Three-way switching question

[Martin Pentreath]

I currently have in my bedroom a five-amp, round-pin lighting circuit for table lamps. It's wired as a spur off the ring main. The spur feeds a junction box powering the sockets. From the junction box a twin-and-earth cable goes to a two-way dimmer switch by the door. From the dimmer switch a three-core-and-earth cable goes off to a two-way plate switch by the bed. Net result: the brightness of the lighting is set by the switch by the door, but I don't have to get out of bed to turn the lights on and off, fantastic.

Now I want to move the bed to another location in the room and I want to put another plate switch in there. Although I tackle most electrical wiring confidently these days, three-way switching is something I don't seem to be able to get my head around.

Could some kind person post a noddy guide to the connections I need to make (colours would be nice ;-) or a link to one? In fact is it possible to have a dimmer (just the one) with three-way switching?

By the way, given the current wiring I'm assuming that my existing plate switch will become the intermediate and I'll have to run three-core-and-earth cable from this to the new switch. Is it possible to do it any other way? It would be much easier to wire the new switch from the junction box or even the existing dimmer, but my attempts to understand three-way switching suggest this is not a runner.

Many thanks for any help.

Martin

[Stefek Zaba]

Asides first: you don't mention this little lot being fused down at any point. I s'pose the dimmer might have a little 1A or 3A fuse hidden in it, but I'd be happier if your description had mention an FCU (switched or not) somewhere early in the circuit.

But to the meat of things: I don't understand why you needed 2-way switching in the original layout, and hence why you'd want 3-way in the new one. AFAIU, the dimmer by the door is the "master" here, which controls whether any, and what, voltage gets to the "slave" sockets into which the lamps are plugged; while the plateswitch by the bed merely interrrupts the flow to the lights. Is it important to you that the lights can be turned on from the dimmer by the door, regardless of the position in which you've left the plateswitch by the bed? If so, I understand the desire for 2-way switching; but if not, the bedside plateswitch need only be 1-way.

Next - do you in fact need both the plateswitch in the current position, and the one in the new by-the-bed position, to remain as active control points? Or can you simply replace the existing plateswitch with a blanking plate, and extend the wiring from there to the new position for the plateswitch?

If neither of these simplifications apply, then you'll need to get out your best sharpened pencil and work out the wiring for an intermediate switch. The basic principle is that it swaps over the two straps which run between the 2-way switches at each end of the run.

HTH - Stefek

[Malc]

By a three way switch I assume you have a centre off switch with two alternatively live circuits coming off? In that case why not just wire up another switch in parallel with the existing switch i.e. common to common, L1 to L1 etc. This way either switch will control the lights on the spur circuit. Of course I could be misinterpreting what you are saying so be careful.

-- Malc

[Dave Plowman (News)]

I'm assuming the dimmer is a push type switch, and that it can be switched on or off at either the dimmer or the switch? If so, the dimmer switch is also two way. So all you need to add is an intermediate. Wiring below.

You can break into the L1 and L2 lines anywhere that is convenient.

L1 L1 0===========0 0===========0============= Line | \ / | C 0================================O C \ / \ / 0===========0 0===========0============= Switch return L2 Optional L2 Intermediate

[Martin Pentreath]

Thanks for the replies, helpful as ever.

In answer to Stefek, the spur which supplies this circuit passes through an FCU fused at 3 amps before going to the junction box.

I do indeed want proper two-way or three-way switching - I like to be able to switch the lamps on as I come in throught the door without having to crouch down by the bed, but to be able to switch everything off without having to stir myself from my horizontal position. (I guess I justify this as labour-saving, but when you actually work out all the time and effort that goes into wiring the damn thing up it looks less efficient ;-)

I take the point about extending the wiring from the existing two-way switch, and that is the easiest way to go. But some part of me DOES want to get on top of three way wiring, so I might just get my pencil out, work out Dave's diagram and try to do it 'properly'.

[Dave Plowman (News)]

It should be fairly easy to work out from the diagram how two way switching works. The intermediate then simply either routes through both the switch lines, or swaps them over.