Replacement oven on 32amp ring

[Necrosis]

So apologies in advance as I'm no electrician.
I had an extension built 7 years ago with new wiring. The more I look, the more I see where my builder cut corners.
I want to replace my single Samsung oven
(BT621VDSD). I have pulled it out to see how it is wired but a double socket was placed there and it is plugged in. There is no red isolation switch anywhere, including in any of the cupboards. I have gone to the fuse board and flicked the 32amp fuse. The oven goes off along with the kettle etc, so it has all been put on a 32amp ring.
We have had no problems in 7 years with the power tripping or anything. Even in the microwave, washing machine and kettle are all on with the oven. Not sure if this is relevant!
The oven, whilst a single, is a dual cook where you can separate the top from the bottom with a solid shelf and have 2 different oven temperatures at the same time, cooking 2 different things at once. Alternatively, pull the shelf out and have one very large single oven.
Looking at newer models of the Samsung flex Cook oven the kW seem the same (3.6) and the same advice is given to old and new style ovens to wire it to a 16amp circuit. In manuals for both the old and new though, it does say "power connection - plug the oven into an electrical socket, or use a multi pole isolator switch". The manuals also give the brown, blue, green/yellow wiring diagram.
Question is, should I replace the oven with a 13amp socket, single oven which wouldn't be ideal for us as a family or can I replace it with a similar Samsung dual flex oven which advises a 16amp connection and do what my builder did, and wire it in to a 13amp plug/socket?
We don't have the money for a separate hardwire at the moment.

 

[stem]

In the UK, ovens that come with a 13A plug fitted by the manufacturer are designed to be plugged into a normal socket, that's how they are designed to be installed and if your original oven came like that your builder didn't do anything wrong.

In Europe where they don't have fused plugs 16A circuits are common place and ovens are rated accordingly, often as with your example in excess of 3kW

If your new oven hasn't come with a fitted 13A plug, and the instructions state that it should be connected to a 16A circuit that's what you should do.

For future reference the 'Your Projects' Forum is for......

Ideally, you should really have posted your electrical queries in the 'Electrics UK' forum instead.

 

[Necrosis]

Sorry for posting in the wrong area and thanks for responding. New to forums and these things.
The original oven has a 13 amp plug attached. It's an old one by the looks of it and completely different colour to the cable. I wonder if we "get away with it" as if you have 2 ovens on or oven and grill which is what we use, it is less then 3000 Watts. When I look at the specification of the oven online it says to hardwire so I know the builder didn't do a good job.

 

[stem]

13A plugs don't take kindly to being overloaded, they often run warm at 3kW. You might get away with it, but personally I wouldn't want to risk it.

 

[ericmark]

As @stem says, over loading a 13A plug is not good. A plug should be in free air to cool it, however with an oven often there are fans cooling anything within the carcase.

The BS 7671 appendix says any fixed appliance over 2 kW should have a dedicated supply, that would include immersion heater, tumble drier, dish washer, oven, and washer/drier, and clearly we don't follow that advice, I have 16 RCBO's in my consumer unit now, to follow that advice it would be up to at least 20, so the electrician has to use some common sense, and although the oven may use 13A for 15 minutes after that it will start cycling off/on so average power is well under 3 kW.

The immersion heater may take 3 hours so that is normally on a dedicated circuit, after that it is the drier, be it independent or washer/drier, compared with the drier the oven takes very little.

Clearly should be under the 3 kW/13 A limit, but as to indepent supply I would not be too worried.

In the main we take a 32A supply to a cooker switch and from there to the cooker connection unit, and you can get a double outlet cooker connection unit so hob and oven are on same 32 amp overload device, however I have seen it where there is gas to the home that there is no cooker supply.

 

[JohnW2]

The BS 7671 appendix says any fixed appliance over 2 kW should have a dedicated supply, that would include immersion heater, tumble drier, dish washer, oven, and washer/drier, and clearly we don't follow that advice ...

Eric, you repeatedly talk about that 'guidance' in Appendix 15 of BS7671 totally out of its actual context.

What it actually suggests (as 'guidance') is that one of the things which can be done to make it unlikley that the cable of any part of a ring final circuit will be 'overloaded for long periods' is to utilise dedicated radial circuits for "cookers, ovens and hobs" with a related power over 2 kW, ratrher than connecting them via a ring final circuit.

