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I installed a new bathroom fan. I ran a wire to the light switch, but when I opened the box, there were only two wires inside. How do I wire it so I have a switch for fan and light?

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What you have is a switch loop. You CANNOT take power from this switch box for another item such as your fan/light. You'll need to find an alternate feed for this to bring to the switch box. In this case you'd leave the existing switch wiring alone.

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I am assuming that by "only two wires..." You mean that there is a single Romex entering the switch box with a black and white wire and maybe a ground wire too? The black and white wire are attached to the terminals on the switch? One wire- probably the black wire is hot and the other wire is connected to the light. If you want to add a switch for the fan, you will need to run a new wire to the switch box.

Another assumption: the light fixture box will have a feed wire and a switch wire only.(if this is not the case, you will have to figure out which wire is for which circuit.) The switch wire will likely lead back to the box with the original light fixture. You can wire the fan like the light is wired. Run a new line to the switch box. Connect the black wire to the black feed wire in the light fixture box. Put black electrical tape around the white side of the new switch return wire. Connect the black taped white wire to the black fan wire. And the white fan wire to the neutral feed wire. Wire the fan switch like the light switch. (Tape the white wire with black tape.)

I am making another assumption that you have attic access or a smooth pull to the source of the wire. If the access to the wire runs is obstructed, you may have to remove a small portion of drywall in some places to get the access you need. Also, be sure that you match the gauge of wire that is in the rest of the circuit. Some older homes were wired with #12 wire- #14 is smaller gauge. Also, again, use best safety practices and make sure the circuit is off before you work on it.

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