Oven & Hob Wiring, Part P etc.

[RealPolitik]

I have a new electric cooker and an electric hob to install - they're replacing a gas/electric all-in-one which looks like it was plugged into a wall socket.

I understand that, for the hob's power requirement at least, I will need a specific supply from the consumer unit and a cooker socket/switch. Because there isn't a socket close enough to the new location I intend to have the oven wired through this as well (and possibly the extractor). The wiring will largely go under floorboards from the consumer unit and won't be channelled and filled.

The CU has old-style push-in fuse blocks with a few spare terminals but no RCD. There is an existing kitchen circuit labelled on the CU.

Do I need to have a separate new fuse box with RCD installed for this?

 

[echoes]

Depending on the power ratings of your oven and hob, you may well be able to supply both from the same circuit. Need to know the ratings to give a more definite answer and also length (in m) of that circuit. Your oven may need fusing down according to the manufacturer's instructions.

A better solution would be to run separate circuits for your hob and oven.

These new circuits will most likely need RCD protection - not necessarily absolutly required, but if any cables are < 50mm depth in walls, or if the cooker control unit has a socket (yuk) then an RCD will be necessary - it's probably unneccesarily and uneconomically difficult to avoid.

 

[RealPolitik]

Thanks. The oven needs a 2.5kW connection, the hob 7kW. The wiring run will be roughly 15-20m from CU to kitchen and none of it will be >50mm in the wall.

 

[echoes]

OK. So you will need an RCD for the new circuit(s), which could be satisfied by installing a new CU. Maybe specify a large one so that your old circuits could be migrated over in time allowing you to ditch the old CU eventually.

You could use a 32A/6mm2 circuit for the hob and just run a new radial for the oven (possibly a 16 or 20A / 2.5mm2 with 13 plug/socket or FCU for the oven connection).

Or one 10mm2/45A circuit could be used to supply both hob and oven.

Or use diversity to convince yourself and LABC that 32A/6mm2 could supply both - it's far too close and not recommended IMHO.

 

[RealPolitik]

Thanks, that confirms my worst fears that £200 of hob and oven are probably going to cost £400 to wire in ...

 

[londonboy]

OK. So you will need an RCD for the new circuit(s), which could be satisfied by installing a new CU. Maybe specify a large one so that your old circuits could be migrated over in time allowing you to ditch the old CU eventually.

You could use a 32A/6mm2 circuit for the hob and just run a new radial for the oven (possibly a 16 or 20A / 2.5mm2 with 13 plug/socket or FCU for the oven connection).

Or one 10mm2/45A circuit could be used to supply both hob and oven.

Or use diversity to convince yourself and LABC that 32A/6mm2 could supply both - it's far too close and not recommended IMHO.

He said it will be run surface, why does he need an RCD?
The radial for the socket outlet will be a dedicated socket too.

 

[RealPolitik]

I thought it needed an RCD because it wasn't channelled?

The actual cable run will be out the top of the CU box directly under the first floor floorboards and then down into the staircase cupboard to the back of the kitchen wall.

 

[echoes]

May be my misunderstanding, but he said largely underfloor and no channelling. Sorry if I misunderstood.
If it's all surface and no socket on CCU then yes, no RCD needed. If dedicated oven radial is also surface then ditto.

 

[echoes]

I thought it needed an RCD because it wasn't channelled?

The actual cable run will be out the top of the CU box directly under the first floor floorboards and then down into the staircase cupboard to the back of the kitchen wall.

Looks like you will not need an RCD then!

RCD's required for unprotected, buried cables at less than 50mm depth and for general-purpose sockets. And TT installations. RealPolitik - do you have a TT earth? Don't want to fall at the last hurdle...

 

[londonboy]

I thought it needed an RCD because it wasn't channelled?

The actual cable run will be out the top of the CU box directly under the first floor floorboards and then down into the staircase cupboard to the back of the kitchen wall.

Looks like you will not need an RCD then!

RCD's required for unprotected, buried cables at less than 50mm depth and for general-purpose sockets. And TT installations. RealPolitik - do you have a TT earth? Don't want to fall at the last hurdle...

I think your pushing it chancing a TT

 

[echoes]

Perhaps! Just the 'old-style plug-in fuse' made me think that it might be of that vintage where a shoe-lace to a water pipe sufficed.

 

[RealPolitik]

No earth electrode that I'm aware of ... CU looks perhaps 1980s to my untrained eye.

So to recap, I can run a new cable from a spare outlet on my existing push-in fuse CU and without installing a new RCD-equipped separate fuse box for it?

As above, the cable will be visible only by the trunking in the stairs cupboard and where it exits the CU in its cupboard into the floor void directly overhead.

 

[echoes]

Yes.

Remaining considerations are your choice of cable size appropriate for load and fuse rating.

Will all this cable be surface clipped, or will it run in trunking where some de-rating will apply?

 

[RealPolitik]

Thank you, seems my electrician's advice about RCDs and Part P was incorrect.

The intention was to cable-clip the cable along the most suitable solid beam or joist, and possibly put it in cosmetic plastic trunking where it's visible in the under-stairs cupboard if it looks neater.

 

[echoes]

Yes.
Remaining considerations are your choice of cable size appropriate for load and fuse rating.

Disregard that if an electrician's doing it - he will know.

Thank you, seems my electrician's advice about RCDs and Part P was incorrect.

Possibly - it's pretty hard to avoid the requirement for an RCD, but by running all the cables on the surface, having a TN earth and not having the cct supply a non-dedicated socket, it isn't an absolute requirement. It's easy to assume that every new circuit needs an RCD, because the vast majority will for one reason or another.