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Discussion Starter #1
First time here; forgive any etiquette or formatting errors.

I removed a fluorescent light fixture in my house and, in my haste, disconnected the wiring and did not pay attention to how it was connected. There are three 12-3 Romex cables coming into the box, and now there are wires everywhere and I can't figure out how to put everything back together with the new LED light I'm installing.

I tried everything I can think of (the obvious black-to-black, etc and even wiring the LED light straight to the switch) but nothing works. I suspect that this means the power is sourced to the light and not the switch, but that just complicates things, as does the fact that there's an extra cable pigtailed in. One further complication: I'm pretty sure that one of the cables had white/black switched but I can't remember which one. I've made a real mess.

So... can I use a multimeter to sort this out somehow? Or do I have to trace wires?
 

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Licensed electrician
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A meter would help you sort out the cables. You could test for power on all the cables. You could also open the switch to see if you have both a black and a white connected to the switch .
 

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Once you figure out the wiring, do you know enough to discern whether the LED replacement you bought for your florescent uses the existing ballast or wires directly to the line voltage?
 

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(By 12-3, do you mean 12-2 + Ground ?!
Unless it is multi-way switching or a 240 V supply, 12-3 + Ground would be unusual.)

As written here:-
So what is the something else that is not working?
because it is probable that
one "12-3" will be the incoming Line and Neutral
one "12-3" will be the outgoing Line and Neutral to another device/light - which is now not working
one "12-3" will be the Line to the switch, with the re-purposed White wire being the Switched-Line back from the switch - which should be marked at both ends with black tape.
A meter would help you sort out the cables. You could test for power on all the cables. You could also open the switch to see if you have both a black and a white connected to the switch .
With all wires disconnected, the Incoming Line and Neutral will show a 120 V difference, when the breaker is ON.
When you have found that, turn OFF the breaker and use an ohm-meter to find the pair to the switch. (Switch ON - short circuit. Switch OFF - open circuit.)

The third "12-3" would be the Line and Neutral feeding something else and you would be well advised to find out what that is before reconnecting.
 
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