3.4KW oven

[Taylortwocities]

I’m guessing it’s this one.

https://assetscdn.loadbee.com/catalogue/n99mswzk5cs85sfy/media3.bsh-group.com/Documents/specsheet/en-GB/CM678G4S6B.pdf

No real detail only total load = 3.4KW.

 

[sparkwright]

It would be interesting to see what the manufacturers instructions say - though no doubt they will be European, and we will pick holes in the UK section.

 

[Adam_151]

I think I know what you're saying - another toasted FCU.

Are you saying diversity should really only come to play for ovens over 16 amp or so?

Its not the rating of the loads, I'm saying that diversity shouldn't really come into play when you only have one or two loads, sure apply the cooking diversity guidelines to the oven cirucit when looking at the kitchen submain or the whole place for example, but not to the individual cirucit or you'll end up with nominal ratings being exceeded on a regular basis.

Applying diversity to the cooking appliance itself can work with a whole cooker, because its a collection of quite a few separate loads, so when it averages out, exceeding the norminal rating is likely to be a rare event, not so with an oven with probably only two separate loads.

I'm also more wary of the slight overload condition with FCUs than I would be with other protection types, as you allude to, I've changed loads that have cooked from the fuse outwards, even when not exceeding 13A.

 

[opps]

We haven't asked/been told what is on the rest of that circuit.
It may be a dedicated oven circuit which just happens to have an FCU?
Or, if it's on the kitchen ring, you may want to keep the washing machine and tumble drier off, until the turkeys done!

...and I would want to use a slightly chunkier flex than 1.25mm2

The cooker switch is on its own circuit. I know that it is 4 or 6mm because I recently had to replace the cooker switch and it had suffered from arching.

Any who, it seems to go from the larger diameter cable at the wall then a runs to two 13 amp fixed spur units, one for the lower oven, one for the upper oven.

The oven, currently, rated at 3.4KW is a like for like replacement for the same branded oven that was absolutely fine for about 7 years..

 

[opps]

That is true.

However the heating of the fuse and the plug need to be considered.

The cold resistance of a BS1362 fuse rated as 13 Amp is 0.0075 Ohms and the calculated heat produced in the cold fuse at 16 Amps is 1.9 Watt.

The resistance of the fuse will increase as the temperature of the fuse wire increases and this means that the amount of heat produced will be more than 1.9 Watts.

This may result in damage to plug and/or socket when 16 Amps is flowing for prolonged periods of time.

It is a £1800 German oven. It does not have a plug or flex. It is however a like for like replacement

 

[danechip]

I would have though full load under worst conditions. Can a manufacturer apply diversity to it's own products? It's made me think.

 

[opps]

BTW, thank you for all of the replies.

I was at my mum's yesterday.

The previous electrician seems to have gone from the 4 or 6mm to a "ring" of 2.5mm for the upper (to be replaced) oven and the lower oven. Behind the upper oven is a 13 amp FSU. I assume that the lower oven has a similar set up.

Given that the oven weighs so much (40kg and has to be lifted to shoulder height) I don't want to have to remove it if the fuse blows. I plan to run "clipped" 2.5mm (as a spur) from the 2.5mm "ring" up to a 16Amp MCB in a small enclosure in the cupboard space above and then 2.5mm flex down to the new oven. The spare gap in the enclosure may be used to feed the lower conventional fan assisted oven (one day).

Is the above overkill. I have to confess that I was under the impression that the new microwave combi was 3.4Kw- it is in fact 3.6Kw. Turns out that the old was 3.4Kw, and not the new one.

In the event that either of the two ovens fail, I am hoping that my mum will be able to isolate them individually, without have to use the main wall mounted oven switch.

I hope the above makes sense...

 

[danechip]

Sorry, It doesn't make sense to me, But I am not a sparky.

