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Hello. I thought I understood three way switches. However, after attempting to trouble shoot one at my new house, I'm not so sure anymore.
I have an intermittent problem that it not solved by toggling the switches. I have changed the switches (but preserved the original wiring pattern) and it did not fix the problem.
I will put all my testing results below. However, the nub of the problem seems to be this: the feed from the panel goes to one switch while the neutral to the light goes to the other switch.
If anyone has any suggestions, I would be grateful.
The whole story...
There is a switch upstairs and a switch downstairs. There is one incandescent light fixture controled by the switches. Usually everything works the way three way switches are supposed to. However, some times it does not and the light simply will not go on. I have not been able to link this to any particular switch position combination.
The upstairs switch is in a box by itself. One cable with three wires (white, black and red), plus ground, comes into the box.
The downstairs switch shares a box with a dimmer switch which is on the same circuit (actually, pretty much the entire livable part of the basement is on this circuit.)
The fixture has its black wire connected to a red wire coming into the box. It also has its white wire connected to two white wires coming into the box. One of these white wires either is, or is connected to, cable 3b but I am not certain about the other one.
Wiring - Downstairs
The box containing the downstairs switch and the dimmer switch has three cables coming in. One two-wire cable and two three wire cables (plus ground in all cases). I will call the three wire cables 3a and 3b to distinguish them.
The dimmer switch has two black wires attached to it. One is joined to the black wire from the two-wire cable and one is Maretted to the black wires from both the three wire cables. The dimmer also has a green wire grounded to the box.
The white wires from the two-wire cable and from cable 3b are Maretted together in the box and don't connect to anything else. I tested these with the continuity tester and they connect to the white neutral running to the light fixture.
The red wire from cable 3b is attached to the common terminal and conects to the light fixture black wire to power the light fixture. (Yes, that's right - the fixture's neutral is off a different cable than the main feed from the panel to the system.)
The red wire from cable 3a is on one traveller terminal and the white wire from cable 3a is on the other traveller terminal.
Wiring - Upstairs
The black wire runs to the common terminal.
The red wire runs to one traveller abnd the white wire runs to the other.
What Powers What
When I disconnect both the switches and test them, only the upstairs black wire has power from the panel.
When I power the other wires one at a time, I find that:
The upstairs white wire is the downstairs white wire on the switch (3a).
The upstairs red wire is the downstairs white wire on the traveller (3a).
Not surprisingly, there is no power to any wires attached to the recepticle at all when the light is off.
There is never power on the white wires running to the recepticle, regardless of whether the light is off or on.
The only things that I can think of that seems odd about this set up are:
(a) the way the neutrals are run off the downstairs switch while the upstairs switch is the one that is powered; and
(b) the fact that no black wires are attached to the downstairs switch.
The problem manifests more like gremlins than like a predictable thing, which I find hard to understand. When the light won't work, it just won't work. I can play with the switch positions all I like and it won't fix it. I just have to walk away and then, sure enough, the next day it will work again. I suppose it could be the fixture, but that seems unlikely.
My next move is to change the fixture and, if that does not work, to call an electrician because I can't figure it out.
I tested all the switch positions and how they affect the circuit. They just confirm what I learned on testing without the switches, but I will put them in just in case it helps somehow.
A) Upstairs switch up Downstairs switch up - light is OFF
Upstairs: red live, black (common) live, white dead
Downstairs: white dead, red 3b (common) dead, red (traveler) live
B) Upstairs switch up Downstairs switch down - light is ON
Upstairs: red live, black (common) live, white dead
Downstairs: white dead, red 3b (common) live, red (traveler) live
C) Upstairs switch down Downstairs switch down - light is OFF
Upstairs: red dead, black (common) live, white live
Downstairs: white live, red 3b (common) dead, red (traveler) dead
D) Upstairs switch down Downstairs switch up - light is ON
Upstairs: red dead, black (common) live, white live
Downstairs: white live, red 3b (common) live, red (traveler) dead
