Wiring Oven, Microwave & Hob – Help Please…..

[active149]

I am about to install a new Neff B1861 single oven and a separate Neff B7881 microwave combi oven above it in a housing unit. At the same time I will also be installing a new Neff T1443 ceramic electric hob. The kitchen has just been built. The electrician replaced my old consumer unit with a new Crabtree mcb type. The hob has been provided with a separate 32A (6mm cable) supply via an isolation switch, so far so good, but it’s the oven & microwave that I need help with.

The single oven unit has a rating of 3.5kw (no cable or plug provided) and the microwave combi oven has a rating of 3.6kw (3 core flex supplied, no plug was attached). Both units in the instructions say they must have an isolation switch and the oven must use a H05VV-F mains connection cable which I have been told is also 3 core flex.
The electrician has provided one double switched socket outlet behind the oven housing which is connected to the kitchen 32A ring main.
From what I’ve read in the forum, both the oven & microwave working at full load can pull 30A? So what are my options?

Can I provide a switched fused spur (FCU) from the 30A ring main (from the double socket behind oven housing) to supply the microwave.
Then, from what I understand, the hob’s electrical connection point allows connection to another/other appliances. Could I then run a 2.5mm T&E cable from the hob to supply the oven via a switched FCU?
If this is possible, what size fuse rating would need to be fitted in the FCU’s?

Apologies for terminology used, not had to do this for a while.
I would be most grateful for any help or suggestions….thanks….

 

[JerryD]

Can I provide a switched fused spur (FCU) from the 30A ring main (from the double socket behind oven housing) to supply the microwave

No, the fcu can only take up to a 13a fuse (your microwave is nearly 16a)

Could I then run a 2.5mm T&E cable from the hob to supply the oven via a switched FCU?

No, same reason

You really need a dedicated 6mm supply to the oven + microwave complete with isolation switch

 

[ban-all-sheds]

the oven must use a H05VV-F mains connection cable which I have been told is also 3 core flex.

FYI - see //www.diynot.com/forums/viewtopic.php?p=55209#55209 - it's probably H05VV-F-3G1.5. Which is a fancy way of saying 1.5mm² 3-core flex. It does look as though that means it does not need to be heat-resistant.

The electrician has provided one double switched socket outlet behind the oven housing which is connected to the kitchen 32A ring main.

Did you not tell him what the load of the ovens would be
Even so, he's let you down - he's not provided an accessible means of local isolation, and if you'd told him there'd be two appliances, even if he assumed each could be connected via a 13A plug, one socket would not have been up to it.

From what I’ve read in the forum, both the oven & microwave working at full load can pull 30A? So what are my options?

Sadly, just the one - get the electrician back to lay in another 32A circuit for the two ovens....

 

[active149]

Thanks for the sound advice – much appreciated. Following up on this then could I ask your advice on a couple of other things please. Firstly, I have checked my CU and there is one spare position for an additional mcb, which is on the right hand side of the CU marked as ‘RCD Protected Circuits’ (all the 32A rings are this side –- lighting and existing cooker are on the left hand side). Will it be ok to connect the 6mm cable here?
Secondly, can I the connect the 3 core flex’s from both the oven and microwave into separate local isolation switches and then run 2.5mm T&E cables from here to connect back up to the 6mm feed?
Also any advice on what types of isolation switch that would be appropriate for the job, and the correct joint box for connecting up the 2.5mm & 6mm cables?
I’m asking these questions in advance of getting the electrician (or another) back so that I’m clear in my mind what I need and what is safe! Thanks again for your time……….

 

[active149]

Can anyone else help me with this please?
Just after some clarification on a few points.........thanks......

 

[plugwash]

yeah putting your ovens on the rcd side should be fine

the loads are above 3KW so you can't use FCUs and i would consider connecting a 16A (roughly) device to a 32A cuircuit without any further protection a bit dodgy (though probablly just about ok)

i would say the best bet is to run 6mm from your CU to a cooker switch and from there run to a small 2 way CU with a pair of 16A breakers for the oven and microwavethe electrician has done the tricky bit (replacing the CU) adding the cuircuit for the two ovens should be pretty easy

 

[active149]

Thanks for the info plugwash…I hadn’t thought about running to a small 2 way CU….how big are they?
I was hoping to fit it in the space at the back of my oven housing…would this be acceptable?
Also, just to clarify a couple of points, would the 3-core flex’s from both the oven and microwave connect directly into the 2 way CU? And would I actually need to run the cable via a cooker switch if each appliance can be isolated at the 2 way CU? Finally, any recommendations for the CU?

 

[JerryD]

The cooker switch is to allow convenient isolation and should be within 2 metres from the oven so you can't do away with this. If you hide the 2 way cu behind the oven it is not 'accessable', also it may well get too hot (I assume you mean directly behind the oven in the housing?)

You don't want to have to pull the oven out if the 16a mcb trips behind the oven.

Also the size of the cu (about 8" x 6" x 5" - the 5" is the protrusion from the wall) will limit where it goes, possibly at the back of a convenient cupboard?

 

[active149]

Thanks JerryD, I see what you are saying; perhaps a 2 way CU would be too big for this solution.
I’ve been giving this some more thought basically because it’s going to be difficult to run a new 6mm cable in due to the concrete floor in new kitchen. As mentioned previously, the electric hob (7kw) has already been provided with a separate 32A mcb supply (6mm cable) via an isolation switch. Would it be feasible to upgrade the mcb to either 40 or 50A so that I can run both the hob and oven (or microwave) from this supply?
Could I then run the remaining appliance from the 32A ring main (take spur from the double socket behind oven housing?). I will only have a fridge/freezer, dishwasher, kettle, toaster etc connected to the ring. Is this possible or am I talking nonsense?

 

[ban-all-sheds]

32A is the most you can use with 6mm cable before you have to take a lot of care doing proper calculations to take account of grouping factors, ambient temperature, installation method, disconnection times.....

 

[JerryD]

As mentioned previously, the electric hob (7kw) has already been provided with a separate 32A mcb supply (6mm cable) via an isolation switch. Would it be feasible to upgrade the mcb to either 40 or 50A so that I can run both the hob and oven (or microwave) from this supply?

Definately not! The mcb is there to protect the cable and must be rated no higher than the cable rating. The cable is rated at up to between 32 to 47 amps depending on installation method so the breaker has to be equal to or less than this. 32a is the safe option, 40a is pushing it, all depends on how the cable is installed regarding correction factors.

The 2 way cu is the best method I think.

Back to the drawing board....................