Always have trouble wrapping my head around 3-way switches. I have a good book that I usually turn to, but didn't see a diagram for this pattern when a junction box is in the middle. How can I wire the junction box and switches in this instance?
A picture is worth a thousand words. This picture only applies to existing wiring, not new construction.
Here is the link for the picture: https://www.how-to-wire-it.com/images/3-way-power-at-light2.jpg
If there is an additional box for the light and the power enters there also, then the power white wire would be hooked up to the light, the power black wire would be hooked up to the black wire of the 14/2 which would be connected to the black wire that goes to the switch on the right of my diagram. The white wire of the 14/2 would go to the "hot" side of the fixture and be marked with some red tape and the other end would go the black wire from the switch on the left of my diagram and also be marked with red tape. The travelers should be marked with yellow tape . If this is new construction, then all switch locations must have a neutral. The cables from the switch boxes to the junction box must be /4 and from the junction box to the light fixture /3. The neutrals can just be capped in the switch boxes.
Harper's Law is Mark travelers yellow with a 5-pack of colored electrical tape they sell for under $5.
Anytime you mark a wire, mark both ends of the wire the same, and at the same time.
I also recommend marking (or choosing) always-hot to be black and switched-hot to be red. Grounds must always and only be grounds, and if neutral is present it must by law use the white wire.
Once you do that, 3-ways are stupid simple, regardless of how weird the plumbing is.
With a wrinkle: law now requires neutral at most switch positions.
On each of your branches, 2 of the wires must be travelers. I usually allocate those to the colors left over after assigning the other functions, that way I can use the "ideal" colors for those functions.
One or both switches must have neutral, based on Code, local amendments, and the layout of your room. This will necessitate /4 cable. If it is one switch, make the neutral one the one with always-hot. If both require neutral, then use /5 cable there - yes, ouch, I know!
You cannot use two cables instead of /4 or /5. All related conductors must be in the same cable! You do have two options, though:
- Smart switches. This moots the entire issue. Simply run /3 to both boxes, and you won't even use all the wires! (Which ones you need will vary by smart-switch model and position, so dual /3 will cover all bases).
- Motion sensors. If the light is automatic, you don't need physical switches. But then, you're married to that.
- Conduit. Then use individual THHN wires. In fact, you don't even need to run neutrals, because it is easy to retrofit additional wires into the conduit later. Further, you can actually use yellow for the travelers, and straightline them right through the lamp box without even stopping! This is what I do in my commercial buildings, and it is very elegant.
But back to doing it with cable and steam-era switches.
One requires neutral
The /4 cable goes to the one where neutral is required. In that one, white is neutral, black is always-hot, and the other two (which happen to be red and blue, not that I care) get marked yellow on both ends.
The other one gets /3 cable (though /4 will suffice). There, red is switched-hot, two of the other wires get marked yellow on both ends, and if it's /4, leave neutral out of it.
Both require neutral
One get a /4 cable. White is neutral, black is always-hot, and the other two (which happen to be red and blue, not that I care) get marked yellow on both ends.
The other one is a nightmare. This one gets switched-hot (red) and two travelers. But it also needs neutral (white), and as a practical matter, neutral is useless without always-hot (black), so AFIAC where neutral is mandatory, so is always-hot. Therefore you need /5 cable. The remaining colors get marked yellow.
On your actual lamp, it will have a black and white. Mark the black wire red.
We are now fully color-coded and need simply match like color to like color. (Travelers, of course, being separated).