Wiring for induction hob

[Seb101]

Currently have an electric double oven and gas hob. The oven is on a dedicated 32A radial circuit from the CU. Wired with 6mm T+E cable running in a bundle through free-air in the ceiling cavity for approx 10 meters, then clipped to thermalite block behind plasterboard for the last 2 meters to the oven isolator.

The prospective new appliances have ratings of 3.6kW for each oven and 3kW for the induction hob, for a total of 10.2kW or ~42.5 Amps. Under the diversity calcs listed here: http://wiki.diyfaq.org.uk/index.php?title=Diversity typically this would equate to:

10A
+ 30% of the remainder = 0.3*(42.5-10) = 9.75A
+ 5A for the socket on the cooker isolator
= 24.75A

Which falls inside the 32A circuit limit, with a fair amount of headroom. However - I know the socket on the oven isolator will not be used as it's about to be boarded up behind some new kitchen units, so for this specific installation there will be another 5A of headroom. Realistically, my typical peak usage might be the 2 ovens on simultaneously with 2 out of the 4 hobs, which would come in at approximately 36 Amps if everything happened to be using max current all at the same time (unlikely).

Given above diversity calculations, it would seem to be acceptable to wire the induction hob into the existing 32A radial by spurring from the existing oven isolator to a new isolator for the hob using 6mm cable? The additional run would be approx 6 meters clipped to thermalite behind plasterboard.

Would you agree?

 

[ban-all-sheds]

I know the socket on the oven isolator will not be used as it's about to be boarded up behind some new kitchen units

It had better not be.

 

[ericmark]

I have argued that although an induction hob can have a higher output to a conventional halogen hob, at the end of the day we are cooking the same food and if less energy is lost and you can't every use full power for anything other than boiling water then in real terms the induction hob uses less power even if the rating is higher.

At home we used the auto boil simmer function so I suppose we do use full power which even then is not maximum power (boost) really don't know why it has a boost, other than boiling water nothing can be cooked using 3.7 kW.

Mothers house a stand alone, 2 kW and default power on turn on is 1 kW I never use it at over 1 kW even that is too hot, 800 Watt is really the maximum setting with normally being 400 or 600 watt. So because no energy is lost, in real terms 500 watt is what the induction hob uses per heat area on average so even if rated a lot higher, unless used commercially where you are swapping pans all the time, then a 4 heat area hob really uses around 2 kW even if the rating is a lot higher.

Except for range ovens where the whole idea is to heat up a huge lump of metal, most ovens do not store much heat, my built in oven mothers house hits 180°C in around 7 minutes, after that it is cycling on/off, at home the stand alone cooker is much faster, it has more elements can select top, back, side etc like the old solid fuel cooker, so up to temp in around 3 minutes, and after it has reached temperature again cycling on/off.

So in real terms although two ovens and a 4 area hob may be rated 12 kW as with my stand alone cooker, in real terms it will never exceed 6 kW and in the 10 years we have had it, it has never tripped the 32A MCB. And that is the point even if some one tries to cause an over load, the MCB will trip before it does any damage. So in real terms no problem.

 

[Seb101]

It had better not be.

Yes, good point, boarded up was the wrong description. I mean it will be replaced with a single flex outlet which will be behind the units, and wired from an accessible isolator.

 

[ban-all-sheds]

a single flex outlet which will be behind the units

Not allowed.

 

[Seb101]

Not allowed.

What about this is not allowed?

 

[muffking]

Will it be accessible?

 

[Seb101]

The flex outlet will be behind the oven unit. It will be accessible if you pull the oven out. The isolator will be accessible on the wall.

 

[ban-all-sheds]

What about this is not allowed?

Sorry - far too brief - and almost as soon as I'd posted I thought "that wasn't very helpful", but I was away from the PC and couldn't get back.

I took "behind the units" to mean it wouldn't be accessible, i.e. it would be as you had said.

The only note of caution I would sound would be to not rely on a hidden accessory to create a safe zone for concealed cables.

 

[Seb101]

Thanks.

Is it a requirement to have a neon on a cooker switch?

 

[ban-all-sheds]

No.

And there's no requirement to even have a switch. Have one if you want one, but if you don't, don't.

 

[Seb101]

OK - there you have lost me. I thought it was a requirement to have a local isolation point for an appliance which was directly wired to the mains. That is not the case?

 

[yorkspark]

Nope, never has been, only councils and sparks who don't fully understand insist on fitting them.
The mcb supplying the circuit is the isolator

 

[JohnW2]

OK - there you have lost me. I thought it was a requirement to have a local isolation point for an appliance which was directly wired to the mains. That is not the case?

No, although they are usually fitted, and, if accessible, are at least usable as 'emergency switches'. 'Isolation' (e.g. for maintenance) can be achieved at the CU.

Kind Regards, John

 

[muffking]

How about on a 40A shower circuit? My shower manual states to have accessable DP isolation, but like you say the CU main switch would provide that.