Replacement Single Oven - 13A or 16A

[JohnW2]

Ah whoops. Missed the h ... It’s still 15A though.

What's 15A? As I said, the self-cleaning cycle is 8.7 - 10A and, as for actually cooking, if one accepts application of the concept of diversity )which apparently yoiu don't), then that's about 11.7A (after diversity).

It's only 15A if one rejects the application of diversity.

Kind Regards, John

 

[AndyPRK]

To me diversity is about a cooker with typically 4 rings, grill and oven.

It’s highly unlikely all 6 of those would be on at the same time. And if some were the thermostats would cycle on and off at different points.

I don’t see how you can realistically apply that to a single element oven.

 

[scousespark]

The oven is rated at higher than 13A so it cannot be used with a SO. It will overload the fuse.

 

[EFLImpudence]

I don’t see how you can realistically apply that to a single element oven.

Such an oven would not have only one element.

However, if it has two plus fan, and they can be on at the same time, it is the same.

 

[JohnW2]

To me diversity is about a cooker with typically 4 rings, grill and oven. It’s highly unlikely all 6 of those would be on at the same time.

If (far from impossible - and certainly can't be 'discounted'), one switched on all of the 'rings' and oven(s) simultaneously from cold, then all would be 'on' for as long as it took them to 'get up to temperature'

... And if some were the thermostats would cycle on and off at different points.

They would certainly cycle on and off, but you couldn't guarantee that there would not be times when all, or most, were on. The 'standard'; diversity calculations must therefore be considered to be acceptable even if that 'worst case' happens.

I don’t see how you can realistically apply that to a single element oven.

One can't, in the sense that you are trying to do. However, as I keep saying, one can have 'diversity over time' as well as 'across loads' - i.e. one can consier the average load over a 'reasonable' (say 30 min) period of time. I fairly recently illustrated (see here) that in relation to my ('single element) 1.7 kW fat fryer, the average draw over the first 15 mins (from cold) was 1,200W, and that settled down to about 600W (and remained at that level thereafter) within the next 5 minutes or so. Extrapolating from that to a ('single element') 3.6 kW oven, one might expect that the average over any 15 min period would quite rapidly settle down to about 1,270 W (i.e. around 5.5A) ......

Kind Regards, John

 

[JohnW2]

The oven is rated at higher than 13A so it cannot be used with a SO. It will overload the fuse.

I respect your view but, as I suggested before, by saying that one has to consider the maximum instantaneous current, rather than the after-diversity (i.e. 'average') current, you appear to be effectively rejecting the application of diversity.

Kind Regards, John

 

[scousespark]

The MIs state the oven is rated at 16A and has no plug. If I was called to quote I'd say this needs a dedicated circuit. For a single oven I would not apply diversity. I would not fit a plug on a 16A appliance.

 

[JohnW2]

The MIs state the oven is rated at 16A and has no plug. If I was called to quote I'd say this needs a dedicated circuit. For a single oven I would not apply diversity. I would not fit a plug on a 16A appliance.

Fair enough, and thanks for confirming that you would not apply diversity.

You say a 'dedicated circuit'. Does that mean that you wouldn't be prepared to put it on a (say, 32A) 'cooker circuit', together with a hob? If you would be prepared to do that, would you then be prepared to apply diversity (to the entire load of oven+hob)?

Kind Regards, John

 

[scousespark]

I have been answering the original question, not a set of hypothetical questions. If a customer called me to fit a single oven, rated at 3.6KW, I would not apply diversity and not fit a plug. That is what I would do as a professional electrician. If there was a hob, I'd look at a single circuit but the single oven would still be rated too high for a SO.

 

[donrkebab]

I have been answering the original question, not a set of hypothetical questions. If a customer called me to fit a single oven, rated at 3.6KW, I would not apply diversity and not fit a plug. That is what I would do as a professional electrician. If there was a hob, I'd look at a single circuit but the single oven would still be rated too high for a SO.

Easy when you know how. And - I'm sure - also realise that in a world where voltage can vary from 200ish to 250 all electrical calcs involving voltage are pretty meaningless, and a responsible person would always use the most onerous figure. Which in this case means 3600w/200v=18Amps, the oven should be on its own 20A circuit. And the graphs and other nitpicking pettiness can be explained to the coroner at the inquest into the fatal fire that was caused when the oven thermostat got stuck and did not turn OFF, and that is not a rare event.

