Single Pole Switch is wired with three wires

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Old 04-12-13, 12:54 PM
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Question Single Pole Switch is wired with three wires

I am replacing switches -- so far, just the simple ones. One switch, one on/off. Single Pole. However, pulled one out of the wall and found three wires: two black and one red. One black went to the bottom screw and I followed suit, assuming it to be the hot wire. The other two were both attached to the same screw: black and red together on one screw. So, I am doing that but feel uneasy about it. What say you, the experts?
 
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Old 04-12-13, 01:04 PM
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Welcome to the forums.

Is there another switch which controls the same fixture as this one? Or does this switch operate more than one fixture?

Do you have a meter so you can determine what's hot coming into the box?
 
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Old 04-12-13, 01:43 PM
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The other two were both attached to the same screw: black and red together on one screw. So, I am doing that but feel uneasy about it. What say you, the experts?
Sounds like it was set up for a ceiling fan with light. Pull the light if both the red and black are connected to the light remove the red and cap. Use just the black for the light.

At the switch use just the two blacks and cap the red. If you have a ceiling fan with light and no remote post back for further instructions.

Note: It is not code compliant to put two wires under one screw. A the very least you would need to pigtail the wires but if only one is needed best practice to cap the unneeded one.
 
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Old 04-12-13, 07:24 PM
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Sounds like the two wires were being spliced by the screw connection. The best method is to pigtail.

Only one wire per screw head.
 
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Old 04-13-13, 05:27 PM
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If its not set up to have 2 switches turn on the same light, (i.e., switch at top and bottom of stairs can turn on/off the stairway lights) then it might be a splice to let outlets in that room get power off the same circuit.

As mentioned by Ray2047, the switch should only have 1 wire connected unless its designed to have more. So the black and red should be wire nutted together with a 3rd wire (called a pig tail) also attached to that wire nut. The other end of the pig tail should attach to the switch and that is the only wire to the switch.

Is the red wire spliced to the black that is hot and feeding power from the the circuit breaker box to the switch, or is it spliced to the load side of the switch? To find out, turn off the breaker and remove the switch and seperate the 2 black and 1 red wire from everything else. You can label the black ones so you know which ones to put back where, once you are done. Don't let them touch anything because one will be hot and cause a short. Turn on the breaker. With the breaker on, use a meter to check voltage between the single black wire, and a neutral (white) wire. Then check the other black wire to neutral. Which ever black wire shows 120 volts is the one supplying power to the switch. The other wires get power through the switch and carry it on the the load. If neither black wire shows 120V when checked to neutral, check the red one. Again, this check is if the switch is the only one used to turn on a light. If you have 2 switches in a room that both operate a single light or all the wall outlets, then the red wire is a traveler between the switches and this check won't really work for that setup.

If the red wire is spliced to the supply side of the switch, then its circuit will get power, even when you turn the switch off. If the red wire is spliced to the load side of the switch, then the switch will operate the outlets (or whatever is down stream from that point on the red circuit). So turning the switch off will kill power to the red circuit.
 
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Old 04-13-13, 05:45 PM
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If the red wire is spliced to the supply side of the switch, then its circuit will get power, even when you turn the switch off. If the red wire is spliced to the load side of the switch, then the switch will operate the outlets (or whatever is down stream from that point on the red circuit). So turning the switch off will kill power to the red circuit.
Since a SPST switch doesn't have a load or a line side, what matters is that you make the 3-wire splice with the pigtail, the red wire and the black wire that was on the same screw as the red wire.

If you get it wrong you'll know. Then the switch will be controlling more than the light, as SolarEd suggested.
 
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Old 04-13-13, 06:26 PM
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And I still suggest:
Pull the light if both the red and black are connected to the light remove the red and cap. Use just the black for the light.
I still think it should be a first move. You might also try the new switch with just the two blacks. If everything works Then you can check the light.
 
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