Installing a new dimmer switch on three way circuit

[Will__]

Hi

I'm about to install a 2 way dimmer switch on a basic domestic 3 way system. (other switch is simple on/off)

Trouble is, although I can buy 3 or 4 way switches, I can't buy a 3 way dimmer. Will it work with the existing system?

The reason I ask is that the dimmer I've bought (a good Siemens one) has three cable points labelled P, 1, and 2.

If I was just using the dimmer and nothing else on the circuit (two swy circuit), I'd connect it through P and 2 (according the to the diagram on the packaging - although the packaging diagram labels them P, 1, and then 2 is a squiggly line with an arrow through it.)

The switch I'm replacing is a three way switch with 3 cables coming out of the wall - two black and one grey (don't ask - here in Spain they just seem to use whatever cable colours are lying around.)

So which cables do I connect to which terminals then?

Do I need to buy another 3 way switch for the other switch I'm not touching? (this will just continue to be another on / off switch, although I presume that the lights will only come on at the level the dimmer was left at?)

I know this is hard to advise or diagnose sight unseen, but any info or advice would be gratefully accepted.

Thanks

Will

[qapmoc]

Hi Will, your descriptions are a bit confusing really. What do you mean by a 3 way system? do you mean that there are three different switches (say at the bottom, middle and top of some stairs) which all work the same light, if so then what you should have in total at the three different switch positions is two x two way switches with three wires at each and one changeover switch with four wires in it.

Or do you mean that the complete unit you are replacing has three different switches in it for three different lights.

[Will__]

Sorry to be confusing - there are two switches that work the same lights (three 60 watt bulbs)

I want to leave one of the switches as is, and change the other for the dimmer. The one I want to replace with the dimmer is the one that has three cables - two black and one grey. I haven't opened the other socket up yet, so I don't know how many cables are in that one.

But I'm hoping that the three connection terminals on the dimmer will correspond with the three cables. Although life's rarely that easy.....

It's a place up in the mountains and there's no internet or anything there, so I can't look and then post - trying to find out everything I need to know this week so I can do it this weekend.

Cheers

[Tim B]

Your dimmer can only affect the input current to the system. There is no such thing as a 2/way dimmer. You 1st need to install the dimmer on the supply side (in series) then you can put in as many 2/way switches as you like but they will only switch the light (on/off), they cannot affect its brightness.

Your greys and black suggest non-uk and your spelling of grey suggest non-US. If you don't understand the above I suggest that you get in a 'lecky.

Merry Christmas.

[Will__]

Well thanks for that - I do understand the answer, but the only problem with this is I don't have a circuit diagram of the house.

Is there a way to test the cables using a ordinary multimeter? I have some fairly basic knowledge.....but need a little hand holding....

Thanks

[stanleyman]

Assuming the switch you have is similar to an MK two way dimmer then this needs to be installed as follows.

http://www.tlc-direct.co.uk/Technical/DataSheets/MK/Logic_Plus_Dimmer.pdf

However, it can only be installed in the switch position that has the live supply. So long as the live is connected to the common, C on UK switchgear or P as you have then the other two can be connected either way round, probably the two blacks. Look at the existing switch which will probably give you the grey for the common live or switched supply.

Just to confuse matters even more there is another way of connecting two way lighting circuits which has five wires at one switch and three at the other. The live and switched lines are wired to both the L1 and L2 terminals respectively and the commons are connected to each other. You would need to rewire as above but the live and dimmer can then be fitted in either position.

Assuming your supply has an earthed neutral then a neon tester will tell you which is the live or switch line. If not then a multimeter can be used between the switches and the return wire at the lights.

[buildersmate]

Tim B - not quite correct regarding observations on colours. Grey/Black/Brown plus earth are the new standard colours for cable used on 3-phase supplies. Electricians will also then use this cable when they need triple plus earth cable - which is what is needed for wiring up multiple way switches. Previously the standard colours were Red/Blue/Yellow plus earth.

[Will__]

Thanks guys.

[robot9]

Hi, check this 3 way dimmer switch guide: https://www.easybom.com/blog/a/3-way-dimmer-switch-basics-wiring-installation
Hope it helps!