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I'm trying to salvage a mess made by someone else. They've got two switches, one for fan, one for light. Power is coming in at the switches. They have a separate 12/2 for each switch is run to the ceiling fan. (I know 12/3 should have been used). How do I get this wired up?

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Wiring this up will depend on the fan and light each having a separate neutral wire.

NEC 300.3 requires all related wires to be in the same cable. Power flows in loops, and what's not allowed is hot current going up one cable's hot, but coming back the other cable's neutral. Also, paralleling is not allowed, so you can't just hook up both neutrals at both ends and "hope" power takes the correct path.

Unfortunately some people do not realize this, and think they can use two /2 cables as a substitute for /3 or /4.

Anyway, this can work if your fan/light gives you a separate neutral for the light, and a separate neutral for the fan. (in fact, this kind of application is why they do that, when they do that, which isn't always.)

So you hook up the light's hot and neutral, using one cable, and then you stop. You power the circuit back on, and confirm the light does in fact work. (so you know you didn't cross neutrals).

Then you hook up the fan hot and neutral to the remaining cable.

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A two 12/2 cables to the Fan and Light each is perfectly suitable for that.

A 12/3 would be needed if you need more than 15 Amps (two hot wires), or branching, or multi function switch and so on.

It is common practice to bundle all whites together, while using one each Black, for Fan and Light.

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