Is 20A double pole switch sufficient for total load rating of two ovens?

[Katie]

Hi there,

I'm installing two ovens, each with a total load rating of 3.5kW. (see details of oven here). The kitchen already has a 20A double pole switch and a 45A dual outlet plate. Is the switch sufficient because I've calculated that the current is likely to exceed 20A? However, I'm not sure if these calculations are correct!

Calculations

• the total current of the two ovens is (3.5kW x 2 ovens)/230V = 30A
• The diversity is therefore 10A + 5A (allowance for socket outlet) + (30% of (30A - 10A)) = 21.1A

This is above the 20A rating of the double pole switch. However, I'm not sure I should have included the 5A allowance for the socket outlet?

​

Hob

I'm also installing a hob with total load rating of 10.8kW. This would be connected to a second pre-existing 20A switch via a second dual outlet plate. Again, I've calculated that this will exceed 20A, even if I omit the 5A allowance for a socket outlet.

Calculations

• the total current of the two ovens is 10.8kW/230V = 47A
• The diversity is therefore 10A + 5A (allowance for socket outlet) + (30% of (47A - 10A)) = 26A

This is above the 20A rating of the double pole switch regardless of whether I include the 5A allowance for the socket outlet.


​

Based on the ovens and hob ratings, what would you recommend? Can I wire two ovens to one dual outlet plate which is connected to a 20A double pole switch? And can I wire the hob to the second dual outlet plat also connected to a 20A double pole switch? Or should I replace those switches with, say, a couple of 45A switches? Is there one in between 20A and 45A that I'm missing?

Many thanks for your advice,

Katie

 

[Darkwood]

When working out the expected demand of a complete multi-circuit installation this is a method of using diversity to ensure you don't end up over-estimating the demand.

When wiring the individual circuit i would advice your cabling, switch gear and fusing be rated for the full load of the accessory taking into account all cable calcs for sizing the install... by applying this to the circuit you will be asking for possible future issues... yes diversity will give average demand but what about when the hob is full up and just been loaded with raw cold food... the stats will all be on for a good duration demanding the 10.8kwuntil the pans/water has got high enough to allow the stat to switch.

 

[Des 56]

You mention existing 20 amp DP switches,what supply does these switches have

Then
Are the 20 amp DP switches necessary for 2 No appliances each or is it because they already feed dual outlets
I can't quite understand Kate what you are trying to achieve and why


You obviously know diversity impacts on your choices
Maybe I could get the drift if you were to describe the existing installation and what alterations you are intending

 

[Darkwood]

I re-read this i think she has a 45amp switch but this doesn't tell us the circuit size or fusing ... it seems shes been perusing the internet for answers and found the diversity factor without really understanding it.

As she in IT i feel she's trying to expand a bit here to save money but ive got a feeling Kate this is out of your area. It would need a competent Electrician to browse it over to confirm what can be done here unless you can give us full details of the circuits, install methods, fusing etc etc than we can advise.

 

[Katie]

Hi both,

Yes, correct - to clarify I'm looking for a second opinion, not wiring up the house myself - already had an electrician wire the kitchen up several months ago, and do everything but now oven's arriving tomorrow and just wanting to check (and by 'IT' I've a degree in electrical engineering! ).

Electrician has run two separate cables to 35amp breakers in the CU, these wires run to the 2 separate 20A switches mentioned above and then to two separate 45A dual outlet plates behind where the oven will be.

So I'm just wanting to check if it's fine to install the two ovens into one of these outlet plates and the hob into the other.

Electrician said it'd be plenty, however I'm just concerned as I can't seem to get the maths to add up - maybe missing something, but just looking for second opinion (rather than finding out mid-way through christmas dinner or something!)

Many thanks!

 

[Hawk]

Hi Katie. From what I make, you have 2 separate 32A circuits supplying 2 20A dp switches, which in turn supplys your 2 connection units. If so I would use these existing circuits for the connection of your 2 ovens and run a new circuit in for your hob.

 

[Badged01]

... or just go buy a 45A switch (without socket) and replace the 20A one?

 

[Sintra]

Electrician has run two separate cables to 35amp breakers in the CU, these wires run to the 2 separate 20A switches mentioned above and then to two separate 45A dual outlet plates behind where the oven will be.

Those 20A DP switches should never have been installed on that circuit if I'm reading this correctly. They need changed to 45A DP.

 

[Katie]

Many thanks Hawk,

I'm hoping to avoid running a new circuit in as it would involve smashing into our newly tiled floor! And all walls in the kitchen are brick :-(

Do you think it would be ok to change the two 20A double pole switches for 45A ones? This would mean I could control the two 3.5kW ovens with one 45A switch and the 10.8kW hob could be controlled with the other 45A switch? Both switches are right next to the ovens and hob, which appears to comply with the IEE Regulations that says a switch may control two cookers, provided both are within 2m of it: http://www.tlc-direct.co.uk/Book/6.5.2.htm

Hope that would be ok!

Thanks again

EDIT - Thanks also Sintra. I didn't see your post when I replied with the above. It looks like what you've said is along the same lines as what I was thinking, but it would be great if you could confirm!

 

[Badged01]

Without checking your cable calcs for correctness to handle the load, it would appear on the surface that if you have both ovens on full blast together cooking the sunday roast for the village then they should only be pulling 30A, well below 45A! So yes you can connect both to the same circuit.

 

[Katie]

Thanks all very much for your kind help. I'll make sure the 20amp switches are replaced with 45amp ones.

Katie

 

[Engineer54]

No-one has bothered to ask what size cable the electrician installed?? Let's hope both are 6mm T&E that's being protected by those two 32A breakers!! lo!!

 

[Darkwood]

No-one has bothered to ask what size cable the electrician installed?? Let's hope both are 6mm T&E that's being protected by those two 32A breakers!! lo!!

Er Ermmm!!! I did put that in earlier ... post 4 but then i drifted to the pub :17:

 

[Octopus]

Back to the OP - what makes you think that the installed DP switches are 20A?

 

[Badged01]

No-one has bothered to ask what size cable the electrician installed?? Let's hope both are 6mm T&E that's being protected by those two 32A breakers!! lo!!

That was my assumption! ......."Without checking your cable calcs for correctness to handle the load"