Leviton Trimatron push on/off dimmer 705
Dimmer: the green wire is for bonding (grounding or earthing) the metal face frame. Do not connect it to the black or red wires. There should be a bare or green wire coming into the box, or the box (if metal) should be grounded through a wire or metal conduit. If the box is metal and you find a bare wire connected to the box, disconnect that wire and replace it with a 6" + (i. e., long enough to extend 6" out of the box) bare wire pigtail. Connect the 6" wire and the green wire to the original bonding wire using a wire nut. The two black wires are electrically interchangeable; they connect the hot wire to the light fixture (in place of a switch).
Existing house wiring: this is a head scratcher. In US residential wiring, one might see a 3/1 non-metallic cable with a black, red and white wire plus a bare wire for bonding coming from a light fixture. This would be used in one of two ways. In one, the black would be hot, the red would be the switched wire to the light fixture, and the white would provide a neutral connection for lighting control devices that need it. In the other (possibly more applicable to your situation), the white would be marked with black tape and would serve either as the incoming hot or as the switched wire to the lighting fixture (the black would have the other function), and the red would serve as a second switched wire for a fan or other load. The wise thing to do would be to remove the lighting fixture and have a look at the other end of the cable - take special note of the connections going to the fixture(s). You are looking for the hot connection coming in to the fixture box and the switched wire going to the fixture. But two black wires? How is it possible to tell which one is which without turning on power? Is there a difference in insulation marking? Or are the connected together?
Another common configuration (and I think this is what you have) is to bring hot and neutral into the box from the branch wiring, then send the switched connection(s) out of the box, and tie the neutrals (white wires) together with a wire nut. (Look for the neutral connection inside the box; if you find one, the job is relatively simple) In this case, one of the black wires is the incoming hot (comes from the cable without the red wire but with the incoming neutral), the other black wire and the red wire go to the fixture in the other cable along with the outgoing white neutral wire. You still need to look at the fixture to find out whether it gets the black wire or the red wire. It is possible that one of the wires (most likely red) is a spare for possible use in a fan/light combination.
Once you have identified which wire is the incoming hot and the switched wire going to the fixture, connect one of the black wires on the dimmer to the hot and the other black wire from the dimmer to the switched wire. What you do with remaining wire depends on what is done with it at the fixture box. If it is an unconnected spare, tape over the end (if stripped) and push it back into the box so it doesn't accidentally poke into a wire not and become energized. If it is connected in parallel with one of the other wires (unlikely), do the same at the dimmer box end. If it is connected to another load (probably a fan), consider replacing the dimmer box with a double gang box and adding a separate switch for the fan (do not run the fan on the dimmer!).
If you have any doubt about what you find, you should hand this job over to a professional electrician. There are a couple of ways to get into trouble with this, especially if the person who originally wired the fixture didn't follow the electrical code. The worst case is you connect the two switched wires to the dimmer and leave the hot wire open and exposed inside the box, and it comes into contact with a metal part in the box. This is potentially dangerous and destructive.
Jun 22, 2013 |