Stick with the high current cooker wiring?

The current oven setup in my kitchen has a thick (6mm2?) cable terminating in a cooker switch (with socket) above the oven. The extractor is simply plugged into the socket. The thick cable continues on to a junction box behind the oven. This circuit is rated 30amp in the consumer unit. I am gutting the kitchen and installing all new appliances.

My aim: separate isolation switches for the extractor and the oven (the hob is gas) at worktop level, and concealed wiring/junction box for the oven and the extractor.

As most single ovens seem to be rated for ring main power, my questions are: am I best to preserve this high current circuit, or should I simply use ring main power for the extractor and oven, or, is it appropriate to use the cooker circuit as though it were a ring main (in other words, perhaps a lower rated fuse in the consumer unit and use of lower rated cable for spurs). Sorry if these questions are unclear - I do plan to have an electrician help with the work, but would like to plan all the channeling, etc now! The most sensible plan seems to be to use the cooker circuit as it's already in place. I think I will spur off to a (3amp) FCU for the extractor and keep the existing wiring for the oven (although terminate this in a mains socket as this seems to be how most single ovens connect now). If I do this, am I right in thinking that all wiring on this circuit (even the spur up to the extractor) needs to be done with the same rating cable? Sorry for rambling, and thanks in advance for any help!

What you describe is a fairly standard arrangement good for about 15kW of cooker - that's assuming you've correctly identified the cable. The "junction box" should be a cooker oulet like this one

http://www.bizrate.co.uk/electricalsupplies/oid66475203.html

If this is the case, I would not do anything to the cooker circuit to affect it's load carrying capacity and I would use it for the purpose for which it was designed - powering the cooker.

For the extractor, I would take the power from the existing kitchen ring main (as you're gutting the kitchen that should not be a problem). As a personal preference, I keep such kit easily removable, so I would create a fused spur from the kitchen ring main with an FCU connected to an unswitched single socket - the FCU placed in an easily accessible place and the socket where the fan can be unobtusively plugged into it. When using FCU's in this way, do make sure that the purpose of the FCU is apparent (some sort of label).

This arrangement will preserve the integrity of the cooker supply and enable you to neaten up the fan power, getting rid of the trailing wire, without creating any of the problems that can arise when you start mixing cable sizes on individual circuits and making the arrangement less clear on first inspection.

Thanks very much for your help. Using the ring main makes a lot more sense, thanks. The link you provide to the cooker outlet is probably what should be in use, but I suspect it has been bodged, as it currently terminates in a standard "connection box"! Cheers