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I am retrofitting a Lutron Caseta dimmer switch to an existing light fixture in an older home. The fixture has line, neutral and ground, but the wire that was run to the switch box does not have a bare ground conductor. It does however possess a third, red wire that is unused on both ends. The Lutron Caseta needs to be grounded (since the fixture is Low Voltage). Can I connect the red wire to ground so the switch is grounded?

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    Tester101 has it right for the question asked. For potential solutions to your problem, it would be helpful to get pictures of the inside of the boxes to determine how it was wired (e.g. Wire in conduit with metal boxes where the conduit could serve as the grounding path). – statueuphemism Oct 26 '16 at 18:09
  • Yes it is a metal box with an armored cable. So the armor should be the ground conductor. My problem is that there is no armored cable emerging from the box in the basement, the box is much more recent (1984) than the house (1950). How can I tell if the box is IN FACT grounded? – Ken from OB Oct 27 '16 at 0:09
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    @KenfromOB -- measure the voltage from neutral to ground -- a grounded box should show a volt or three with the circuit loaded due to voltage drops in the neutral, while an ungrounded box should show nothing whatsoever or a high voltage. – ThreePhaseEel Oct 27 '16 at 3:18
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Not unless you wrap the entire length in green electrical tape, or you wired the circuit with 4 AWG or larger conductors. National Electrical Code says that equipment grounding conductors can be bare, covered, or insulated. It goes on to say "Individually covered or insulated equipment grounding conductors shall have a continuous outer finish that is either green or green with one or more yellow stripes..." (250.119).

If you wired the circuit with 4 AWG or larger conductors, then you can reidentify the red as ground. You just have to permanently identify the conductor at each end, and at every point where the conductor is accessible.

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