In this particular thread the starting point is a single oven rated at 16A. As such, my sop is to not apply diversity.
Fair enough.
The MI for the oven state the oven characteristics are 3600W, 16A and no plug. This indicates the circuit requires a 16A MCB. Applying diversity would indeed drop the current to 12A, but this doesn't change the MCB rating.
Again, fair enough - but this is a bit different from what you say in the case of multiple cooking appliances, when you say that the required MCB rating should be determined from the after-diversity, rather than pre-diversity, load. I suppose this comes down to the fact that you don't believe that diversity should be applied to a single oven (although I still wonder whether a "3.6kW" one really does have only one element!).
I would also, referencing Appendix 15 ( Fig 15A iii which advises that cookers/ovens/hobs over 2KW should be on their own circuit).
To be fair, you are talking about the notes to Fig 15A, which relate specifically to
ring final circuits. Since such circuits are unique in that they are allowed to be wired with cable whose CCC is less than the In of the circuit's OPD, there is a clearly a risk that some parts of the cable could be overloaded even if the total load was less than the In of the OPD. In order to minimise that specific risk, the notes to which you refer advise against connection of cooking loads >2kW (and also immersion and space heaters etc.) to a
ring final circuit. There is no such advice in relation to radial sockets circuits (Fig 15B) - although I suspect you would probably be saying the same even if it were a radial circuit?
Having said all that, (a)... I do agree that, as a general concept, it is preferable not to have any 'large fixed' load supplied by
any sockets circuit (whether radial or ring), but (b) there are, of course, a good few (~3kW) single ovens out there which came supplied with 13A plugs, most of which are probably plugged into ring final circuits (whatever I might think about that).
If there were other appliances I'd apply diversity over the total load and use that value to determine the MCB rating. This would account for the fact that not all of the load would be in use at any one time.
It really comes down to a question of the degree of 'gamble' one is prepared to take, since, if most bits of the cooker (oven, hobs etc.) are turned on, there is a very much a finite probability that there will times when everything is 'on' simultaneously - so it's a question of how high a level of that probability ('risk') you are prepared to accept.
There is certainly a potential 'issue' at switch on from cold. If (far from impossible) a 3.6kW oven plus three 1.6kW hob plates were switched on from cold simultaneously, about 36.5A would flow until at least one of the bits got up to temperature. If (as is common) the ciurcuit were protected by a 32A MCB, that would represent about 14% over the MCB's In - which is almost the same as the ~15% by which a 13A fuse's In would be exceed whilst just the 3.6kW oven was heating up from cold (if supplied by a 13A plug).
I'd look to install a single circuit for these appliances (ref. Appendix 15).
See above. 'To be fair' what Appendix 15 advises is that such loads should not be connected to a
ring final circuit, even though I agree with you that it is prefereable that large fixed loads are not supplied by
any sockets circuit.
This is how I interpret the regulations and guidance. If anybody here, or elsewhere takes a different position, I'm not saying they are wrong to do so. Maybe I am over cautious. I take the view I will only do for a client what I'd do in my own home.
I have no problem at all with cautious - unless it becomes 'over-cautious' to the extent that it's hard to justify additional work/costs on the basis or perceived 'benefits' (which I'm not really suggesting is the case here).
I think the only real difference between us is that, given that (despite my views) people are buying 3kW ovens with 13A plugs and plugging them into sockets circuits (and I'm not going to tell them not to), I can't get too excited about a 3.6kW one being supplied similarly (particularly given that one can 'support' that practice on the basis of what diversity guidelines actually say, even if we can debate about that).
Kind Regards, John
