Extension: wiring requirements

[Brian Sharrock]

I'm intending to add an extension onto the rear of my 1935 built house. The 'house' has suspended timber floorings (joists and floorboards) over a void which is ventilated. The void is a space within which electrical cables are routed for the outlets. I understand that the extension is likely to have a solid slab as the 'foundation layer for its floor (there'll be ducts from the existing 'airbricks' to the outside world).

How should electrical cables be routed for this situation? There'll be a requirement for outlets on at least three walls. One wall however will have a large-ish (3.2 m) 'bi-fold' door opening. Should I route the cables (first fix) then have the slab cast over them [burying them for eternity ... ] or should the cables be laid on top of the slab and 'below' the insulation layer? Or; should cables be routed above the floors and fed down the walls - in the approved zones? What is the practise?

Advise ... please.

[Lobster]

You put all the wiring below the upstairs floor, and drop the cables down to each socket/switch as required.

David

[Owain]

is the (easiest) correct answer.

Owain

[Brian Sharrock]

Thanks for the rapid response .... is this the definitive requirement? Currently the bottom floor ring main is one complete circuit fed off the CU down through the floor and 'pops up' at the socket locations. This requirement will necessitate a feed being taken up - by some means - to the upper floor . The 'drop the cables down to each socket /switch as ' I can cope with. :) seems more work than I'd anticipated. :(

[<me9]

You could always feed the extension sockets from the upstairs ring (suitably re-labeled) or install a new ring for the extension only.

[Lobster]

What, the dropping-down of cables from above the ceiling to the accessories, or the fact that you need to get your wiring up to

1st-floor level in the first place?

Shouldn't be too bad... are you planning on extending the existing downstairs ring, or adding a new circuit (which might be required depending on overall floor area, loading etc?)

Assuming you're extending the existing ring - are there any ground-floor sockets on the external wall which will form an internal wall between the house and extension? If so, one tack would be to fit a new socket on the reverse side of the wall to the old one, right behind it; feed the cable through from the existing ring main, then run it up the wall into the ceiling space, before you plaster.

David

[Christian McArdle]

Yes, with the drop lengths taken into account, these extensions often need a new circuit to avoid exceeding circuit length. It can be better if you drop a single cable from a junction box, as the ring length stays low, giving much better earth loop impedence. Obviously, this has the disadvantage that any such drops can't can't run multiple sockets (at least without an FCU involved).

Christian.

[Brian Sharrock]

I'm intent on having a 'Room in the Roof' {which is AIUI the nowadays preferred nomenclature for 'Loft Conversion' plus a 'single storey ground floor extension'. The _present_ (oops, I nearly said 'current') installation has a (1973) fuse-wire consumer-distribution-unit mounted under the stairs with a down-stairs loop, upstairs loop, kitchen (oven) feed; plus two lighting circuits. I realise that a replacement CU will be required. The 'existing' house has parquet flooring on the ground floor which I do not wish to disturb. The single storey extension will, practically, remove the exterior wall and result in a combined (present) Dining Room, Kitchen, Extension, single space. Introducing a new ring for the extension (only) will result in the proximity of outlets in the present dining room and extension being on different rings (similarly the kitchen/extension). Extending the 'upstairs-ring' down into the extension may result in a situation where the 'breaker' for the upstairs needs to be pulled to render safe something downstairs (in the extension). [I'll know ... but will somebody in 2050 know?]

At this early stage it's merely pencil marks on paper, so I'm asking for advice and definitely open to suggestions over what is best practise.

The 'Room in the Roof' situation is a no-brainer as I intend to have that serviced by a dedicated breaker.

[Brian Sharrock]

Thanks Dave; this sounds eminently do-able. I have a socket on [presently] the internal face of an external wall. Actually I have two; at either side of the present house. It will be feasible to extend the ring from either, or both, from these sockets and running the cables up into the ceiling then down for the other outlets. After all, why else did The Almighty allow SDS bits and chisels to be invented?