Wiring in new Oven and induction hob.

[JimboJet]

Hi, first post, although have used the site from time to time for good advice so go easy on me!

I'm refitting a kitchen that was fitted around 10-15 yrs ago. My consumer unit is a Crabtree box (with RCD) and the cooker circuit is on a 32 Amp MCB at the unit.

This then leads to a Switched cooker and single socket unit on the wall, which in turn leads to a surface mounted junction box.

The old gas hob (electric ignition) and single electric oven used to both connect to this junction box.

I believe the cable is 6mm


So onto my queries:

I plan to fit a Siemens single oven rated at 3300w and a Siemens Induction hob rated at 7200w(!). Both the oven and hob have to be hardwired.

7200/230 = 31.3 Amps
3300/230 = 14.35 Amps

1st option:
I'm not sure I can connect both of these to the junction box without any changes. If I were to do so and change the cooker switch/socket unit for just a cooker switch and swap out the 32a MCB for a 45a MCB would this be ok?

2nd option:
Connect the Hob to the existing junction box with the existing 32a MCB protection in place, change the cooker switch/socket unit to just a cooker switch.

There is an existing switched 13amp supply which was used to power a fridge freezer (fused switch above worktop, unswitched socket below) Can I replace this socket with a junction box to connect the new oven to and change the switch unit for a more obvious dedicated (red) oven switch.


I will add a photo of the existing setup if it helps to explain. Apologies for the vague terminology in parts!

Existing socket/switch layout

• JimboJet
• 23 Feb 2009
• 1

Proposed changes to allow new electric oven and induction hob

 

[17thman]

Hi

After you have applied divesity to the appliances a 6 mm2 conductor should be fine as 6mm will take 46amps at best. If you know that the cable runs through any insulation for a parts of it run, especially if embedded in it and not touching any other surface then it may get above its maximum allowed operating temperature and you may want to opt for 10 mm2

I personally think that if you connect both to the same ccu then you have an mcb in your consumer unit protecting potentially a low current carrying capacity cable to the oven.

I dont like applying the onsite guide figures on diversity myself to cooking appliances, as i often have oven on full with 3 or maybe 4 of the plates in action and on quite high

This believe the work, as it is in the kitchen is notifiable. If you change the 13amp socket on the ring final in the kitchen to a higher rated cc unit then this is not changing like for like.

 

[JimboJet]

Ok so I could connect them both to the existing cooker outlet and leave the 32A MCB in place. This would be fine until all four rings and the oven were on at full whack where the MCB may well trip?


Not ideal in my opinion, but essentially still safe is that what you're saying?

 

[ColJack]

the second option is your best bet..

hob onto the old cooker circuit, oven onto the ring using a 20A switch above counter and a flex outlet below..

 

[17thman]

Hi again, what i am saying is that a 32 mcb would not trip with a few more volts in the event of full usage and the conductors get hotter than they should.so the cable needs to be rated correctly. In the event of an oven and hob wired together you have a 32mcb protecting potentially smaller conductors as the appliance is rated at 3300w your circuit protection should be equal to or greater than the design current but smaller than the lowest current carrying capacity cable.

 

[JimboJet]

Bingo, that's what I thought, many thanks.

And as far as current regulations and such that would satisfy the various bodies that the installation was safe and above board? Would this be notifiable under part P?

As I plan on renting the flat in the near future I would like to have the necessary permissions/consents in place so I don't run the risk of being nobbled for it down the line.

Would a detailed photographic storyline be sufficient for someone to sign it off? Unscrewing faceplates for inspection of connections is one thing, removing walls, units and worktops is quite another!

 

[17thman]

as coljack says, but this maybe then notifiable

 

[JimboJet]

@ 17thman Thanks, I understand you. Essentially the worse thing I could do is up the 32a MCB to 45a without uprating the cable accordingly. That is what I first thought but wanted clarification, thank you.

Going back to the proposed changes...

I want to have the oven built in to a tower unit, the existing switch will clearly need to be repositioned, but the existing socket if replaced by a flex-outlet will be immediately behind the oven.

Is this acceptable or should I move this flex outlet over a few cms to behind the adjacent 600mm base unit so it isn't in the void behind the oven tower unit?

