Double electric oven - Wiring

[stanrog]

Hi
I will be replacing my gas double oven with an electric one, one like a Bosch HBM13B150B. Total load rating of 4.4kw.
Hob is to remain as Gas
My gas oven electrical circuit is fed by a 13amp fused plug into a single switched socked fed by 2.5mm cable on a ring main.
From info gleaned on this site and others, it would seem that I need to replace the 2.5mm cable with 4mm cable on a single circuit. Would this be suitable. Also, advice on the socket would be appreciated, I cannot find a single switched socket for a double oven. Also what should be the fuse rating in the fuse box.

many thanks
Stanrog

 

[PrenticeBoyofDerry]

Hi
I will be replacing my gas double oven with an electric one, one like a Bosch HBM13B150B.

One like or the same?
If you know what model you are wanting, the best advise would be read the manufacturers instruction supplied with the appliance.

 

[riveralt]

Your oven rating before diversity comes in a just under 20Amps so 2.5mm2 T&E would be sufficient though you might want to consider 4/6mm2 or 10mm2 T&E to future proof. You will also have to consider the size of the cable from the oven to the cooker connection unit and when calculating diversity whether or not you have a socket in your cooker switch. The final thing regarding cable size will be the cable run and whether or not it is covered by insulation or carried in trunking.
Your MCB/Fuse needs to be lower than the current carrying capacity of the cable but sufficient for the load.
Your circuit will also need to be additionally protected by an 30mA RCD.
Not sure what you mean by needing a socket? This oven will need to be wired direct to the cooker connection unit and then to the cooker control switch and from there to the consumer unit.

Also this work is notifiable - so if you intend to do this work yourself you will need to contact your Local Authority Building Control office before you start the work, pay their fee and provide them with the necessary test certificates and test results for the new circuit

 

[stanrog]

Thanks riveralt.
Cable run from electrical switch to fuse box approx 10 to 12 metres. 1st 2 meters in conduit in wall then aprrox 6 metres under floorboards then 3 metres between cavity wall. Will i need insulated cable. Will use not less than 4mm2. Cable length from power switch to cooker less than 1 metre.
Your comment [This oven will need to be wired direct to the cooker connection unit and then to the cooker control switch and from there to the consumer unit.] has a layman like me a little confused. I assumed cooker might be already wired to plug or otherwise I could run cable from power switch to a single connection on the cooker.

Stanrog

 

[riveralt]

Thanks riveralt.
Cable run from electrical switch to fuse box approx 10 to 12 metres. 1st 2 meters in conduit in wall then aprrox 6 metres under floorboards then 3 metres between cavity wall.

Will i need insulated cable. Will use not less than 4mm2.

No the cable must not pass through insulation or conduit otherwise it will need to be derated.

Cable length from power switch to cooker less than 1 metre.

What do you mean by power switch?

Your comment [This oven will need to be wired direct to the cooker connection unit and then to the cooker control switch and from there to the consumer unit.] has a layman like me a little confused. I assumed cooker might be already wired to plug or otherwise I could run cable from power switch to a single connection on the cooker.

This is a cooker control switch:
http://www.screwfix.com/p/45a-sw-skt-neon/25231
This is a cooker connection unit.
http://www.screwfix.com/p/45a-cooker-outlet/16686

It is highly unlikely that a 4.4kw oven will be wired to a normal fused plug - the maximum load for a 13Amp plug is under 3kw.

To be honest your replies do not instil confidence that you will be able to carry out this installation safely.
You have not mentioned anything about RCD's whether you have them for example? What type of Consumer Unit do you have? How you intend to test the circuit and complete the certificates for the Local Authority?