EDIT This calculation is incorrect, see next post from EFLImpudence for the correct calc, but my argument remains the same.

 

[EFLImpudence]

a responsible person would always use the most onerous figure. Which in this case means 3600w/200v=18Amps,

That's not how it works.

A quoted 3600W @ 240V is 15A

but at 200V would be 2500W and 12.5A.

The most onerous would be at 250V - 3900W and 15.6A

Only the resistance of the elements is constant. The Wattage and Amperage rise and fall with the Voltage.

Of course, should the resistance of the elements increase with the temperature rise, that too will reduce the Wattage and Amperage.

 

[JohnW2]

I have been answering the original question, not a set of hypothetical questions. If a customer called me to fit a single oven, rated at 3.6KW, I would not apply diversity and not fit a plug. That is what I would do as a professional electrician.

I realise that, and I'm grateful for your giving us your professional opinion. The 'hypothetical questions' have been my attempt to gain a fuklkl understanding of what your professional opinion (as regards the application of diversity) actually was.

If there was a hob, I'd look at a single circuit but the single oven would still be rated too high for a SO.

I was not suggesting that the oven would be supplied through a 13A plug. What I was asking was, if the oven and a hob were hardwired into the same circuit, whether you would apply diversity when designing that circuit and, if so, whether you would include the 3.6 kW of oven load in your diversity calculation.

As I said, all I'm trying to do is to gain a complete understanding of your professional opinions regarding diversity.

Kind Regards, John

 

[JohnW2]

Easy when you know how. And - I'm sure - also realise that in a world where voltage can vary from 200ish to 250 all electrical calcs involving voltage are pretty meaningless, and a responsible person would always use the most onerous figure. Which in this case means 3600w/200v=18Amps, the oven should be on its own 20A circuit.

As EFLI has explained, you have got all that entirely wrong.

And the graphs and other nitpicking pettiness can be explained to the coroner at the inquest into the fatal fire that was caused when the oven thermostat got stuck and did not turn OFF, and that is not a rare event.

Are you suggesting that a fire could result from ~15A flowing through a 13A fuse (even if for a long period)? If so, I wonder if that has ever happened (particularly given that it takes ~22A to blow a 13A fuse).

I have absolutely no problem with people 'erring on the side of caution'. However, the concept of diversity has been a part of accepted electrical design practice for decades, so I'm interested to hear how some electricians are interpreting and applying it (or not).

There is at least one electrician here who regularly points out that, on the basis of the 'standard' diversity calculation, large cookers (or oven+hob combinations) with maximum possible power consumptions up to about 19 kW (about 83A at 230V) can be supplied from a 32A circuit. How do you feel about that?

Kind Regards, John

 

[donrkebab]

That's not how it works.

Absolutely correct, I screwed up on the calculation.

However the point of my post remains the same, cable ratings and protection must be sized for the most onerous situation, ie when things go wrong. And that this branch of electrical engineering is not an exact science due to the unknown variables.

 

[JohnW2]

However the point of my post remains the same, cable ratings and protection must be sized for the most onerous situation, ie when things go wrong.

As far as the cable is concerned, in the situations we have been discussing, it has been assumed that the cable would be adequately protected by the circuit's protective device. This means that the cable is considered to be 'safe' no matter what current flows through it for how long, for whatever reason - since the protective device will cut off the supply if the magnitude/duration of the current reaches a level which might damage (or seriously overheat). The concerns voiced therefore relate only to the currents that will sometimes flow through protective devices, plugs/sockets, accessories etc.

And that this branch of electrical engineering is not an exact science due to the unknown variables.

That is precisely the point. Concepts like diversity are statistical in nature, so one could describe them in lay terms as 'intelligent/informed gambles' - but they are gambles based on what are (or were) believed to be reasonable/sound foundations, and have stood the test of time. However, as you imply, there will never be certainties in a probabilistic situation.

It's perfectly healthy to question these concepts, particularly given the changes (e.g. in cooking appliances) that have happened over time - and that's the reason I've been trying to explore what people now actually believe is appropriate application of diversity.

I'm not yet convinced that people are necessarily consistent in their view. For example, I'm not sure how rational it is to be more concerned about, say, a ~15% 'overload' of a 13A fuse for a few minutes when a 3.6 kW oven is switched on from cold than the 50%-100% 'overload' of a 32A MCB which could occur for a few minutes if many/most of the bits of a 12-15 kW cooker were turned on simultaneous from cold (which certainly isn't impossible).

Kind Regards, John