hob oven and combi microwave wiring

[JohnW2]

It has a quick heat up function which uses all three to get it to temperature. During that process, it's quite clear that the unit will be consuming a significant amount of power continuously for several minutes.

Whether one likes it or not, the concept of diversity does, of course, take on board the fact that currents in excess of the calculated 'after diversity' current will often flow for relatively short periods - certainly for 'several minutes', sometimes appreciably more than that.

Kind Regards, John

 

[slup]

There are plenty of large built in combi units which have just as big (if not bigger) power requirements than a standard single oven.

There are, indeed - and, of course, many of those 'standard single ovens' are, themsleves (thanks to diversity!), supplied via a 13A plug.

That's bugger all to do with diversity, it's to do with the fact that the cheaper ovens don't let you use more than one element at once and will therefore be rated at something significantly less than 3000W

 

[slup]

Whether one likes it or not, the concept of diversity does, of course, take on board the fact that currents in excess of the calculated 'after diversity' current will often flow for relatively short periods - certainly for 'several minutes', sometimes appreciably more than that.

This is a moot point until someone shows that you are even able to apply diversity to a 13A plug/socket arrangement. I find it difficult to believe that you can.

 

[JohnW2]

Diversity isn't real world though is it, it's a fudge, ....

It's certainly a controversial topic, but its based in pretty sound statsitical principles (given the 'leeway known to exist in circuit design), and seems to be 'generally accepted'.

... and partly relies on the fact the the circuit protection device and cable are actually overrated for just about every installation, and will easily be able to cope with slight overloads during the brief time that all four hob elements are being heated simultaneously. ... Using the calcs we've just done above, it's clear that the above combi microwave will indeed be consuming 3400W with the grill and microwave on, and may do so for large chunks of time at once.

Yes, it partly relies on that, but it doesn't actually require any 'over-rating' in the design, since a lot of 'leeway' already exists in the criteria to which we design. For example, we know that a cable (and associated OPD) is deemed to be able to safely carry 1.45 times its tabulated CCC for one hour. I do admit, however, that there is more uncertainty about the over-current capabilities of a plug/socket - although, again, we know that an (obvioulsy 'allowed') 13A BS1362 fuse will allow considerably more than 13A to flow for an appreciable time.

As I've mentioned in a recent post, many 'standard single ovens' are supplied (seemingly satisfactorily) via 13A plugs, something which would not usually be possible unless one invoked diversity.

Kind Regards, John

 

[slup]

That's very interesting - my normal fan oven (13A plug) doesn't have the bottom element only the top grill and rear element with fan.
I am intrigued to know what the make and model number of your combination microwave oven is?

Just about any mid range to high end product has multiple elements - including ones in the floor of the oven. It's not at all unusual.

Microwave is a HME9750, can't remember the model number of my oven off the top of my head.

 

[JohnW2]

There are, indeed - and, of course, many of those 'standard single ovens' are, themsleves (thanks to diversity!), supplied via a 13A plug.

That's b*****r all to do with diversity, it's to do with the fact that the cheaper ovens don't let you use more than one element at once and will therefore be rated at something significantly less than 3000W

Some don't, but some do.

Kind Regards, John

 

[slup]

As I've mentioned in a recent post, many 'standard single ovens' are supplied (seemingly satisfactorily) via 13A plugs, something which would not usually be possible unless one invoked diversity.

I don't agree - I've just looked at half a dozen random cheap single ovens, and they all have power consumption in the 2.5k - 3kW range - so they are fine on a 13A plug. This isn't application of diversity - it's just a smaller load!

There are - equally - lots of high end single ovens which are over 3.2kW and therefore need hard wiring.

 

[JohnW2]

Whether one likes it or not, the concept of diversity does, of course, take on board the fact that currents in excess of the calculated 'after diversity' current will often flow for relatively short periods - certainly for 'several minutes', sometimes appreciably more than that.

This is a moot point until someone shows that you are even able to apply diversity to a 13A plug/socket arrangement. I find it difficult to believe that you can.

