Step-by-step for unravelling 3 and 4-way switches?

[phenix]

This might help with wiring a 3-way.


http://www.youtube.com/watch?v=yxRmbi6uuzk

 

[beelzerob]

Well, I have no fear now of wiring my own 3-ways.....but figuring out how someone ELSE wired it is still the mystery to me.

I'm going to try and measure those voltages again, as they didn't make much sense to me.

 

[beelzerob]

Ok, so I confirmed it:

With Switch A down:
Switch B: Red: 110 V
White: 0 V

With Switch A up:
Switch B: Red: 110 V
White: 77 V <= <_<

On Switch B, black is connected to the common screw, and it was always 110 V.

Sooo then. If white on switch B is the one that is affected by the changing of Switch A, then that means i'd wire nut together the red and black one, correct? And that makes Switch A a normal switch?

 

[dBeau]

Hmmm...well, I had hoped to be able to do this without taking the light fixture down too. Is that really the only way to know for sure? For some of these (the foyer light, for example) it's just not going to be possible.

If you take the bulb out of the fixture it might help to remove some of the confusion. You dont have to take the fixture down.

Ultimately, each three-way is doing one of two things. It is either feeding power to one of it's travelers, or connecting one of it's travelers to neutral. At either end (that is, not on a traveler), there can be (well, better be) a load in the circuit. (either between the breaker and the common of a three-way or between the common of a three-way and neutral). If the load is in play, you'll see voltage in more places. That could explain the 75v you were seeing.

I find it also helps when figuring things out to check the voltage not between a red, white, or black, but against the bare copper ground. At the breaker box, the neutral and ground are tied together. But since the ground is never (supposed to be) used to return current to the panel, it's potential should always be 0v. Note, that the voltage potential between two hot wires is 0v but if you touch one you'll know it!. Measuring against ground helps here.

If you take the load out of the picture and measure the voltage at each point in the circuit under all four switch positions, you should be able to reconstruct the actual wiring. I promise that if you go through this exercise a couple of times you will either be thoroughly confused or will be able to piece these things together quickly in the future.

 

[beelzerob]

Ya, I did measure between the copper ground and the points...it was easy to clip the alligator clip onto it so I wasn't trying to hold 2 probes to screws at the same time. I swear, everytime I touched one of the screws, I was just waiting for a *kapow*.

Ok, i wondered if maybe the load was causing the strange voltage reading.

Taking the bulbs out is still something I want to avoid....in this current case, it involves 3 light bulbs...and while that's not SO bad, again when I do the foyer light, it's impractical to remove 9 bulbs from the chandelier. And when I do the front lights, it'll again be a pain to open up the 4 fixtures to remove the bulbs.

So am I correct then that I need to wirenut the 2 wires that always had voltage on them in switch B? I guess the worst case would be wirenuting a hot wire to neutral, right? But that shouldn't be possible, as I'm connecting 2 wires that always HAVE 110V on them.

 

[dBeau]

So am I correct then that I need to wirenut the 2 wires that always had voltage on them in switch B? I guess the worst case would be wirenuting a hot wire to neutral, right? But that shouldn't be possible, as I'm connecting 2 wires that always HAVE 110V on them.

I could easily be missing something here, but if they "always" have 110v on them, what's to gain by connecting them together?

 

[beelzerob]

Hmmm...I had thought that was what Sokolov was suggesting to do. It made sense at the time....

 

[dBeau]

If your goal is to eliminate one of the three-way switches, Sokoloff had it right in his original reply. It really doesnt matter which switch or which traveler. The switch should make it obvious which are the travelers and which is the common. Usually the common is the one on the side by itself. Connect the common of one switch to one of it's travelers and the other switch will control the load. The only implication of which travel you choose is the position required of the remaining switch to power the load.

What's left unanswered is whether the three-ways control powering the load or connecting it to neutral. I suspect that the answer to this question is more important to the installation of your new switch. If you could remove the bulbs you'll see that you are either feeding power to the load or getting nothing at the switch. Without removing the bulbs, when the load is on, if you are seeing less than 120v at your switch, you most likely controlling the neutral side. This might be problematic for your new switch.

