another 'replacing hob' thread

amjl2000 · 17 Nov 2010

Hello!

I have just bought a 7.2kW induction hob to replace an old ceramic hot-plate 5.5kW hob (which is a pile of crap, 5 mins to heat up, 20mins to cool down, i have no idea why anyone would want one).

On the same ring is a 2.4kW oven.

The CU has a 32A breaker in it. There is cabling in the wall between the CU and the cooker switch and then there's t+e to the oven and hob - I don't have the exact details of what size any of this is.

Now from what I have read here a 7.2kW unit by itself is fine with a 32A fuse, and 6mm^2 cable, but as I have an additional oven there too, how much beefier do I need things to be?

Plan B would be to take the oven off the cooker ring and put it on with the sockets as it only needs a 16A fuse and 1.5mm^2 t+e (as quoted from the installation manual) - this would be similar to in my old house when I had the kitchen done that the fitters put the oven on the sockets ring.

I haven't taken into account diversity yet, but wanted to guage opinion before I go any further - I'm not averse to calling a spark out but I know I'm competent doing the work (I worked with an electrician a few years back), so would rather learn about it too.

Many thanks for your time

Alex

(just to add, distance from CU to cooker switch is around 2.5m, and less than 2m then to cooker - the cable length that is)

echoes · 17 Nov 2010

amjl2000 said:
(which is a pile of crap, 5 mins to heat up, 20mins to cool down, i have no idea why anyone would want one).

Me neither. But wife's mother swears by hers. I swear at it.

amjl2000 said:
On the same ring is a 2.4kW oven.

Cooker circuit will be radial.

The CU has a 32A breaker in it. There is cabling in the wall between the CU and the cooker switch and then there's t+e to the oven and hob - I don't have the exact details of what size any of this is.

amjl2000 said:
Now from what I have read here a 7.2kW unit by itself is fine with a 32A fuse, and 6mm^2 cable...

Yes... most likely will be fine.

amjl2000 said:
but as I have an additional oven there too, how much beefier do I need things to be?

Best practice here would be to add an additional radial circuit for your oven. Other options may exist.

amjl2000 said:
Plan B would be to take the oven off the cooker ring and put it on with the sockets as it only needs a 16A fuse and 1.5mm^2 t+e (as quoted from the installation manual) - this would be similar to in my old house when I had the kitchen done that the fitters put the oven on the sockets ring.

A fixed appliance like an oven (> 2kW) is best served by its own radial circuit.

amjl2000 said:
I haven't taken into account diversity yet, but wanted to guage opinion before I go any further - I'm not averse to calling a spark out but I know I'm competent doing the work (I worked with an electrician a few years back), so would rather learn about it too.

If diversity can be applied, then you might arrive at a discomfortingly low current. 2 separate circuits would be far better than relying on diversity to lower the current demand, IMveryHO.

EDIT: unsure as to whether diversity can be applied to induction hobs, since the coils modulate. ??

amjl2000 · 18 Nov 2010

echoes said:
amjl2000 said:

but as I have an additional oven there too, how much beefier do I need things to be?

Click to expand...

Best practice here would be to add an additional radial circuit for your oven. Other options may exist.

Would I be right in saying that if I created a new circuit from the CU then it would have to be certified? - i.e. get someone qualified in to do it.

echoes said:
amjl2000 said:

I haven't taken into account diversity yet, but wanted to guage opinion before I go any further - I'm not averse to calling a spark out but I know I'm competent doing the work (I worked with an electrician a few years back), so would rather learn about it too.

Click to expand...

If diversity can be applied, then you might arrive at a discomfortingly low current. 2 separate circuits would be far better than relying on diversity to lower the current demand, IMveryHO.

EDIT: unsure as to whether diversity can be applied to induction hobs, since the coils modulate. ??

I haven't done enough research on how exactly diversity is calculated (apart from the basic knowledge of what it is)... but you could be right that induction hobs are modulating - it sounds very plausible.

Thanks for your advice
Alex

echoes · 18 Nov 2010

Would I be right in saying that if I created a new circuit from the CU then it would have to be certified? - i.e. get someone qualified in to do it.

Yes that's right. It would have to be notified to building control either by you DIYing it which would incur a fee, or by an electrician who is able to notify building control. It would be cheaper I suspect to get an electrician to do it.

