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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 3x3 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.

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Thinking harder, They are PAR-20 sockets, and no more than 50w bulbs. But when I want the room bright, its not quite bright enough, but I'm OK with that, but am unwilling to go with lower output bulbs. And no guarantee that even if I did, someone else wouldn't replace them up to the brightest possible. The circuit doesn't need additional capacity. I need two dimmers is all. The alternative plan would be to drive two dimmers off the 1 incoming circuit, and run another 2-wire cable in the attic. –  Jeff Warnica Jun 9 '12 at 22:01
    
Two dimmers isn't much of a concern to me. I've updated my answer to address that. By my math, 50W x 18 bulbs = 900VA. A 15A breaker at 80% gives you 1440VA. That should leave room for a few more lights on the same circuit, but not many. –  BMitch Jun 9 '12 at 23:55
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2 Answers

up vote 3 down vote accepted

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.

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To change your question to get another answer, you can do this. Make sure the lamps are 75 watts or less. 9 X 75= 675, unless there are transformers involved. For every transformer involved add 20 of the lamps wattage.

Most dimmers in the US are 600 watt if incandescent, so with 75 watt lamps that is too much wattage. You can always leave the circuit alone and add a 1000 watt dimmer, which would cover your load at 75 watts a lamp.

To be honest, to get the correct answer for this too is for you to answer the questions @BMitch is asking plus this question. What lamps are you dimming? i.e. 75 watt Par 30's or 50 watt MR16.

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18*75. Actually, as above, 18*50. Two lights per box. –  Jeff Warnica Jun 9 '12 at 22:12
    
Looks like @BMitch nailed it pretty good. Unless you have something else, he should get the answer. –  lqlarry Jun 10 '12 at 3:11
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