replacing conduit wiring in part.

[urchaidh]

I'm replacing the lighting spur in my kitchen as part of a bigger renovation (Scotland, building warrant, etc). The current (boom! boom!) wiring is twin conductor in steel conduit. I don't know how old this is but the insulation on the wire seems like plastic rather than rubber so I guess it was replaced at some point in the not too distant past.

The supply for each light ruins trough the switch, rather than a switched live back from the rose, so I need to replace it. Building regs require a mains powered smoke alarm (run of the lighting circuit) so I need a permanent live to the rose.

Anyway, I'm not keen on a total rewire back to the CU (the conduit is quite narrow and pulling twin/earth all the way through might be problematic) and so wondered about terminating the conduit in a new junction box where it enters the room (Under the floorboards) and rewiring from there. I can't see any obvious problems with this so long as I get a good earth from the new junction box onto the exisiting conduit.

Any comments, am I missing something obvious, is this kind of "mixed mode" wiring acceptable in the 21st centrury?

Thanks

[Vaci]

If you do it that way, you need to make sure you actually have a good earth.

I'm in a similar situation - all my wiring is single cores in conduit. My electrician wasn't very happy with the continuity of the earth provided by the conduit, and recommended adding an earth core all the way back to the CU. He also added earth clamps to connect all the conduit pipes to the earth block, rather than just relying on the threads on the conduit box.

Although the conduit might be quite narrow, you should be able to pull three single cores through quite easily. You don't need (and probably won't succeed) to pull twin and earth through the conduit.