If you are looking to save money, then it would be acceptable, albeit bad practice, to plug the oven into a socket... but it wouldn't happen in my house

On diversity, induction hobs can 'boost' the power also which will throw out the usual diversity formula. The instructions may give guidance, but to be on the safe side, go with a circuit designed for the hob's full load.

amjl2000 · 18 Nov 2010

I would prefer to aim towards the full load on the system - aka the Christmas day loading.

Will umm and ahh over what to do with the oven. Its a fixed appliance so should be wired in properly, though it's power requirement is less than my kettle...

Thanks for the advice echoes

echoes · 18 Nov 2010

No problem!

Religious festivals are not respectful of diversity formulae anyway!

PrinceofDarkness · 18 Nov 2010

'Echoes', You've been over-cautious with your advice on diversity for induction hobs. Such hobs are no different to ordinary hobs where diversity is applied according to the Regs - whether they 'modulate' or not......

This oven and hob with a combined load of less than 10kW is well within those rules for 30/32A domestic cooking appliances. Therefore, the existing cooker circuit cable is more than adequate to serve both appliances via a common-or-dual connector - even on Christmas Day!

Moreover, the final connections to the appliances would not be notifiable to the local council.

Provided that the oven and hob are within 2M of one another and the common control switch, they may be regarded as being one appliance similar to a free-standing cooker.

Lucia.

echoes · 18 Nov 2010

@PrinceODarkness. I was unsure regarding diversity for induction hobs and have noticed there's been a bit of discussion about it on here before. Thanks for clarifying.

PrinceofDarkness · 18 Nov 2010

OK, understood, Echoes - but I'd hate to think that the O/P has gone to the unnecessary expense of installing a new circuit for the oven and paying an unfair fee to his council, on the strength of your advice.........

Lucia.

amjl2000 · 18 Nov 2010

Hi

Thanks for the further input Mr Darkness. Our hob came with a mail-in offer of free pans, so have to wait for them to arrive before I can get stuck in - I was getting in now to do the preparatory 'think' stage, before the second 'think' stage (I read that in the forum guide

).

Provided that the oven and hob are within 2M of one another and the common control switch, they may be regarded as being one appliance similar to a free-standing cooker.

This is what I have.

So as along as the cable is at least 6mm^2 that already exists then I'm good to go with replacing the old hob with new?

Cheers
Alex

riveralt · 18 Nov 2010

amjl2000 said:
I would prefer to aim towards the full load on the system - aka the Christmas day loading.

Will umm and ahh over what to do with the oven. Its a fixed appliance so should be wired in properly, though it's power requirement is less than my kettle...

Thanks for the advice echoes

7400+2400 = 9800/230 = 42.6Amps.
Diversity = 10Amps +(32.6*.3) = 19.78Amps + 5Amps if you have a socket as part of your cooker switch. Total 24.78 Amps.

MCB = 32Amps - tick
Current Carrying Capacity of 6mm2 (assuming no derating factors) = >45Amps (haven't got my tables to hand) - tick.

Get cooking

echoes · 18 Nov 2010

The 2 other posters have eloquently given you a valid other option with the diversity calcs. It looks like this is the easier and cheaper option for you, so best of luck with your new hob and oven!

I still lean towards 2 circuits as preferable - there are quite a few debates on here about cooking appliances and diversity, and it's largely a matter of opinion. Interesting though.

EDIT: daft typo!

PrinceofDarkness · 18 Nov 2010

amjl2000: You seem to have a "standard" 30/32A domestic cooker circuit. So, odd that this might seem to you: the Regulations allow this peculiar arrangement for domestic cooking appliances up to 15KW.

This rule has endured in the Regs for nearly sixty years, and there are no plans to amend it - simply because it works.

Your combined oven/hob load of less than 10kW is well within those 'rules' and furthermore: you are quite at liberty to make the final connection to each appliance without notifying your local authority.

Lucia.[/i]

amjl2000 · 18 Nov 2010

I thank you all for your inputs - I did read other forum topics based on the same thing - and yes I agree that there appears to be more than one way to skin the cat though (thankfully) illegal/dangerous practices/ideas will be leapt on by the more knowledgeable. I can be confident in your replies that I can cook away without worry!

Alex

echoes · 18 Nov 2010

8) I think the only thing we haven't discussed is the likely requirement for fusing down to 13A your oven. Via plug-with-fuse or an FCU between oven and cooker switch.

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another 'replacing hob' thread

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