How to determine common wire in a 3 way?

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  #1  
Old 12-19-04, 01:07 PM
uncoiler
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How to determine common wire in a 3 way?

I am replacing an old 3 way dimmer with the pushable rotating knob. The new dimmer is a slider w/rocker switch for on/off. Anyway the old dimmer has "fixed" wires coming out of its guts, the wires are not screwed on at all and just connected to the wall outlet wires with mar connectors. None of the wires are marked and niether is the back of the old dimmer switch. Now my problem is to hook up the new dimmer I need to know which is my common wire that goes to the dark screw?

Here is what I have for wires. There is 1 red and 1 black wire coming out the wall plus another black wire that comes from another (regular) light switch that mounts beside it, which I think is "hot". Also the old dimmer did not have a ground wire to the box if that matters.

So how can I tell which of the 3 wires is the common which goes to the dark screw on the new dimmer? and should I also put in a ground wire from green screw on the dimmer to a ground screw inside the metal box? Thanks.
 
  #2  
Old 12-19-04, 05:20 PM
J
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It will only take 3 tries to get it correct. The only wire you need to be concernd about is the common. The travellers are intchangeable.
Put one of the wires on the common of the new 3way. Try it. If it doesn't work try a different wire.
 
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Old 12-19-04, 10:11 PM
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The one coming from the other light switch is the common wire.
 
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Old 12-20-04, 06:31 AM
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Generally (but not always) the common wire (wire that connects the two switches in a three way set up) is red.I would try the red wire as the wire to the dark screw on the switch.This should also be the case on the other switch in this pair so you also might take a look at it.The wire coming from the switch beside it is NOT the common wire.
 
  #5  
Old 12-20-04, 10:10 AM
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The common wire does not connect to the other switch. It connects to the power or the light. The 2 traveller wires connect to the other switch.
 
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Old 12-20-04, 11:33 PM
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Originally Posted by joed
The common wire does not connect to the other switch. It connects to the power or the light. The 2 traveller wires connect to the other switch.
Here's what he said: "Here is what I have for wires. There is 1 red and 1 black wire coming out the wall plus another black wire that comes from another (regular) light switch that mounts beside it, which I think is "hot".

Remember that he also says there are two switches in the box. I see what he describes as being a pair of travelers "coming out the wall", and the hot jumped off of the hot to the switch next to it. The hot jumper would hook to common on the dimmer.
 
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Old 12-21-04, 09:22 AM
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That makes sense. I guess I misread the post.
 
  #8  
Old 12-21-04, 09:30 PM
uncoiler
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Cheyenps your visions are correct. I got it hooked up tonite and it works great. I put the black wire coming from the regular switch beside it to the black or "common" screw, and the two wires from the wall to the brass screws.
To be honest though I don't quite understand whats going on, partly because I don't know the technical defintion of a common wire. Hec when you think about it all the wires are common, it all relative, and depends on how you look at it. The way I see it that black wire from the next switch is a "hot" wire, if thats the case then why don't the instructions just say find your hot wire and wire it to the black screw? Had it said that I would have been done with it in 5 minutes. Next issue I don't get is that I have always been taught that the "hot' wire go to the brass screw. Well obviously this is a contradiction.

Anyway because of that thinking on my part this was the one wire combination I didn't try for fear of frying my $30 switch or $250 light. As I have found out though you can hook a 3 way dimmer anyway without harm, it just won't work right until you wire it correctly as joed stated.
 
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Old 12-21-04, 11:16 PM
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Originally Posted by uncoiler
The way I see it that black wire from the next switch is a "hot" wire, if thats the case then why don't the instructions just say find your hot wire and wire it to the black screw?
Because there are two three way switches in a three way switch setup. The "first" switch (the end you are working on) has the hot wire connected to the common screw. The "second" switch has the switch leg going up to the light on the common screw.

Had it said that I would have been done with it in 5 minutes. Next issue I don't get is that I have always been taught that the "hot' wire go to the brass screw.
True for receptacles, not necessarily true for switches.

Glad it worked out.
 
  #10  
Old 12-23-04, 12:33 PM
uncoiler
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Good info, thanks Cheyenps.
 
 

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