3.4 or 3.6Kw is neither here or there. 200w is negligible here. 2.5mm is fine. for EACH appliance. 3.6KW plus what ever your 2nd oven is (13A?) is over what 2.5mm can carry (16+13A). I think 2.5mm is about 23A, Without 'diversity'. you would need 4mm to your pair of contactors from your 'ring'

You mentioned 'ring' twice. how do you know it is a ring? Is it part of the kitchen sockets or a dedicated cable as a 'radial' with it's own fusing in the CU.

You will need 2 isolators (switches, one for each appliance) to be able to isolate them separately. The 'wall switch would kill them both. Maybe 2 isolators in that cupboard. IF the ovens have their own fuse in the CU, It can be isolated from there, you do not need a wall switch, but can be a pain in the event of an emergency.

 

[opps]

Sorry, It doesn't make sense to me, But I am not a sparky.

3.4 or 3.6Kw is neither here or there. 200w is negligible here. 2.5mm is fine. for EACH appliance. 3.6KW plus what ever your 2nd oven is (13A?) is over what 2.5mm can carry (16+13A). I think 2.5mm is about 23A, Without 'diversity'. you would need 4mm to your pair of contactors from your 'ring'

You mentioned 'ring' twice. how do you know it is a ring? Is it part of the kitchen sockets or a dedicated cable as a 'radial' with it's own fusing in the CU.

You will need 2 isolators (switches, one for each appliance) to be able to isolate them separately. The 'wall switch would kill them both. Maybe 2 isolators in that cupboard. IF the ovens have their own fuse in the CU, It can be isolated from there, you do not need a wall switch, but can be a pain in the event of an emergency.

I believe that it is a radial that turns in to a "ring" to serve both ovens. By that I mean that there is 4 or 6mm running from the "led" cooker switch to 2.5mm set up as a ring., ie. 2.5mm from one device down to the other and back to the cooker switch.

Admittedly, I am guessing. I have never seen the set up behind the lower oven. The fact that the upper oven had 2 lots of 2.5mm running in to the FSU leads me to assume that they ran a ring off the higher rated cable.

Yeah, I appreciate that I am making assumptions but, hey, every thing was fine for years, The only reason that the top oven is being replaced is down to the fact that it rusted (because of a lack of free air flow).

 

[opps]

Sorry, It doesn't make sense to me, But I am not a sparky.

3.4 or 3.6Kw is neither here or there. 200w is negligible here. 2.5mm is fine. for EACH appliance. 3.6KW plus what ever your 2nd oven is (13A?) is over what 2.5mm can carry (16+13A). I think 2.5mm is about 23A, Without 'diversity'. you would need 4mm to your pair of contactors from your 'ring'

You mentioned 'ring' twice. how do you know it is a ring? Is it part of the kitchen sockets or a dedicated cable as a 'radial' with it's own fusing in the CU.

You will need 2 isolators (switches, one for each appliance) to be able to isolate them separately. The 'wall switch would kill them both. Maybe 2 isolators in that cupboard. IF the ovens have their own fuse in the CU, It can be isolated from there, you do not need a wall switch, but can be a pain in the event of an emergency.

Apologies if I am not being clear.

There is a single cooker switch on the wall that has either 4 or 6mm running in to and out of it. That isolates both ovens. The circuit has it's own MCB in the main consumer unit.

I suspect that 4(?)mm then feeds two lengths of 2.5mm T&E that are run as a ring, that is to say, one leg goes to the lower fused spur, the other leg goes to the upper fused spur, there is an additional 2.5mm that runs between each fused spur (thereby creating a ring).

AFAIK a single 2.5mm is rated at 27amps (if clipped). If configured in a ring, that goes up to far more than 32amps. I potentially need, as you say, about 23amps.

As things currently stand, each of the inbuilt ovens need to be pulled out (fully) to get to the fused spurs.

I am hoping to change that so that each individual oven can be independently isolated via (dedicated) MCBs in an enclosure in the cupboard space above the two ovens.