 

[stanrog]

Thanks riveralt and I can appreciate your concern.
What I was hoping was to replace the single gang 13amp switch socket next to the oven housing with something similar to a 45A DP Sw + Neon but 1 gang size. Clearly this is not possible.
I do not have RCD's. The house is 1960's with old fashion fuses ie with fuse wire. Already fitted next to the main fuse box is a single 30amp fuse box with off/on switch presumably used by the previous occupants for a electric cooker. This is not in use now although I was considering using it for the new cooker. I will not now and will replace with a RCD unit. From this I will run 4mm2 cable (not insulated). At the cooker housing end it seems that I need to connect the 4mm2 cable to the cooker control switch unit and then using 4mm2 cable to the cooker connection unit and thence still using 4mm2 cable to the oven.
Regarding the MCB/fuse, is this in the connection unit and if not, how is this fitted to the circuit.

As far as testing etc, I will need to pay an electrician to do this.

Sorry if I am testing your patience.

regards
Stanrog

 

[EFLImpudence]

Already fitted next to the main fuse box is a single 30amp fuse box with off/on switch presumably used by the previous occupants for a electric cooker.

This is not in use now although I was considering using it for the new cooker. I will not now

Why not?

and will replace with a RCD unit.

You could do that and still use the circuit.

4mm2 cable (not insulated).

What does that mean? Sounds dangerous.

Regarding the MCB/fuse, is this in the connection unit and if not, how is this fitted to the circuit.

I don't think you should be contemplating doing this work.

As far as testing etc, I will need to pay an electrician to do this.

Also this work is notifiable - so if you intend to do this work yourself you will need to contact your Local Authority Building Control office before you start the work, pay their fee and provide them with the necessary test certificates and test results for the new circuit

You're not listening (reading and inwardly digesting).

 

[stanrog]

Re last message, Part of old circuit was removed at least 20 years ago to allow new 2.5mm2 cable to feed gas cooker's electrical demand.

Re insulated cable this is my misunderstanding of what riveralt advised, I thought he was referring to the cable but I now suspect floor/wall insulation.

Having done a bit more research I feel that the following would be suitable.
At consumer unit end (CU) (terminology new to me to indicate the fuse box end), fit a 32A MCB. Run 4mm2 T&E cable from CU to cooker housing. 4mm2 cable is rated 7.75Kw and the max load on it will be 4.4kw. Understand that load of 7.75 needs to be derated because thermal rise in cable is not free to dissipate in 2 short sections of its run.
Undecided as to whether a RCD (40A 30mA Double Pole Type AC) is necessary. Several other sites I have looked at do not include an RCD on cooker circuit or in the CU; only a MCB at the CU. One reason undecided is that my CU needs to be updated to MCB's & RCD at some time in not to distant future.
At cooker housing end, 4mm cable to be connected to Marbo 45A DP 1G Switch + Neon. Run 2.5mm2 cable from this switch to the cooker unit.

Re contacting my Local Authority Building Control office, I do not need to consider this until I have decided whether I do the work or employ an electrician.

regards
Stanrog

 

[riveralt]

Just to get the LABC issue out of the way - if you do it yourself you must contact the LABC before you start the work. If you intend to employ an electrician you must speak to him before you start any work. He is the one who will sign of the work as designed, installed and tested by him. He will not be able to just test the work and sign the Electrical Installation Certificate - that would be illegal.

Back to the issue.
RCD is necessary unless you have surface mounted cable from the CU to the oven. If the cable is buried then it must be buried more than 50mm below the surface or be mechanically protected. So I would suggest that it SHOULD be protected by an RCD.
This does not necessarily mean a brand new CU. You could use a garage unit (search on google) or if you have room on your current CU for a 32Amp MCB then the RCD could be run inline.

If you want to go with 4mm2 T&E okay but run it all the way to the cooker connection unit and to the oven. Though for the bit from the cooker connection unit to the oven you will need to be guided by the manufacturers instructions.

 

[17thman]

but also put in an isolator

 

[stanrog]

Thanks riveralt, your advice has been taken onboard.

stanrog

 

[EFLImpudence]

If the cable is buried then it must be buried more than 50mm below the surface or be mechanically protected. So I would suggest that it SHOULD be protected by an RCD.

This does not mean you must or indeed can bury the cable more than 50mm. deep.

Wall depths are limited by the Building Regulations to -

one sixth the thickness of the leaf horizontally and one third vertically.

So, in a 'normal' building this is not possible.