UK Wiring new Hob / Cooker into circuit

[joshimoto]

Hi folks, DIYer here, hoping for some advice before most likely calling in a spark. I'm looking to wire in new cooker and hob (separate units). Max draw 3.5kW and 7kW respectively. I believe the circuit is radial from consumer unit (32A RCD) to 45A DP switch plus socket. Have done diversity calcs and got 25.125A which seems sufficient for the switch. Looking at the cable it seems to be 6242Y 2x10+4 mm to and from the switch over about 18m or so, runs through roof space and down behind plaster wall.

However, at some point in the past the cable has been cut, then joined to 2.5 t&e in a connector box with 3 double sockets spurred off, as well as a fused switch to an external oil boiler through the wall outside. All of this seems to be on the cooker circuit - i.e. switching off the cooker switch isolates all of the above. I've got the impression this doesn't seem right and am looking at ways of rectifying for safety. As I say above, most likely will get in a qualified electrician, but the 2.5mm spurs and boiler connection are giving me concern. Looking at fitting FCUs or a Dual cooker connection unit to the appliances as they'll both be hard wired. Any advice greatly appreciated

 

[Lucien Nunes]

If multiple socket outlets are connected via one length of 2.5mmsq then it is not protected against overload by the 32A device in the CU. (You mentioned an RCD but I guess you mean RCBO, or MCB, as an RCD doesn't provide overload protection.) If each point has its own 2.5 back to the junction with the 10.0 then they can't readily be overloaded and they are probably OK for short-circuit protection, but it's a non-standard configuration that would be hard to defend. Plus, you need most of that circuit's capacity for the oven and hob.

The cooker isolator should also be exactly that; one does not want to be 'hesitant' to isolate a dangerous situation or over concern that the heating boiler will also go off. If your new oven turns out to have moisture in the elements that causes RCD nuisance tripping, again you need to be able to isolate it individually.

So it fails on two or three accounts. Solutions might include running a 20A radial out to meet the orphaned sockets, breaking them into an existing ring if present, or turning them into a fused spur with a fused connection unit spurred of the existing ring, which is something of a last resort in a kitchen as the 13A fuse is a limiting factor and likely to get blown if the sockets are heavily used. If you were running a new cable, there is something to be said for running two and separating the heating supply entirely.

Re. the job you actually wanted to do in the first place i.e. connect the oven and hob, double-check that the oven instructions don't specify a 20A (Max) fuse. No FCU's for either of these as they are both over 13A.