As a further thought, the oven is listed on the Siemens site as being 16A. Are there 16A protected switches or is a 20A still ok?

 

[ColJack]

since you already have a gas hob installed, why not just replace it with another gas hob?

this solves all you problems, removes any need to contact the LABC and since you'll need a CORGI registered plumber to remove the gas to the hob anyway, will not cost that much more than you'd be spending anyway..


17thman, you can use smaller cables than the protective device allows as long as certain criteria are met..
one is if it's less than 3m..
another is that the load is limited downstream of the change in conductor size by either a fixed load or the use of fusing down...

( this is why you can spur off a 32A ring with 2.5mm cable rated at 27A.. )

 

[17thman]

sure mate, i know, i just wanted to point out that personally, i dont like it.

 

[JimboJet]

since you already have a gas hob installed, why not just replace it with another gas hob?

this solves all you problems, removes any need to contact the LABC and since you'll need a CORGI registered plumber to remove the gas to the hob anyway, will not cost that much more than you'd be spending anyway..

I hear ya, and after seeing that its not as simple to fit in the induction hob as I had first thought I'm tempted to agree with you. It's just that I like the flush look of the induction hobs and more importantly they are so much easier to wipe clean!

I take it that the 20A FCU to the oven is not going to provide the protection I need? Doing the 3300w/230v calculation comes out at 14.35A so would a 13AMP FCU be close enough or would I still get nuisance trips/blown fuses?

 

[JimboJet]

On looking a little more into this, I wonder if I may be ok putting both the Oven and the induction hob onto the existing cooker supply junction box protected by the 32A MCB.

Ok this would be with using Diversity and I do see the arguments against applying these rules where it comes to Cooking appliances - however, this is a small 2 bed flat and I think its reasonable to assume that even on Christmas day not every hobplate and oven will be working at max Wattage.

Recapping:

Fan Oven with grill 3300w
Induction Hob 7200w total broken down as 1800,1800,1400,2200w plates.

Diversity Calculation:

Oven 3300/230 = 14.35A

10A+(4.35x0.3) = 11.305A


Hob 7200/230 = 31.3A

10A+(21.3x0.3) = 16.39A



11.305+16.39 = 27.7A Designed Load.

Again, to recap the existing Cooker/gas hob has a 32A MCB at the CU.


Even if this still isn't sufficient/ideal then applying the same diversity assumptions to the Oven if wired to the Ring through a 13A FCU and flex outlet should be acceptable?

Please don't get me wrong, I'm not trying to create an unsafe situation but clearly if I can wire both units into the existing cooker junction box then I believe I am correct in thinking I don't need to seek Part P approval.

If on that one day of the year it MAY cause the MCB to trip but at all other times it is going to cope then I'm tempted to take the existing wiring solution. Alternatively if I'm advised that it is a definite risk, then I'm more likely to seek the appropriate notifying route or simply go with a gas hob (installed by a corgi plumber!)

 

[17thman]

Jimbo, i concur that your diversity calculations make for a safe circuit and technically i feel that you are correct in applying this and connecting them both through the ccu to the existing 32A mcb. I was just pointing out some areas for you to double think about so that you can sleep at night.

 

[JimboJet]

Firstly, thanks again for all of your comments and help.

Secondly, I haven't started any of this work and I haven't yet purchased anything with which to do the same. The reason being is that I am simply researching the correct methods of doing something before launching into it. If it seems that the best course is to get a pro in then so be it.

So far, we've established (as is usually the case) that it would be "okay" to connect both the oven and hob to the existing Cooker circuit.

I think we can safely agree that in an ideal world, running a 10mm2 cable from the CU to the cooker would be optimum.

I haven't mentioned yet that the CU has no spare slots for another dedicated circuit.


There is still the option of a separate FCU for the oven and having the hob on the existing cooker circuit. Where do I take the feed from though? From the cooker circuit with a 20A or 13A fuse or from the ring via a 13A fuse?

Is this an alternative solution?

 

[JimboJet]

Can anyone offer guidance on the proposed oven spur, or should I just wire them both into the existing cooker circuit?