This guidance is therefore specifically about the reducing the risk of large loads (near one end of the ring) overloading one leg of a ring final circuit, does not apply to radial circuits and, despite what you suggest above, does not say anything about putting "immersion heaters, tumble driers, dish washers or washer/driers" on dedicated circuits, even if they are >2 kW.

For the sake of those who rely on what they read here for knowledge of what BS7671 (or Appendices thereof) actually says, I think we should always try to correctly report what is written, and in what context.

Kind Regards, John

 

[ericmark]

OK I agree plugging in the impedance meter and checking that the socket is reasonable near to the centre of the ring final and not close to the ends you can be reasonably confident that it will not overload either end of the ring final for any extended time.

Or you can use a low ohm meter again to work out how close to the centre the socket to be used is.

In the front kitchen of my house I am sure due to position relative to consumer unit there would not be a cable overload, but the same could not be said about my back kitchen.

So yes

The load current in any part of the circuit should be unlikely to exceed for long periods the current-carrying capacity of the cable (Regulation 433.1.5 refers). This can generally be achieved by:
(i) locating socket-outlets to provide reasonable sharing of the load around the ring
(ii) not supplying immersion heaters, comprehensive electric space heating or loads of a similar profile from the ring circuit
(iii) connecting cookers, ovens and hobs with a rated power exceeding 2 kW on their own dedicated radial circuit
(iv) taking account of the total floor area being served. (Historically, limit of 100 m² has been adopted.)

However I will allow you to explain what that means in real terms, and how the DIY guy can ensure all is OK without tools to measure the low ohms or loop impedance.

So not so much as the the electrician has to use some common sense, to the electrician should carefully measure first. Maybe I should not have said I would not be too worried may be I should say careful measure the position of the outlet around the ring final and work our how the load will be shared, however with most ovens within 15 minutes they are cycling, OK as part of a range cooker it may be a problem because of the extended time it takes to warm up, but most ovens there will be not problem.

With a tumble drier yes possibly a problem, but unlikely for an oven.

 

[JohnW2]

OK I agree plugging in the impedance meter and checking that the socket is reasonable near to the centre of the ring final and not close to the ends you can be reasonably confident that it will not overload either end of the ring final for any extended time. .... Or you can use a low ohm meter again to work out how close to the centre the socket to be used is. .... In the front kitchen of my house I am sure due to position relative to consumer unit there would not be a cable overload, but the same could not be said about my back kitchen.
However I will allow you to explain what that means in real terms, and how the DIY guy can ensure all is OK without tools to measure the low ohms or loop impedance.

It depends upon whether you want to look at this in terms of common sense/pragmatism or something else. I agree that things may be a little different in other situations, but in the (very common) context of a 32A 2.5mm² Method C ring final circuit, I really do not personally think that there is anything to worry about at all.

Method C 2.5mm² cable has a CCC of 27A, and we know from the required characteristics of an MCB that this means that the cable is deemed to be able to carry 30.5A (27A x 1.13) indefinitely without coming to any harm, and to be able to carry 39.2A (27A x 1.45) for 1 hour without coming to any harm.

Assuming someone didn't overload the circuit with more than its design current (32A in this case) (which is obviously possible with any sockets circuit, but beyond the control of the designer/electrician), the worst that could happen would that a 'full' 32A load might be applied very close to one end of the ring, such that virtually all of that 32A flowed through the 'short' leg of the ring. However, (a) such an occurrence would be extremely unlikely, (b) it's even more unlikley that such a situation would persist 'for a long period' and (c) even if it did happen, per the above figures the cable would not come to any harm, anyway.

I would be very surprised if, in the actual world, any cable in a ring final circuit has ever come to harm as a result of this theoretical consideration - at least, in the case of a 32A 2.5mm² Method C ring final circuit.

So not so much as the the electrician has to use some common sense, to the electrician should carefully measure first. Maybe I should not have said I would not be too worried may be I should say careful measure the position of the outlet around the ring final and work our how the load will be shared ...

It's all very well saying that an electrician should make "careful measurements", but don't forget that what we're talking about is a householder plugging things into sockets, now, and in the future, so the very most the electrician could do on the basis of his/her measurements would be to advise the consumer about 'plugging things in'.

If a designer or electrician really were concerned about this theoretical issue, they should ensure that there are no sockets closer than, say (for 2.5mm² Method C), about 16% (of ring length) from either end of the ring - but I doubt that (m)any would be sufficiently concerned to do that.

