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I installing new bathroom fan and light. It has two wires coming in, one is 12/3 from the switch and in the switch it's only this wire with no wire from the circuit. Another 12/2 coming from the circuit.

The fan/light has 2 black wires for light and fan 2 white and one green.

Please help me out with wiring it.

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    Connect the same as the old fan/light. If old fan just had one black and white pair, can connect the two black/white fan pairs as one(keep black and white separate).
    – crip659
    Sep 7, 2022 at 10:49
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    Some pictures and exactly how you want the switch to work the fan/light will help. Right now it seems to be a single switch so the fan and light will come on together, it is both or neither.
    – crip659
    Sep 7, 2022 at 11:19
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    Have you disconnected the original wiring yet, or did you stop when you realized you didn't recognize what was going on here? If you stopped, then good for you! Get pictures of how it is currently wired (turn the power off, pull all the devices out of the boxes, but DO NOT disconnect any wires), then edit the pics into your question. Someone will help you out.
    – FreeMan
    Sep 7, 2022 at 11:51

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If not overly weird, the "incoming cable fom the circuit" should be ground, neutral, unswitched hot.

If the old unit had only a single switch, your right, proper and legal method for what's next depends on what code applied at the time the house was built or the bathroom last remodeled.

Why? Depending on the code that applied, you may need to run another cable to be legal, or you may not. Under current and recent code versions, you are required to have a neutral at each switch box. In that case, you can either switch the fan and light together on a single switch, or you can run a new (/4) or additional /3 cable to the box.

If and (legally) only if the last time this area was worked on was at a point before that requirement came into force in your local adopted version of the code, you can use a switch loop with no neutral to the switch box, and have two switches. That seems unlikely to be the case, if the old unit only had one switch, yet has /3 cable installed.

If subject to recent codes and not wanting to run a new cable, you would connect like colors in the cables, and connect the whites from the fan/light to the cable whites, while connecting the blacks from the fan/light to the switch cable red. Fan ground connects to cable grounds. Black and red would connect to the switch. If the switch is a "smart switch" or sensor switch that needs neutral, white would also connect to the switch - if not, white would be capped in the switch box. The fan and light would operate together.

If running a new /4 cable, the black in the /4 would connect to both switches, and fan black would connect to one of blue or red, while light black would connect to one of red or blue; red and blue would each connect to only one of the two switches at the switch end. Otherwise as above.

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