Okay, I have a 40+ year old house. The wiring is a mish-mash of old stuff and whatever the handyman the previous owner happened to hire that day put together.
So, I have a hallway light. I’m replacing very old switches with new ones. I hooked the new ones up the way the old one were and no joy. I bought a bunch of switches so I changed them out (just in case - I’ve bought new switches that were defective and still no joy).
So I started moving things around. Big mistake.
I now have the following (switches are two brass screws at top, 1 black screw on bottom):
Switch Alpha: Wires A, B, C.
Switch Beta: Wires D, E, F
Alpha is wired: Beta is wired:
B A F D
C E
So, I’m going to use “up” as towards the two screws and down towards the single screw:
Alpha Up. Beta Up Light Off, Beta Down Light On.
Alpha Down. Beta doesn’t matter. No light.
Multimeter tests:
Alpha UP, Beta UP (light off): BA, BC, FD, FE HOT
Alpha UP, Beta DOWN (light on): Nothing HOT on either Alpha or Beta
Alpha DOWN, Beta UP (light off): BA and BC HOT
Alpha DOWN, Beta DOWN (light off): BA and BC HOT
I’m not following your code, but here’s what you need to know. Of the three screws on each switch two need to be connected to each other. The third screw goes to the power source at one end and the light at the other. Use your multimeter as a continuity checker (assuming that it has such a function, if not you can improv one from a flashlight) and establish which of the three screws on each switch is the “common”. Then, since you’ve probably mixed up the wires in each box, try to id which one doesn’t seem to go to the other box and screw that to the “common”. You don’t have to worry about shorting anything, so just keep track of which wires you’ve already tried as the “common”.
I had this happen with a stairway light when we first moved into our house. You could turn it on and off at the top as long as the bottom switch was in the “right” position. If it wasn’t, then flicking the top switch did no good whatsoever. Then you could turn it off at the bottom, but would need to turn it back on before going back upstairs. This was 20+ years ago so I don’t remember which switch was wired wrong but the fix was fairly straightforward.
Take everything apart. Find the hot wire, there should only be one. Connect it to a black screw, connect the other two wires in that box to brass screws.
Other box,
One wire should be hot here now, it goes on a brass screw. Flip the switch in box A. A second of the remaining wires should become hot, it goes on a brass screw. Last wire goes to the black screw.
ETA: Killing the power while terminating wires keeps the power from killing you.
There are however, several ways to configure them. panel-switch-switch-light, panel-switch-light-switch, panel-light-switch-switch and a few others. I think I have a chart of 6 of them around here somewhere. These are the most common. But the basics of one hot, one neutral and two runners are common to all.
It’s done. Knocked it out in about 10 minutes this morning.
K2500 That was the clearest explanation I’ve ever seen for troubleshooting a 3-way installation. And helped tremendously when I moved on to the next one.*
And for the love of all that is holy, would it really be so hard to come up with a standard for where the damn common screw is?! :mad:
I was only changing one of these so I did as follows:
Cut power.
Disconnected all three wires.
Repowered
Found 1 hot. Went to 2nd switch and flipped it. Same wire stayed hot. Got my common.
Cut power.
Hooked it to common on the dimmer switch (which put common in an entirely different place than the others - hence the complaint above).
Hooked the other two wires to the other two screws.
Repowered and we have light!
Your post has been printed and added to the reference stuff I keep in my toolbox!
There are at least five and a half different ways to wire up such a circuit, even beyond such details as whether the light is on the hot or neutral side. There are good reasons why the standard one is the standard, but the others will still work. It’s possible that the original wiring was one of the nonstandard ones, and this is causing confusion.
What I did was to take out the old one carefully recording what was connected to what and just reconnected the new one in the same way. Power off the whole time and it worked the first time.
When two of the six wall switches in the house broke, I decided to replace all of them one bright summer day (so plenty of natural light). I discovered something astonishing: all six switches were of different design and were from 4 or 5 different manufacturers. My house was built in 1942 during WWII and I conjecture that these were all used switches recycled from even older houses. Saved a lot of effort replacing all six.
Confession. I am an electrical engineer who has designed control wiring for substations and power plants and even I have stumbled when rewiring 3 way switches.
Been there, done that… Some “post mortem” advice that may help the next guy looking for help - Look at the backs of the switches. If there are markings indicating the common terminals, follow that and ignore the color of the screws. I have fallen into the trap of switches with two black screws and a brass screw, but the commons were one of the blacks and the brass.
I knew a very senior EE who would ask a circuit design candidate how a three way switch worked as one of his standard interview question. That knowledge was not a job requirement, the idea was to see how the candidate would work through the problem. I appropriated the question after the senior EE retired - it amazed me how many people had trouble getting to the right answer. If they did get to the right answer, I would ask how they would add a third switch location (four-way switch).
For a third switch, you need a different switch design, one which takes two input wires and swaps them. Once you’ve got that, you can put in any number of them you’d like between the switches on the ends. And you could even use those switches for the ends, too, if you don’t mind the occasional hot empty terminal, but I imagine they’re a bit more expensive.
I’m not an electrician and don’t totally understand how these things work but I ran into a situation where it had been wired wrong but worked correctly for years. It was in the house I grew up in.
There were two light fixtures, one at the bottom and top of a stairwell. There was a two switch wall panel at each end so you could control both lights from either end.
My mother asked me to replace the downstairs bulb which had burned out. I climbed up a metal ladder, removed the cover, and felt a tingle when I touched the metal fixture. Thinking the switch must be on I flipped it the other way, climbed back up, and still got a tingle. At that point I told her that I wasn’t going to risk electrocuting myself and that she should call an electrician and have him sort it out.
He confirmed that it was wired wrong (the ground and live were swapped or something) and fixed it.
