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Here's what we have in the box:

(2) Black wires
(1) Yellow wire
(1) Capped cluster of white wires.

Here's how it was wired:

As stated above, the three white wires were capped. One black wire was attached to one screw of the light switch. The other black wire was taped to an opening in the middle of the yellow wire. The yellow wire was then connected to the other screw of the light switch.

I was trying to replace a regular switch with a dimmer and knew I should've dropped when I saw this, but went ahead and things got mixed up. I tried to return the light switch to at least the originally goofiness and now other circuits are being controlled by this light switch.

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A photo would be extremely helpful. Also, electrical wires are usually bundled in larger cables with anywhere from 2-4 (or more) wires inside the larger insulating sheath (also called wires but lets call them cables for clarity). Identifying how many cables you have and how many wires and the color of them in each cable would be helpful. –  The Evil Greebo Sep 16 '12 at 9:37
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Different countries use different color codes for wiring so knowing where you're located would be useful. (But if you're in the USA, please say that by "yellow wire" you don't mean the bare copper wire that's supposed to be used for ground.) –  Niall C. Sep 16 '12 at 14:20
    
Until you get the rep to inline a photo yourself, upload it to imgur.com and include a link in your question. –  BMitch Sep 16 '12 at 14:25
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It would be nice to know where you are in the world. –  staticsan Sep 17 '12 at 2:50

2 Answers 2

Since your other circuits are now switched, you have reversed the black wires. You must also eliminate the taped connection. Since your dimmer probably has wire leads instead of screw terminals, this could be easy. The thing to ensure is if you cut the yellow wire back to where it was taped, will it still be long enough? It should be if the dimmer has wire leads. Also ensure there is enough room in the box for all the wires and the new dimmer, as it is larger than a standard switch, it's easier to run out of room.

If it all looks good, first turn off the power to the switch box. Mark the black wire now attached directly to the switch as power. This was originally taped to the yellow wire until you got mixed up. Remove it from the switch and straighten the wire end.

Remove and straighten the other black wire now taped to the yellow. This originally went directly to the switch. This goes to the light you want dimmed. Trim it so the exposed wire is the same length as the wire stripped for the dimmer lead. Attach it to one of the dimmer leads with a wire nut.

Cut the yellow wire back to where it was taped, leaving an appropriate length of exposed wire. Trim the insulation as needed so the new end is free of any insulation. Connect the power wire, the yellow wire, and the free dimmer lead together with another wire nut. Be sure the nut is big enough to accommodate three wires.

Ensure any ground wires are all connected together, but not to any other wiring.

Turn the dimmer up to full power, and turn it off if you can tell the difference. Restore power to the circuit. Turn off the dimmer if it's on. Ensure the other circuits are still powered. Enjoy your now dimmable light.

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You might have some non-standard wiring. My guess is that the yellow is hot, and the black is tapping for power to the remainder of the circuit.

What you may have is a hot yellow to the switch, and the black from the switch to the switched device. The black taped to the yellow is then carrying on to feed power to other devices on the same circuit.

You need to get a non-contact voltage tester, and test each wire (disconnected) and figure out where the power is coming from.

If this is indeed the case, you need to mark the yellow as hot by wrapping electrical tape around the end of the wire.

I'd also like to see the connection pigtailed rather than wired in the middle. Cut it off at the bare spot, strip both ends of the piece you cut off, and wire the black, yellow and pigtail together with a wirenut. then connect the yellow pigtail to the switch.

But first test with a non-contact voltage tester and verify that I am correct.

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