Which wires are backwards in this wall switch?

I replaced a switch (switch B in gang box 2 in the picture below) - now, the two 3-way switches don't exactly work as they should

I'm convinced i have two wires reversed on Switch B - but from this diagram below do you have enough information to tell me which ones?

The blue trapezoids indicate wires leaving the gang boxes but i can't be certain where they go without disconnecting leads one at a time

I don't know where exactly the wire(s) go from the light itself, it's in the ceiling and pretty hard to get to

i'm reasonably sure my setup goes:  power source > switch a > lights > switch b - based only on the fact that there is only one 12/3 wire connected to switch b, and because switch a seems to be nutted to hot which feeds another (unrelated) switch

I'm not an electrician but i have replaced a lot of standard outlets and switches, moved breakers around in the panel, wired a 220v run from a new breaker to a dryer (first 3-way here though) and i'm only looking for advice, not going to take your suggestions as legally binding instruction.  i understand this is the internet, information can be wrong especially not given all of the information, consult a qualified electrician, etc - just looking to see if an expert here can tell based on what i can see in the boxes, which wires might be reversed and if it's reasonably safe to switch them (power off, of course).

the positions of the switches and how they control the light, are like this:


D = switch toward traveller screws
U = switch toward common screw

A-U B-D      off
A-U B-U      off

A-D B-U      on
A-D B-D off

how the wires are connected in the two boxes
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Gary CaseRetiredCommented:
It's not clear exactly how you've wired this, but if you've only changed switch A then clearly you've connected something different than it originally was.

In a 3-way setup, the hot is switched at BOTH switches ... so if you they've both got it connected the light is on;  and if either is changed the light goes off until either switch is thrown -- which reconnects the electrical path.

It's hard to say based on what you've said/shown, but I THINK you have the traveler connected at the wrong spot in switch A.    This would normally be connected at the same relative position on both switches in the 3-way setup (and it is not on your diagram).

But without a complete picture (i.e. showing the actual wires from the light) it's impossible to say for sure just what you need to change.      Note that trying other configurations won't "hurt" anything -- you may just blow a circuit breaker if you've connected something wrong and cause a short.
I can help if you provide additinal info as folows:
- in your switch rectangle symbolm, could you add the "internal" detail/symbol of how the switch is made on/off between input/output wires which come/go from switch?
- are all 3 switches a 3-way switch? If not specify which one is what type and draw the symbol of the switch in your diagram.
- If we name the switches SW-A, SW-B, SW-U and the light L-C, then could you specifiy one more time in a different way, as a table, how would you like to have the switches-light actions. I do not understant clear from mention above. For example SW-A has 2 postions up and down. Let's consider up is on and down is off. Then something like:
           up(on) down(off) L-C

In case of a 3 way switch of course there is no on/off, is just a switching from one normal open to one normal close or the other way around.
Here are some links how a 3-way switch works:
I analyzed your circuit and I processed it a bit.
You should check the cables to the lamp and how the switches work.
If you respect the diagram below, then the circuit should work.
C #1 – cable number 1, main power cable which supply the electricity
C #2 – cable number 2, goes from the “unrelated switch” to “unrelated lamp”
C #3 – cable number 3, most probably goes from the 3-way switch A to the Light C
C #4 – cable number 4, most probably goes from the 3-way switch B to the Light C

You should double check if the 3-way switches, inside symbols drawn by me below are as in your real circuit.

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FocISAuthor Commented:
Thanks for the replies

viki2000 i think you have commented exactly my situation

is there something in this diagram that would explain why switch B only affects the light when Switch A is "NC" ?

swtich A = NC, switch B = NC, light = on
switch A = NC, switch B = NO, light = off
switch A = NO, switch B = NC, light = off
switch A = NO, switch B = NO, light = off

would it matter if i have switch B travellers reversed from what they were originally?
If by mistake you reversed for switch A or B the NO with NC there is no problem, but if you reversed Common with NC or NO then you may end with your unwanted situation described in the table above.
FocISAuthor Commented:
Hmm so since the breaker isn't tripping, anything else you can think of that might make switch B only functional when switch A is "on" ?
I think you just need to inverse on Switch A the Common wire with the NC.
Here is an example:
FocISAuthor Commented:
I see, and laid out like that it makes so much more sense

Sorry for all the questions, but since i didnt actually change anything in switch A (same as the day it was installed years ago), would the same apply to switch B - say, switching the red and black wires in switch B?  Would that electrically make sense based on how i described the physical switch positions?