To facilitate that, I am now thinking that I will replace both fused spurs with a suitable "connector block" (maintaining the "ring") and run 2.5mm T&E up to each MCB and 2.5mm flex back down to each appliance.

I hope that makes more sense.

 

[winston1]

Apologies if I am not being clear.

There is a single cooker switch on the wall that has either 4 or 6mm running in to and out of it. That isolates both ovens. The circuit has it's own MCB in the main consumer unit.

What size is it.

I suspect that 4(?)mm then feeds two lengths of 2.5mm T&E that are run as a ring, that is to say, one leg goes to the lower fused spur, the other leg goes to the upper fused spur, there is an additional 2.5mm that runs between each fused spur (thereby creating a ring).

Why do you think that. It would be very unusual.

AFAIK a single 2.5mm is rated at 27amps (if clipped). If configured in a ring, that goes up to far more than 32amps. I potentially need, as you say, about 23amps.

As things currently stand, each of the inbuilt ovens need to be pulled out (fully) to get to the fused spurs.

I am hoping to change that so that each individual oven can be independently isolated via (dedicated) MCBs in an enclosure in the cupboard space above the two ovens.

To facilitate that, I am now thinking that I will replace both fused spurs with a suitable "connector block" (maintaining the "ring") and run 2.5mm T&E up to each MCB and 2.5mm flex back down to each appliance.

I hope that makes more sense.

Why do you want MCBs? There is one in the CU though you have not told us the size.

 

[opps]

What size is it.

Why do you think that. It would be very unusual.

Why do you want MCBs? There is one in the CU though you have not told us the size.

I made assumptions that turned out to be incorrect.

It turns out that the electrician that was part of the team that installed the kitchen, put the main conventional fan assisted oven on the cooker circuit an then decided to put the higher rated microwave combi oven on the kitchen ring (that explains why I could see two 2.5mm T&E running in to the 13amp fused spur).

I have since removed the fused spur and (in the plastic back box) used 3 way Wago 221 connectors to run some 2.5mm T&E up to an enclosure housing a 16 amp B type MCB. From there I will run 2.5mm flex down to the oven, which I am hoping to lift in to place later this evening.

BTW, in answer to your question about why I would want additional MCBs, when I thought that the cooker circuit was powering both ovens, I wanted a way that would allow each device to be independently isolated. I had hoped to fit two MCBs in the enclosure, one for the top oven and one for the lower oven. I guess I will now need to buy a blanking plate for the enclosure. Annoyingly, the blanking plates will cost more than enclosure if I buy them over the web. Fortunately CEF sell them, which means that the next time I visit my local Toolstation, I shall pop in to the nearby CEF to spend £0.78 for a blanking plate to fit a £3 enclosure.... or just use white electrican's tape...

 

[Taylortwocities]

I have since removed the fused spur and (in the plastic back box) used 3 way Wago 221 connectors to run some 2.5mm T&E up to an enclosure housing a 16 amp B type MCB. From there I will run 2.5mm flex down to the oven, which I am hoping to lift in to place later this evening.

But that is WRONG.

Any connection to a ring final must be fused at 13amp. That is what the FCU is(was) there for. It needs to be reinstated.

 

[EFLImpudence]

Any connection to a ring final must be fused at 13amp.

Why do you say that? It is simply not the case.

 

[opps]

But that is WRONG.

Any connection to a ring final must be fused at 13amp. That is what the FCU is(was) there for. It needs to be reinstated.

I don't understand.

Surely the FSU was there to protect the flex running from it to the old 3.4Kw oven? I have no idea what size flex was previously used.

I am now using some rated at 27amps (2.5mm) but I am concerned that the 13 amp fused is under rated for the maximum load. I don't want my mum complaining that each time she runs the oven at max, it trips the 13 amp fuse.

From my understanding of what I have read here, the primary purpose of fuse protection is to protect the cable supplying the device. I assume that the oven has internal safety features that protect the internal wiring.