3-way switch voltages????

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Old 06-08-09, 01:11 PM
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Question 3-way switch voltages????

I have a 3 way switch circuit (power into one switch, two lights between the switches) that I'm going to change into two circuits, each switch controlling one light. In figuring out the circuit I noticed that if the lights are on, I get a reading of about 20 volts between the two powered terminals and the unpowered terminal on each switch. What I mean is the three reading are 20,20, and 0. Looking at the circuit diagram I came up with, the unpowered terminal is connected to wires that should be just 'floating'.

How come there's 20 volts there? Have I missed something?
 
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Old 06-08-09, 01:19 PM
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Measuring voltages at switches is meaningless. Do you want to retain three way control of the lights when you separate them or are you going to abandon one of the three way switches? Note in some cases three way switches are required. Where are the lights?
 
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Old 06-08-09, 01:37 PM
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Ray--Thanks for the reply.

I figure I'll abandon the 3-way switches for regular off-on types, one controlling a light, and the other a fan. Replacing one light with a fan is the whole reason for the change. I didn't want to have the other light come on every time the fan was turned on, and I figured getting power to the fan through a switch would be about as easy as just providing power there and using pull chains to work the fan.

I did the voltage measuring to help figure out the circuit.

The lights are in the ceiling, and there's an attic above them.
 
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Old 06-08-09, 02:06 PM
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Does either of the three way switch boxes have more then a single cable with three wires + ground coming into them? If not then the power probably comes in at one of the two luminary Jboxes. To keep down confusion I'm just going to ask only that question for now.
 
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Old 06-08-09, 03:28 PM
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Yes, the switch box near the door (one light is for that door landing) has power coming in. But so does the box in the kitchen, where the other switch box is located (with two other switches).

I'm pretty sure the circuit is like this: power comes into the box at the door, and to that 3-way switch. A bwr+g cable goes to the door light. A second bwr+g cable goes to the 3-way switch box in the kitchen. The second light, where I was thinking about putting the fan, is fed by a bw+g from the first light's box.

So I tbelieve there's enough existing wire to add the fan. Maybe the simplest thing would be to replace both 3-way switches with on-off switches, forget about using any red wires to get power to the kitchen box, and just power the fan from what's already available in the kitchen switch box. This would keep power coming into the kitchen box from only one circuit.

But I keep wondering if there's something I'm missing because of those 20 volts. I think I'll check terminal-to-ground voltage on those terminals next.
 
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Old 06-08-09, 04:17 PM
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Yes, the switch box near the door (one light is for that door landing) has power coming in. But so does the box in the kitchen, where the other switch box is located (with two other switches).
One of those is not power in for the three way switches. Instead of making conclusions tell us exactly how each switch box is wired. If there is more then one cable call one "A" and one "B". So it would go like this Black "A" to single pole switch for kitchen light and.... well you get the idea.

Probably the power in the kitchen light switch box is for just the two single pole switches and is not connected to the three way circuit at all or the two single pole switches could be on switch loops. We need all that info though about both boxes before proceeding..

You say this is for a stairwell. That means we must keep the three ways functional to meet code.
 
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Old 06-09-09, 03:04 PM
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I was misleading about the door. It's not in a stair well. The door light just illuminates the area in front of that door.

OK. Here's what's in the door light box. Three cables come in. Cables A and B are bwr. Cable C bw. It looks like A comes from the door S3, and B from the kitchen S3. C goes to the light which I'm thinking about replacing with a fan. Connections are as follows: Cable A black to Cable B white. A red to B red. A white to C white and light white. B black to C black and light black.

In the kitchen switch box there is an S3 and two S1's. Five cables come in, cable B, and four bw cables, D, E, F, G . Cable B connects only to the S3. D...G whites are connected together. F and G black, and two black pigtails are connected together. The pigtails go to the lower terminals of the S1's, and D and G blacks connect the upper terminals.

I can get some info about that's in the door switch box if it would be helpful.
 
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Old 06-09-09, 04:24 PM
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OK. Here's what's in the door light box. Three cables come in. Cables A and B are bwr. Cable C bw. It looks like A comes from the door S3, and B from the kitchen S3. C goes to the light which I'm thinking about replacing with a fan. Connections are as follows: Cable A black to Cable B white. A red to B red. A white to C white and light white. B black to C black and light black.
Cable "C" is most likely Power in. My suggestion is that you use the existing leads to the light for the light on the fan. That way there will not need to be any changes in the wiring, Just the adding of a switch loop for the fan motor..

First you need to record where cable "C" is connected, turn off the breaker and then temporarily disconnect the black and white. Restore power and check for voltage across the black and white of "C". If "C" is always hot run a 12-2 to where you want the fan motor switch. Remark the white red and wire to the switch. At the light remark the white of the new cable red. Redo "C" connection adding the red to the black connection. The black of the new cable goes to the black (motor) connection of the fan.

There are other ways to do this but I think the above is simplest. But if you want an alternative read on. Power most likely comes into the luminary box from the switch box with three switches. You could add the fan motor control there by replacing one of the SPST switches with a duplex switch and replacing the 2-conductor cable from it with a 3-conductor cable.
 
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Old 06-09-09, 05:54 PM
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Not to butt in, but the odd voltages you were receiving were "phantom" voltages. If you used an analog VOM, you would get more accurate readings.
 
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Old 06-10-09, 08:49 AM
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Ray--Thanks for all the info. I think I'll take a look at the door switch box before I make any changes in the situation. I'll let you know what I find.

Chandler--Interesting. I did use a digital VOM. I'll give it a try with an analog and see what I get.
 
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Old 06-10-09, 10:47 AM
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I wrote:
Measuring voltages at switches is meaningless.
Just to clarify I meant measuring across switch loops or on three-way travelers. Checking a cable in a switch box to see if it is always hot is a meaningfully measurement but the cable must be disconnected before measuring across the wires.
 
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Old 06-14-09, 12:23 PM
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Chandler--I checked the (phantom) voltages with an analog VOM. I got voltages that varied depending on which range the VOM was in, but strangely, the needle deflection on the meter was the same for all ranges. I don't really know what that means electrically, but it doesn't sound to me like there's some real but sneaky load hiding somewhere in the circuit. That was what I was thinking might be going on.

Ray---The door switch box has two 3-way switches in it. It also has a rat's nest of wires. I'll get into that box later.
 
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Old 06-14-09, 12:45 PM
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Ray---The door switch box has two 3-way switches in it. It also has a rat's nest of wires. I'll get into that box later.
Because of fill problems it may be beast to use a separate box for the new switch.
 
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Old 06-21-09, 11:59 AM
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Ray---Thanks for your help. I got the S3's out and replaced them with on-off's, did some circuit changes, and things seem to be OK. There is now one red wire that's entirely disconnected, but that's to be expected.

Chandler--Thanks for the note on "phantom voltage." Turns out if you know what the correct name is, there's a lot of info out there.

In working on this project I came up with another question, about two circuits sharing a white, but I'll start a new thread for that one.
 
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