Or, if there's an order i can test the wires in switch B to for sure confirm which is which, i can do that... although i can confirm that with switch A "off", there is no electricity on any of the 3 wires in switch B - if that helps
Theoretically, the effect of the wrong connections that you see home can have as cause:
- 2 possible wrong connections if you reversed by mistake the wires in Switch A (Common with NC or Common with NO)
- 2 possible wrong connections if you reversed by mistake the wires in Switch B (Common with NC or Common with NO)
And 4 more possible wrong connections if you reversed by mistake the wires in both Switch A and Switch B.
In your case, if you know for sure that you never touched Switch A, then are only 2 possible wrong connections if you reversed by mistake the wires in Switch B.

" would the same apply to switch B - say, switching the red and black wires in switch B?  Would that electrically make sense based on how i described the physical switch positions?"

Yes, but not red-black. Only if you reversed white-red or white-black, the Common with one travelers. If you reverse travelers between them makes no difference.

And you see, the circuit breaker does not trip off because the neutral never comes in contact with the hot wire to make short-circuit no matter how do you reverse by mistake the wires in the 3-way switches.

i can confirm that with switch A "off", there is no electricity on any of the 3 wires in switch B - if that helps"
What you tell me is that Switch A is wrong wired, assuming that there is no problem with the wiring in the wall and no failures with the 3-way switches.
The correct wiring of the Switch A means that any position has switch A, either up(let's say on) or down (let's say off) the electricity should be present in one of the travelers wire and should arrive to switch B.

If you want to solve the problem faster, you have to add a bit of “reality” to the description:
     1) Please add the last table above that you made, the physical position of the switches, up or down.
     2) Would be very helpful if without electricity and without front cover of the switches you can make some pictures of the switches. Would be nice be close and clear.
I want to see if one of them is mounted “up-side-down”.
The real position of the switch in the wall with correct identification of which is the Common and which are the travelers is the first important step.
     3) Could you tell me the type/code of your switches? A Google image will be helpful.

The way how I would solve the problem would be with the help of an basic instrument as multimeter, ohmmeter or at least a continuity tester, either a battery with a small lamp, bulb, LED or a battery with a buzzer.
I would shut off all the electricity.
I would open the lamp(s) and both switches, disconnect all the wires from them and I would try to identify the wires and put small labels on them.
I would check the functionality of the switches with the instrument, to make sure they have no failure and to identify correctly the Common and the travelers, which is NC, which is NO corresponding to up and down position, I would write them down in a table remembering the physical position, to see if they were or not up-side-down mounted. IN that way the “picture” of the circuit would be clearer in the mind when compared with theoretical diagram on the paper.
When all the wires are correctly identified, maybe you have one wire broken in the wall or not tight to one screw of the one 3-way switch, then I would connect proper all the wires back correctly to the switches and the lamp.

The good technical practice based on experience tells that you should check first the failure where you worked/touched last time the circuit.
If you know for sure that the circuit worked good in the past and the Switch A was not touched, but Switch B was changed/touched, I mean the wiring, and also the wiring to the Light C was not touched then it may be very possible the problem only at the Switch B. Either is a bad switch, new or old does not matter, or most probably you reversed the Common wire with one of the travelers. It is not so difficult, you have only 2 trying.
But there is more: can be, if you replaced Switch B with another new or just checked the wiring, due to the work, one wire is not well tight with the screws of the switch, bad contact or no contact at all.
It might be also the situation when one wire is broken, sometimes only the conductor and the isolation is still OK, so you do not see it with naked eye. You can detect that only by measuring the wires from one end to another or without electricity check their end with the hand on the length 5-10 cm of the wire near the switch and see if they move abnormal as it would be broken the conductor inside. That is a common problem especially for the single conductor wire.
More confused than before?
Or, are any of my above explanations helpful?
Gary CaseRetiredCommented:
A couple basic questions ...

=> Are you CERTAIN you didn't change anything at switch A ?

=> Did the 3-way switches work okay before you changed out switch B?  ... i.e. you could toggle either switch with the other switch in either position and the light would change state

=>  Are you CERTAIN that no wire at switch B is getting electricity with switch A in either position?   If you think you are, then describe exactly how you're confirming this => i.e. what two points are you measuring between with your multimeter to determine this?

As has already been noted, it would be VERY helpful in isolating your exact issue if you could expose the wires in the light fixture;  disconnect ALL wires; and then use your multimeter (with a long extension) to do a continuity check to confirm exactly which wires run where.
FocISAuthor Commented:
Hi all, sorry for the lack of response so far, this is all really good information i just haven't had a chance to go open the faceplates again, but i will tonight

i for sure didnt change anything in "switch A", and i can say that "switch b" is "upside down", meaning travellers are on the bottom where switch A has the travellers on the top.  