 

[beelzerob]

Well, the goal here is to convert a 3-way switch scenario to a normal 2-way so that I can then replace THAT switch with an OnQ switch. So, the resulting position of the first switch doesn't really matter...I'll just have to know which is hot, and which is switched hot, which should be pretty easy to tell.

Usually the common is the one on the side by itself

That's kind of my whole problem...I don't want to rely on "usually", because (while I have good regard for the people who wired the place) I have no way of knowing for sure if they did it the usual way.

Does it mean anything that the light switch has a screw labeled "common", and it's NOT the screw that's by itself on one side? It's the black screw on the same side as the brass screw.

So, based on the voltage measurements I made on switch B while flipping switch A, can i say with certainty which wires to connect and which to leave disconnected?

 

[dBeau]

Does it mean anything that the light switch has a screw labeled "common", and it's NOT the screw that's by itself on one side? It's the black screw on the same side as the brass screw.

The job of of the three-way switch is to connect the "common" to one of the other two terminals. The connection made depends on the position of the switch. So yes, it does mean something. You can take the switch out of the circuit by connecting the wire connected to the "common" to one of the other two wires. The remaining wire should then be capped. A three-way switch will never connect the "other two" terminals together, only one of them to the common. In this configuration, the wires connected to the other terminals are usually called travelers.

Now in the box with the second switch, are there only three wires (ie one 14/3 coming into the box)? If so, that really simplifies things a lot.

 

[beelzerob]

Now in the box with the second switch, are there only three wires (ie one 14/3 coming into the box)? If so, that really simplifies things a lot.

At switch B, yes...only the 14/3 comes in there, and that's where I measured those voltages.

 

[123]

The goal of the technique I used was to determine, without doubt, the path and purpose of each wire. I disconnected the load because, by measuring continuity, it allowed me to determine how the wires were routed between the two switches and the load. In my case, I learned that power entered the load and then continued to the two switches (i.e. circuit breaker ---> load ---> top of stairs ---> bottom of stairs). It was completely different from my initial guess; it was fortunate that I didn't act on my hunch.

If you plan to act on incomplete information, omit the ALC switch and use a standard wall switch initially. If something is miswired, it won't damage the ALC switch. Once you confirm the circuit works, swap the standard switch with the ALC device.

 

[beelzerob]

If you plan to act on incomplete information, omit the ALC switch and use a standard wall switch initially. If something is miswired, it won't damage the ALC switch. Once you confirm the circuit works, swap the standard switch with the ALC device.

Ya, that's been my plan...I want to remove one of the switches so that the remaining switch is a normal working 2-way switch. Then I'll replace that one with an ALC switch. One step at a time. But this first step seems the hardest.

I realize the power could be coming in anywhere....so that's why I'm trying to figure out how to cut out the 2nd switch without killing anybody. How do I know which wire going off the switch really is the "common" one? Is it the one that's on the side of the switch all by itself, or is it the one that is marked "common" on the front of the switch?

 

[dBeau]

Now in the box with the second switch, are there only three wires (ie one 14/3 coming into the box)? If so, that really simplifies things a lot.

At switch B, yes...only the 14/3 comes in there, and that's where I measured those voltages.

If the 14/3 is the only cable coming into the second box the picture becomes much clearer as that would imply the cable leading to the load must be in the other box. So unless you want to put your new switch in the second box, you can pretty much ignore the 14/3 that is connecting the two three-way switches. Everything you need is in the first box.

With this set up, you should be able to identify the 14/3 in first box because two of it's wires are connected to the "travelers" on three-way there. The other wire goes to either the hot or neutral of the load. And that, believe it or not, identifies the 14/2 running to the load... which is what you really want. Because with that, a hot and a neutral, you can wire your new switch.

 

[dBeau]

How do I know which wire going off the switch really is the "common" one? Is it the one that's on the side of the switch all by itself, or is it the one that is marked "common" on the front of the switch?

It's definitely the one marked "common". I regret my previous "usually" comment. I should have said simply that the common is usually marked as such!