Kind Regards, John

 

[ericmark]

I think if I remember right you need to use 2.5 mm² unless mineral when it can be 1.5 mm², and the installation method must be so it can carry 20 amp. As you have said clipped direct 2.5 mm² 624B twin and earth LSZH cable is rated at 33 amp Batt cables spec the table 4D5 is for thermal plastic cable (624Y), there is no table for thermal setting LSZH to match the 4D5 and the problem is how do you know if cable is 624B or 624Y throughout its length. The same goes for installation method, method 101#, A and C are all above 20 amp with 2.5 mm², but 100, 102 and 103 can't be used with 2.5 mm² it would need 4 mm² or 6 mm².

So if you know the installation method and type and size of cable, then we could state the amps permitted, but often we have to assume it was wired to installation method 101 or better, we don't actually know.

As I said to start with I see no problem in real terms, but there are exceptions, the lollipop ring for example, using 4 mm² or 6 mm² to take the supply to kitchen then form a ring means the sockets close to the cooker connection unit or other unit used to change from 6 to 2.5 where ring starts can cause an overload. I don't expect this is the case here, but we don't know, so we can say unlikely to cause a problem, but can't say it will not be a problem.

 

[JohnW2]

I think if I remember right you need to use 2.5 mm² unless mineral when it can be 1.5 mm², and the installation method must be so it can carry 20 amp.

True, even though I don't understand the reason for the distinction between PVC and MICC cables!

...but often we have to assume it was wired to installation method 101 or better, we don't actually know.

I acknowledged that things would be a bit different if it were not Method C 2.5mm² T+E.

As I said to start with I see no problem in real terms, but there are exceptions, the lollipop ring for example, using 4 mm² or 6 mm² to take the supply to kitchen then form a ring means the sockets close to the cooker connection unit or other unit used to change from 6 to 2.5 where ring starts can cause an overload.

Sure, but you're merely talking about a (rare) reason why some sockets might be close to the end of a ring.

I'm not really sure what you are really suggesting. I may be wrong, but I doubt that you are saying that you would advise (maybe even try to 'insist' on!) a householder having separate dedicated circuits for each tumble drier, dish washer, washer/drier, cooker, oven and hob etc., are you? I see only a limited number of options for a designer/electrician

1. As above, 'insist' on separate dedicated circuits for each tumble drier, dish washer, washer/drier, cooker, oven and hob etc.
2. Install only radial circuits
3. Install a ring circuit with avoidance of any sockets close to either end of the ring (minimum distance from end depending on installation method)
4. Install a ring circuit 'in the usual way', with sockets wherever the householder has requested them
5. As (4), but combined with advice to householder as to what should (or should not) be plugged in where

Which of the above are you advocating - or are there others I've overlooked? I suspect that (4) is by far the most common [or (2) for some electricians].

Kind Regards, John

 

[BS3036]

True, even though I don't understand the reason for the distinction between PVC and MICC cables!

Probably because copper does not melt at such a low temperature as PVC.

 

[JohnW2]

Probably because copper does not melt at such a low temperature as PVC.

I'm not sure I buy that one. The CCC figures (for various installation methods) we work with obviously indicate 'maximum current' figures way below those which would result in PVC melting!

In any event, the point is not so much about the PVC/MICC distinction but, rather, that the minimum CCC requirement (for a ring final) for any cable is 20A - but that, although 2.5mm² (PVC) T+E with a CCC of 20A (e.g. Method 100/102) is allowed, 1.5mm² (PVC) T+E with a CCC of 20A (Method C) is not allowed - despite the implication (of the CCC tables) that both cables (with respective installation methods) would reach roughly the same conductor temp with 20A (or any higher current, come to that) flowing through them.

Kind Regards, John

 

[EFLImpudence]

I think you should ask the question the other way round.

Why is 1.5mm² T&E not allowed for ring final circuits?

 

[JohnW2]

I think you should ask the question the other way round. Why is 1.5mm² T&E not allowed for ring final circuits?

Eh? I thought was the question which I was asking

Same material and configuration, hence presumably same conductor temp when both (because of their respective installation methods) have the same CCC - so, if "20A" 2.5mm² is allowed (Method 100/102) why not also "20A" 1.5 mm² (i.e. Method C)?

Kind Regards, John

 

[EFLImpudence]

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