Tonight i will just change the switch B, common wire and one or both of the travellers and see what happens

switch a and b both controlled the light as normal before i replaced switch b (not really having any experience last year, including putting it upside down lol)

when swich A is "off" and i take an electricity checker tool (with a green light, when you get close to a live wire it turns red and beeps) - touching all 3 wires in switch B yields no beeps or red lights on the tester.  however, with switch A "on", the tool lights red and beeps inside switch B

i may go the route of exposing the wires in the switch if reversing switch b doesn't work - but it's a lot of work, up on the ceiling, a flourescent tube light set within a wooden decorative box cover, 6 feet long requiring two people and two ladders to take it down, etc.
Gary CaseRetiredCommented:
r.e. confirming the electricity at the outlet:

=>  First, if the power's been cut off before you work on this, you shouldn't see anything anyway !!

But if you're checking with the breaker on ...

=>  If you're testing with a non-contact tester (i.e. a single probe) be sure it's got a good battery anthat you've set the sensitivity correctly at a known-hot outlet or switch.

=>  If you're testing with a 2-probe device be sure you're checking ALL possible combinations ... including any ground wire that may be simply wire-tapped together instead of being connected to the switch.   Just in case something wasn't wired as it should have been (the switch was, after all, installed upside down, so who knows how much care was taken in the actual wiring).

In any event, if the only thing you've changed is Switch B then clearly if it's wired correctly it's going to work UNLESS you replaced the wrong switch  [i.e. perhaps Switch A is the defective switch].

... which brings a thought to mind:   HOW did you decide that switch B was defective?
["switch a and b both controlled the light as normal before i replaced switch b (not really having any experience last year, including putting it upside down lol)"

Maybe you just found out your problem. If you maintained the same position for the wires and the switch was mounted up-side-down, then you just mixed up the Common with one travelers as in my diagram above with error on Switch A.
Why don't you put the Switch B in normal position?
FocISAuthor Commented:
I changed the battery in my non-contact tester and found power in switch B (with switch A "off") on red

i changed switch B common screw (was white) and the red traveller (red traveller on common screw, white wire on traveller screw) - the breaker didnt blow, nothing caught on fire, but now switch A can only control the light if switch B is "on"

so i'm going to remove this now-stripped screw and just try every combination inside switch B, with it mounted upside-right
"I changed the battery in my non-contact tester and found power in switch B (with switch A "off") on red"

Next logical step is to see on what wire color do you have power in Switch B when Switch A is "on".
Then the red wire and this second wire above are the travelers in Switch B.
Please tell me what position has Switch B and if you have power on Common in Switch B when "on" or "off".
FocISAuthor Commented:
with switch A "off", and with switch B "down" (towards the travellers) there is only power on red in B
with switch A "on", and with switch B "down" (towards the travellers) there is power on red and white in B
with switch A "on", and with switch B "up" (towards common) there is power on red/white/black in B (and oddly the ground screw and metal housing of B indicate power with the no-contact tester, and nothing connected to the ground screw)

was just about to finish this when the wife walked in and demanded electricity :)
You have to test one more time, also: with switch A "off", and with switch B "up", BUT no lamps in their socket, because the electricity may return back through the lamps in the wires from switches.
So, one more time please, without lamps, all 4 positions.
And please be consistent and use "up" and "down" beside the "on" and "off" for Switch A and Switch B.
Gary CaseRetiredCommented:
No need to remove the lamps -- these are fluorescent fixtures, not resistive incandescents.

Now that your tester is working (I suspected it might not be -- that's why I suggested checking the batteries) ... you need to determine which two wires are bringing the power from switch A.

=> Remove ALL wires from switch B.
=> Turn Switch A ON and determine which wire has power in switch B.
=> Turn Switch A OFF and determine which wire has power in switch B.

You now know which two wires need to be switched between.    These need to connect to the bottom of the switch on opposite sides.    The other wire is the output.     This should be all you need to wire the switch correctly.

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FocISAuthor Commented:
Thanks for the help and direction here, all - not only do i now have a properly working pair of 3-way switches but i learned a lot which will of course help down the road

I'll split up points as fairly as i can, but the important thing is passing on knowledge and for that i'm greatful

i did in fact have in switch B, the white (previously on common) and the black (previously on traveler) reversed

in switch B, putting black on common (which makes common sense now that i understand how a 3-way works), and red/white on traveler, both switches function perfectly, and without tripped breakers or blown fixtures :)

the original goal of replacing the 3-way last year was simply to change from beige to white, and to just dig in and learn things one way or the other
Gary CaseRetiredCommented:
Glad it's working :-)
And the most important of all is a happy wife with the electricity in the house :)
FocISAuthor Commented:
If you guys are interested, i have a new switch question posted, this time from the attic, posted below, now with even more fun :)

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