My recent comments are a bit ironic, given that I'm usually the person arguing against the invocation of diversity!

As I've recently written, there is some uncertainty (at least in my mind) about the wisdom of applying diversity when a plug/socket is involved - even though one would like to think that a plug/socket ought to be able to safely cope with anything that a 13A BS1362 fuse will 'allow' to happen (in terms of current x time).

One problem,of course,is that there is very limited guidance about diversity. The regs themselves say absolutely nothing, and the OSG (which is what everyone seems to go by) just gives some figurse without any detail or explanation.

Kind Regards, John

 

[JohnW2]

I don't agree - I've just looked at half a dozen random cheap single ovens, and they all have power consumption in the 2.5k - 3kW range - so they are fine on a 13A plug. This isn't application of diversity - it's just a smaller load! There are - equally - lots of high end single ovens which are over 3.2kW and therefore need hard wiring.

You may be right. If/when I have the time, I'll maybe have a look, but I certainly was fairly sure I'd seem ovens with a theoretical maximum load >3kW which came with a 13A plug. Of course, it's not necessarily easy to ascertain what the maximum possible load is, since what one finds on-line doesn't necessarily clearly indicate 'what bits can be drawing current simultaneously'.

Kind Regards, John

 

[harrythehat1]

So thanks again for the helpful posts my fine gents and here's a little more of this conundrum:

This afternoon I fitted a 13amp plug to the JL micro wave and got on and cooked some jacket spuds which took 18 - 20mins. I repeatedly tried the temp of the plug and it didn't even get warm. Oh and incidentally the attached black cable looks to be 1.5mm which I guess is heat resistant flex.
Outwardly it would seem that this beast will run happily on what is a proven ring. Though I'm not completely comfortable due to the Mis being so vague. As previously said a call to JL may be helpful.
Again how do we fuse something at 15amp without a breaker in a neighbouring cupboard and ugly too.
Referring back to my earlier post:
If I were to run all of these appliances from the 32amp cooker circuit which has a 6-7m cable run, under the suspended groundfloor and not in any insulation would it be too much/ borderline?
7.4kw + 3.45kw +3.68kw = 14.53kw. If our mate diversity is or should be applied we only get a load of 26amp.
As the OSG states a 32amp cooker circuit can feed appliance/s up to 15kw.
Also just to recap...........the 32a mcb is protected by a 30ma rcd in the board.
If I were to feed it from the kitchen ring should I use a 13a FCU or a 16a mcb to connect the flex too?

I hope I'm not getting to be a bore on this topic?

Cheers all

Harry

 

[JohnW2]

This afternoon I fitted a 13amp plug to the JL micro wave and got on and cooked some jacket spuds which took 18 - 20mins. I repeatedly tried the temp of the plug and it didn't even get warm.

Which 'cooking mode was that- microwave plus 'hot air'/convection?

Again how do we fuse something at 15amp without a breaker in a neighbouring cupboard and ugly too.

If there is a 13A fuse in the plug, the 15A thing really becomes an irrelevance - at least, in electrical and common sense terms, even if not in terms of the MIs!

Referring back to my earlier post:
If I were to run all of these appliances from the 32amp cooker circuit which has a 6-7m cable run, under the suspended groundfloor and not in any insulation would it be too much/ borderline? 7.4kw + 3.45kw +3.68kw = 14.53kw. If our mate diversity is or should be applied we only get a load of 26amp. As the OSG states a 32amp cooker circuit can feed appliance/s up to 15kw.

... actually about 19kW. There are, however, differences of opinion as to whether one should/can apply diversity across multiple appliances - in giving the "10A+30%" diversity calculation, the OSG talks in terms of "household cooking appliance" (singular), without any clarification or detail.

If I were to feed it from the kitchen ring should I use a 13a FCU or a 16a mcb to connect the flex too?

