I am trying to connect a dimmer switch.There are 3 large black wires coming from the box(they all look alike).The dimmer has a red and green from the right,and a red and black wire from the left.How do I connect them?

I am trying to connect a dimmer switch.There are 3 large black wires coming from the box(they all look alike).The dimmer has a red and green from the right,and a red and black wire from the left.How do I connect them?


NOTE: I AM MAKING ASSUMPTIONS HERE. IF THEY ARE INCORRECT, DON'T TRY MY SUGGESTION!!

I'm counting four wires from the dimmer, and three wires in the box, so I'm guessing you have a light that is controlled by two switches, such as top and bottom of a stairwell, opposite ends of a hallway, or two entrances to a room?

If so, I am also guessing you are replacing one of the "3-way" switches with a 3-way dimmer. The green wire on the dimmer is grounded, the black wire is the switch common, and the two red wires are "travelers."

When you removed the old switch, did you notice that the screw on one of the terminals was a darker color than the other two? The "common" wire will be connected to this terminal. The two travelers should be connected to the other wires.

If you didn't notice when you took the switch out, all is not lost. Take a piece of white tape, and label the wires in the box 1,2, and 3, for reference. Connect the dimmer to these wires, and see if it works in conjunction with the other 3-way switch. If it is wrong, it will work in only one position, just keep trying different combinations until it works correctly.

Note: Only do this if all the assumptions I made were correct. Next time you do electrical work, remember to make a diagram of how everything is connected before you take it apart. Note the color of the wires, which ones are "wire-nutted" together, and to what terminals of the switches and outlets they are connected to... it will save you a lot of trouble when you put it back together.