I have a room with 9 pairs of halogen lights, currently all wired to one switch, and one circuit. So 18 total lights, with pairs sharing appropriate boxes. This might actually be more then is allowed for 1 circuit, but is for sure more than a single dimmer can handle. I'm resigned to rewiring things, wondering however what I can get away with. Physically distributed in 3×3 pairs, it doesn't ascetically easily break in two, except breaking up each pairs of lights.
Would it be acceptable to use 3 wire in this case? i.e. wire all "left" lights with the black hot, all "right" lights with the red hot, and a common neutral? Side-by-side dimmer switches, and input side of dimmers only with the existing single hot from the panel.
Best Answer
3 wire cable (black, red, white, ground) doesn't give you any additional load capacity unless the wires are on separate circuits and each circuit is on the reverse phase. This is because the neutral is returning all of the current for both hots. This is something you'd likely only see in kitchen outlets to easily run 2 circuits and distribute the load. In other parts of the home, the third wire is usually for running a switched and non-switched power source in the same cable (e.g. an outlet with one half switched.
Can you run two separate dimmers, one on the black, one on the red? I don't see any reason why you couldn't. Double check the packaging on the dimmer switches to see if there are any spacing requirements for heat dissipation. I would go with the deepest J box you can fit in the wall to give yourself lots of space to cram in the wiring. And if the wire holes in the back of the switch clamp via the side screws (rather than spring loaded), I'd use those to reduce any risk of the side screw shorting out with the neighboring switch.
To tell you if you're exceeding the capacity, we'd need to know the wattage for each fixture, wire gauge, breaker amperage, and what else is on the same breaker. This post explains the math and rules of thumb to ensure you don't overload the circuit.