hob oven and combi microwave wiring

[JohnW2]

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.

That's obviously true, and is clearly the reason why fused spurs >13A do not appear in their diagram. However, there is obviously also the matter of potentially excessive loading, or excessive 'point loading' of a ring final cicuit. As you go on to mention, it is this very same ('informative') Appendix which suggests that one of the ways of avoiding overloading of a ring final is to avoid connecting "cookers, ovens or hobs" >2kW to a ring final but, instead, to supply them via their own dedicated radial circuit.

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.

Whether or not a spur protected by a 16A mcb is compliant with regs is, I suppose, debatable. In common sense terms, since an unfused spur supplying a double socket is allowed (which, avoiding arguments, represents a potential load of 20A), it's hard to see how there could be an objection, load-wise, to a fixed wired load of 3.4kW - and any OPD included in that spur would only be there to protect the cables ('fixed' and appliance flex) adequately, so I can't see why there should be any objection to that, either (even if 16A or 20A). As for the manufacturer's requirement for DP isolation, as you say, that could be satisfied by a 20A DP switch. Indeed, to avoid littering the place with even more accessories, if an MCB were to be used, it could perhaps be put in a 3-4 module enclusure, including an 80/100A 'main switch'

Question is which one is safer?

I'm not sure what we're comparing with what here - do you mean a 13A FCU (or plug) vs. a 16A MCB? As far as I can see, the only reason for hesitation about application of 'OSG diversity' to a combi-microwave is the uncertainty about the ability of the plug itself to cope with occasional currents >13A. Would you say that the same uncertainty applied with an FCU? As far as cable protection is concerned, both approaches are obviously equally safe, provided that the appliances flex had large enough CSA to be protected by a 16A OPD (which it would, just, be if it were 1.5mm²)

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

Quite. Mine certainly comes with a (moulded on) plug, and no hassle

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.

I'm not sure what you are saying here. Are you merely saying that "cookers, ovens or hobs with a rated power exceeding 2kW" does not explicitly include mention of combi-microwaves? If so, that is clearly true, but I would have thought that most people would have assumed that it applied to any 'cooking appliance' >2kW.

Kind Regards, John

 

[ban-all-sheds]

This afternoon I fitted a 13amp plug to the JL micro wave and got on and cooked some jacket spuds which took 18 - 20mins.

That's a barbaric way to do them. Oven all the way - no microwaving at all, that's the right way.

 

[ban-all-sheds]

Whether or not a spur protected by a 16A mcb is compliant with regs is, I suppose, debatable.

It isn't, really, when you look at what may be supplied by a ring final.

 

[JohnW2]

Whether or not a spur protected by a 16A mcb is compliant with regs is, I suppose, debatable.

It isn't, really, when you look at what may be supplied by a ring final.

I've already conceded that, since an unfused spur is allowed to supply 20A via a double socket, the regs ought not to have any problem with a OPD of any In (even 50A, if one wanted!) being inserted in a spur supplying a single 3.4kW fixed load. But 'ought to be compliant' and 'compliant' are not always the same thing.

As has been discussed, at 3.4 kW one is already well beyond the 'informative' guidance in Appendix 15 as regards the maximum power of a 'cooker, oven or hob' (which I take to include combi-microwave) that should be supplied from a ring final circuit. I agree that contravening 'informative guidance' does not actually constitute 'non-compliance', but ....!!

Kind Regards, John