I presume you're talking about the microwave. If so, I suppose it depends (per discussion above) whether one feels that it is appropriate or acceptable to apply diversity to that 3.4 kW. If one decided that is acceptable, then a 13A FCU is obviously equivalent to using a 13A plug (without any uncertainties about the plug itself). I know what I'd do if it were my house, but, as discussed, I think you really ought to see what the manufacturers have got to say about this. To use a 16A MCB 'in a cupboard' would certainly be an unusual and messy approach, and would cause you to have to make sure that the flex was adeuately sized to be protected by a 16A device.

Kind Regards, John

 

[riveralt]

Again how do we fuse something at 15amp without a breaker in a neighbouring cupboard and ugly too.

You can't use a plug or an FCU - your only other option is to run a 2.5mm ² T&E from the consumer unit with a 16A MCB and connect to the combi via a 20A dual pole switch. That circuit would normally need to be RCD protected.

Referring back to my earlier post:
If I were to run all of these appliances from the 32amp cooker circuit which has a 6-7m cable run, under the suspended groundfloor and not in any insulation would it be too much/ borderline?
7.4kw + 3.45kw +3.68kw = 14.53kw. If our mate diversity is or should be applied we only get a load of 26amp.
As the OSG states a 32amp cooker circuit can feed appliance/s up to 15kw.
Also just to recap...........the 32a mcb is protected by a 30ma rcd in the board.

You could do what your propose - however I'm not sure how you would to meet your version of the MI's (As I said I cannot find reference to 15A protection in the ones shown) with regard to the combi fuse rating.

If I were to feed it from the kitchen ring should I use a 13a FCU or a 16a mcb to connect the flex too?

Hardwired it would be a 13A FCU (As implied by my version of the MI) if Lewis's tech support clarify the position or as I have outlined above regarding its own circuit - or as you have suggested, maybe 2.5mm ² T&E as a spur to the breaker in a neighbouring cupboard - which I think is OTT.

 

[JohnW2]

Hardwired it would a 13A FCU (As implied by my version of the MI) if Lewis's tech support clarify the position or as I have outlined above regarding its own circuit - or as you have suggested, maybe 2.5mm ² T&E as a spur to the breaker in a neighbouring cupboard - which I think is OTT.

I meant to also comment on that. Assuming we're talking about a ring final circuit, as well as being 'OTT' (and 'messy') I'm not even sure that it would be compliant with the regs - a "16A 'fused' spur" is certainly not something mentioned in the infamous ('informative') Appendix 15 of the regs.

Kind Regards, John

 

[riveralt]

I meant to also comment on that. Assuming we're talking about a ring final circuit, as well as being 'OTT' (and 'messy') I'm not even sure that it would be compliant with the regs - a "16A 'fused' spur" is certainly not something mentioned in the infamous ('informative') Appendix 15 of the regs.

I am assuming that FCU's are limited to max 13A via BS1363-4 and the only reason they are put in the drawing because that is one of the 'standard' ways of extending a ring final circuit.

Compliance can sometimes be a flexible friend.
It could be argued that the installation of a 16A mcb in its place does not comply with the regulations and also with the manufacturers instructions for dual pole isolation - though I added a 20A DP switch to cover that element.
Question is which one is safer?

I suppose the other, rather drastic option, is to split the ring to 2 * 20A radial circuits and introduce the 'mcb spur' that way.
Or incorporate the 16A MCB unit into the radial cooker circuit.

Seems a lot of messing about when compared to plug and play combination microwave oven.

One other issue that might need addressing by the IEE is Appendix 15 (iii) regarding the definition of an oven. They must has known that a lot of combination microwaves exceed 2kw in rating (perhaps not when used) but chose not to include this particular appliance in BS7671.

 

[harrythehat1]

Rivalt,
I just read the info on the microwave on the JL site and it does say its rated at 3400w and mentions a plug is fitted or it can be hard wired if permanently fitted.
Confusing or what?............. On the one hand we have a 3400w appliance which the blurb clearly states can be simply plugged in or the case of the one "I have here" which is the same model number as the one I've just viewed the blurb on JLs site which straight from the box has no plug fitted.
There's consistency for you.
On that happy note its dinner time but not cooked in the new m/wave.

Cheers to all


Harry