In a bathroom where a switch powers a circuit running to a light, then to a fan, is it allowed to replace the switch←→fan circuit with a 3-wire cable, so that I can add a switch dedicated to the fan?
In this location, the back of the wall on which the light and the switch are mounted is easily accessible (front is ceramic), so I thought the switch could be replaced with a dual gang box and the 2-wire cable with a 3-wire cable, the third wire only powering the circuit going to the fan.
It seems to me like it should be allowed, as the circuit is only powered from one branch; there is no risk of overloading the neutral (or any of the live); at any time in any branch, the return current is equal to the source current.
Best Answer
Go right ahead
Since all the current-carrying wires will be in the same cable, you'll be fine as long as the new cable is the same AWG as the old cable. Your analysis of the current flows is correct, by the way.