I replaced a single pole light switch with a single pole dimmer switch. I was surprised to see two black wires and one white wire. Since there are only two screws to connect, I connected one white wire to a terminal screw, and I managed to fit the two black wires onto the remaining screw.

I realize this was not a good idea, and don't feel like they are on there good since the terminal is designed for only one wire. What should I have done? Should I have connected the two black wires to the two terminals, and kept the white wire completely disconnected?

When you removed the old switch which wires went to which screw. A switch interrupts the power so normally both blacks go to the two screws. A white is frequently a neutral and would not be connected to the switch. It is important to know if either or both black wires are hot when disconnected from the switch, you never know what previous diyers have done to the wiring

What wires were connected to what on the old switch??
If it was all three, each on a separate screw, then you have a 3-way switch.

And NO, two wires on one screw is NOT good.

The white wire was connected to the top screw, one black wire was connected to the bottom screw, and the second black wire was inserted into the hole in the back of the switch. There is only the one switch that controls the light. Wiring is from the early 70's.

Disconnect all wires and use a tester to see what is hot, most likely just on of the black wires. Once you figure out which wire(s) is hit label it or mark it. Connect a hot to a switch and one of the "dead" wires to the other screw, flip the switch. If it lights then look around and test other lights and receptacles, I am guessing the spare black was running power to something else.

OK... I will try that as soon as I can. It's my friends house so I will try it next time I am there.
So you think that one of the black wires is definitely dead? Shouldn't the white wire be dead or neutral? Should I connect the white and the hot black, or should I connect the hot black and the dead black? Thanks!

If you had a white and a black on a switch I know for sure it was not wired by an electrician. White SHOULD be neutral but if you put a white and a hot black on a switch you get a dead short when flip the switch. I think that white should have been black but some diyer messed not knowing how to wire. Do you know if the switch box is metal and or grounded, is there a green or bare wire anywhere in the box

Scarey.

Did not see any green or bare grounding wire anywhere. I think the box is plastic, but I don't remember or sure. The dimmer is working fine, but I know I should not have two blacks on one screw. Guess the only way to find out is to test the wires like you said.

Yep and get back to us. Two wires on one screw is another indicator that a non electrician did this work.

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If you had a white and a black on a switch I know for sure it was not wired by an electrician. White SHOULD be neutral but if you put a white and a hot black on a switch you get a dead short when flip the switch. I think that white should have been black but some diyer messed not knowing how to wire. Do you know if the switch box is metal and or grounded, is there a green or bare wire anywhere in the box

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This is not true, and I am surprised you would say this.
It is very common to see w white on a switch. It's called a switch loop. The power is at the light and feeds down on the white and back up on the switched black.

Now... the very odd thing is the extra black, which I agree is feeding out to something else. In this case there should be no whites on a switch.

Duh, my bad , was not thinking of a switch loop at all. I jumped to too many conclusions once I heard he had two wires on one screw.

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Originally Posted by stanfortyman

This is not true, and I am surprised you would say this.
It is very common to see w white on a switch. It's called a switch loop. The power is at the light and feeds down on the white and back up on the switched black.

Now.....the very odd thing is the extra black, which I agree is feeding out to something else. In this case there should be no whites on a switch.

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Well I'm not an electrictian, but I do know I have seen white on a switch.
As far as the wiring... all I know is the dimmer works fine, but I know it's not right. I have a white on one screw, and two blacks on the other... maybe one is dead.

Okay,

Whoever set this up originally was controlling two devices with the switch.

Normally, a switch loop brought power to the switch with the white wire that was marked with black tape or marker.

The power was returned to the circuit via the black conductor and grounds were connected together. So, the best way to isolate the wiring is to get a tester or multi meter and then shut the power down to the circuit, disconnect the conductors from the switch. Now turn the power back on and test each wire between ground (bare wire) and each conductor.

I would expect that the white conductor is hot and the black conductors are not.

Next, cut the power off again, connect white to the switch and only one of the two blacks.

Turn the power back on and verify the correct operation of the switch. If it does not turn the light on, than cut the power and remove the black from the switch and replace it with the other black wire. Retest the circuit. If it works, cap off the remaining black.

Now see what is not working. Something is connected to the other end of the remaining black. It will no longer work because of the disconnected conductor.

Don, thanks... that sounds like something that will work, and I will try it as soon as I am able to get over there. I wonder what the other device is. I guess there's no way to connect both wires safely to a dimmer switch, right?