Is this an acceptable way to add a light to a 3-way circuit?
My whole-house fan is currently on a dedicated circuit. I want to convert the single-pole switch to a 3-way. My plan is take the existing switch leg to fan and make it the traveler; the hot remains in the existing box. On the other end I can add a 3-way switch and run a new switch leg back to fan. Since the existing switch is downstairs, I cannot get a neutral from the fan circuit. Can I use a neutral from another circuit as-log-as it comes from the same phase side of the circuit panel?