Connecting 4KW Oven and 7.5 KW Hob to same CU?

[DazElecNovis]

Hi, can somebody tell me if the following is safe to do....

I have a 4KW double oven and a 7.5KW Induction hob. I want to wire both appliances straight into the same 45AMP switch. For clarity 6mm2 cable runs from the 45AMP switch to the consumer unit where there is a 32A fuse.

Q1) Can I run a 4mm2 cable from the oven and a 6mm2 cable from the hob and connect them into the same 45AMP switch, assuming they will physically fit?

Q2) If the answer to Q1 is 'yes', then is the 32AMP fuse in the consumer unit ok for this demand?

Q3) If none of the above is safe then what can I do? Note: Replacing the 6mm2 cable from the consumer unit to the 45amp switch is not an option, in case that is suggested.

Many thanks for any replies....

 

[Taylortwocities]

Q1) Can I run a 4mm2 cable from the oven and a 6mm2 cable from the hob and connect them into the same 45AMP switch, assuming they will physically fit?....

No. You will need two supplies. The existing supply would be enough for the Hob and a you'll need a new one (16A) with its own isolating switch for the oven.

As an aside, if the cable from the CU is 6mm then all connections to it must be at least 6mm.

Q2) If the answer to Q1 is 'yes', then is the 32AMP fuse in the consumer unit ok for this demand?....

Probably not enough. On a light day it might just cope, on Christmas Day, you'll have cold turkey.

Q3) If none of the above is safe then what can I do? Note: Replacing the 6mm2 cable from the consumer unit to the 45amp switch is not an option, in case that is suggested.....

See answer to Q1

 

[ericmark]

You need 32.6 amp for the hob and 17.5 amp for oven that’s 50 amp which exceeds every device you have so you will need separate supplies. Do check with hob what protection is required you may need a fuse as well as MCB because of the semi-conductors and a special semi-conductor fuse not a standard one.
Eric

 

[The Baker]

As a new comer to electrics may I ask if there is any diversity factor that would apply of am I getting mixed up with free standing ovens?

Tim

 

[plugwash]

I would think some diversity could reasonally be applied here yes. 32A seems a bit low though.

How is the existing cable run? Is there any thermal insualtion arround? Assuming the cable is run clipped direct to a surface of burried in ordinary plaster for it's entire run and the run length is reasonable it should be ok to protect it with a 40A breaker.


What if anything do the do the manufacturers instructions for the oven and hob say about fuse/breaker ratings? If the manufacturers tell you that they need to be on breakers lower than 40A then I would fit a small two way CU between the cooker switch and the devices.

 

[DazElecNovis]

Thanks for all the replies.

Answers to the above questions…..The existing cable is clipped around the kitchen wall behind the base units. Cable length from the consumer unit (in the cloakroom) to the 45amp switch in the kitchen is about 7 metres.

The manufactures (Neff) instructions for the oven is pretty useless, I can’t find any information about fuse/breaker ratings. The hob hasn’t arrived yet due to a useless courier, however should be arriving next week, so don’t know what info that will provide me with, although being Neff to, I’m not holding up any hopes.

So back to a possible solution, if I connect the 7kw Hob only to the existing 45amp switch with 6mm2 cable, I understand this is fine. Can I connect the 4kw oven to a spur of a mains socket, with an isolation switch, if so what cable size could I get away with? Or have I just broken all the rules/regs/common sense by suggesting this……?

Thanks in advance for any replies…

 

[ericmark]

I did look at Neff web site and only instructions for use are available which I agree seems daft as they will come with cooker and only installation instructions will be required before it arrives there are however phone number to call and I will guess the semi-conductor fuses will be built into hob but worth asking first.
As to oven far too big to come off ring main and will need its own supply.
One thing the web site did say is the hob has a boost available when not all rings are used so I would say no diversity is really allowed with these.
Eric

 

[Taylortwocities]

Do your ohms law
Work out the current for a 4KW load at 230volts

Now, you decide - what will happen to a 13amp fuse in the plug when you switch it on full load?

 

[DazElecNovis]

Even if it was a 20amp fused spur?

Ok, last question before I decide to either ditch the induction hob or call up a sparky to fit me another supply!

If I was to change the fuse in the consumer unit to a 40amp*, connect the 7.5kw hob to the current 45amp (switch) control unit and then connect the oven to the hob (so its in series), all using 6mm2 cable, is that ok to do? I have read that 6mm2 cable can be used upto 12kw, so in theory this should be ok, as the combined kw for the appliances is 11.5kw.

*Even though the the max demand of the combined appliances are 50amps, is a 40amp fuse ok for this, bearing in mind that it is unlikely that full load will ever be used, even at xmas.

If all of the above is not a good/safe idea, is my only option to run another supply (16A) from the Consumer Unit to another control unit for the oven?

Many thanks in advance again for any replies.

 

[GaryMo]

Even if it was a 20amp fused spur?

If they existed......
A FCU takes a BS1362 fuse with a maximum rating of 13A.

 

[DazElecNovis]

I think I really meant a 20A Switch or fused outlet unit? Can this be connected off a spur from the circuit main, then to the cooker?

If not then an answer to my previous question(s) would be greatly received.

Thanks.

 

[GaryMo]

Circuit main? Do you mean ring final (or ringmain as some people call it)?
If so then you're stuck with a maximum rating for a fused spur at 13A.
In any case, heating loads over 2kW should be on a dedicated radial circuit and NOT connected to a ring final.

 

[BS3036]

Cookers always seem to cause problems on forums.

Most replies seem to ignore diversity altogether. Some mention it then dismiss it. However there are rules specified in the On-site Guide and one has to assume that the calculations are valid.

There are two ways to treat a hob + oven. The most benign way is to treat them as one combined 'cooker' appliance, and the cautious way is to treat them as separate appliances.

________________________________________________
As combined appliance:

7.5+4kW = 11.5kW
current = 11500/230 = 50A

allowing for diversity: 10 + .3 x 40 = 22A[/b]
________________________________________________
As separate appliances:

7.5kW
current = 32.6A

allowing for diversity: 10 + .3 x 22.6= 17A

4kW
current = 17.4A

allowing for diversity: 10 + .3 x 7.4= 12.2A


total for both appliances = 29.2A

________________________________________________

So either way 32 A cable and breaker will work.

If I had a choice I would do a new install with two separate cables capable of taking each max load without allowing for diversity. Just because it is easy and gives complete peace of mind if you don't trust the IET On-Site Guide.

But for an existing cable which meets the IET rules - how can anyone say it is wrong?

 

[DazElecNovis]

Thanks BS3036. If I was doing a new install then I agree two separate cables from the Consumer unit would be ideal, however without ripping out walls and taking up floorboards etc, I can't do this. So if the IET rules states that 6mm2 and a 32amp MCB is ok then I'm not complaining.

One question though, would you connect both the hob and oven 6mm2 cable into the same 45amp CU (I'm guessing it would be a tight fit!) or would you connect them up in series, i.e. oven connected to the hob which is then connected to the CU?

 

[Taylortwocities]

The diversity discussion in the On Site Guide is a GUIDE, not a rule.
It is up to you, as designer, to decide if diversity applies to your application.

You'll not get 2 x 6mm in most 45A switches. If you decide to run both devices off the one feed then use one of these:

http://www.alertelectrical.com/Wiri...asyfit-Dual-Appliance-Outlet-Plate-PRW217.asp