit.

It does not change.
In fact, 4mm² would suffice - assuming it is buried in masonry which, if it is 'old' it will be.
Nearly every cooker circuit in the country is 32A with 6mm² cable.
There is no difference between a separate oven and hob and a cooker.

If not, I'm worried that 2 electricians have said similar things to me. Most important is that the electrics are safe. However, I'm not paying out for works that don't need doing.

I can only advise you to keep trying until you find one who knows what he is doing.

 

[JamesK]

Here's a couple of pics of the socket. Click on them to enlarge. Not sure if they're helpful or not.