How to replace a regular switch with a dimmer

[robinsonpr]

Hi,

I have a light switch in my dining room and I want to put a dimmer in. This light is only controlled by the one switch.

I removed the existing switch faceplate and I was surprised to see lots of wires in there, 3 separate lots of 3 core wire. The green earths just go to the backbox, and I know I'll need to run an earth wire from that to the new metal dimmer case, that's fine.

But there are loads of reds and blacks! Here's a photo (sorry, a bit blurry):

2 reds go to the top of the switch, and 3 reds go to the bottom of the switch. The terminals are not labelled at all on the existing switch so I don't know what they are.

All the blacks seemed to be joined together in a block.

Not sure what to put in my dimmer now. It has a Common, L1 and L2 inputs.

Can anyone help me not get electrocuted?

Thanks!

 

[rjm2k]

Can anyone help me not get electrocuted?

Firstly, you should turn the power off so you don't get electrocuted!

When you say 3 core, do you include the earth as one of those cores?

Looks like it's looped at the switch (check the wiki for the description)

How are the terminals on the existing switch labeled?

2 of the cables coming in will be the loop in from the light before and loop out to the next light. 1 of the cables will be going to the light itself, however I would have expected only 1 red into that one, does this switch operate 2 lights at the same time?

It will probably be a case of connecting the 3 reds to COM on your new switch and the other 2 to the L1 but we need more detail as above before you do anything, particularly we need to verify where there are 2 reds into the terminal rather than just 1

btw I'm not a sparky so follow at your own risk

 

[robinsonpr]

Thanks for the reply!

lol yes I am turning off the power while I'm working!

Yes it certainly looks like it's looped at the switch, I found this: http://www.flameport.com/electric/lighting_circuits/lighting_loop_at_switch.cs4

And at the ceiling there is definitely no loop there, it's just got one cable with brown/blue and earth.

The terminals on the existing switch are not labelled at all unfortunately.

And no this switch only operates 1 light, and that light is only operated by this 1 switch.

I haven't got a multimeter but I've got one of those screwdrivers with a light in it.

I left the power on and probed the terminals on the exiting switch. The top one (with the 2 reds) was live. The bottom one (with the 3 reds) was not, presumably this becomes live with the switch on, but I didn't test that.

 

[rjm2k]

neon screwdrivers are frowned on by the experts here, but may be worth testing that bottom terminal with the switch on too. is there any strange behavior with lights in your property? are there any blanking plates around at all as though something has been removed?

The 2 that are live will be your loop in and loop out so they should goto COM on the new switch, the other 3 will be switched lives which will connect to L1 on your new switch you would only expect 1 not 3 though

 

[flyingsparks]

The live feed wires you tested go to common and the others go to l1. By the way neon screwdrivers need to go in the bin !

 

[robinsonpr]

Thanks rjm and sparks.

I thought I would get shot down about the neon screwdriver What's the problem with them? Are they dangerous?

Why do you think the bottom one (switched?) currently has 3 wires? As rjm says, 1 would make sense. But 3???

There doesn't appear to be anything weird with this light or any others in the house. And there are no blanking plates around.

Cheers,

Paul

 

[rjm2k]

Could be that there were wall lights at one time and they've been plastered over with the cable in place. You may need to investigate more. If you are careful you could identify which of the 3 feeds your working light (put the other 2 in a connector block whilst you try them one at a time). Then it's a case of working out what the other 2 do/did.

On the other hand, if you are happy to just wire as originally with the 3 into L1 on the new switch.

 

[robinsonpr]

Thanks rjm good idea, I'll try all 3 in L1 with the dimmer and see if it works.

I could identify which of the 3 reds goes to the switch but having done that I wouldn't know what to do with the other 2! I don't think I'd stand much chance of finding out what they do. And it's quite an old house so like you say, there could have been more lights once upon a time.

 

[robinsonpr]

All done, all working.

I went with the top 2 into COM and the bottom switched 3 into L1.

I reckon there must have been a couple of wall lights in there once upon a time.

The dimmer buzzes a bit but I guess that's normal.

Thanks again!

 

[flyingsparks]

Buzzing is normal yes. Sorry I didn't read your post earlier to carefully. Really u should find put what the other 2 do / disconnect them. Easiest way is to probably connect one at a time (of the 3 reds) until you have the light working and then put the other two in a connector block. That way if there is wires hidden in a wall somewhere or something they won't be live and pose a danger.

 

[robinsonpr]

Yes I think you're right, it's a bit worrying having extra wires that are live and not know what they do!

 

[joe-90]

Especially if you start hanging pictures.