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I am switching an old rotary style single pole dimmer for a new single pole dimmer switch. The old dimmer had only two wires, with no identifiable ground wire. There are two other wires in the box, which appear to be pig-tailed together and wrapped with black electrical tape. How do I decide which of these two wires I should attach to the green ground wire on my new dimmer switch. I am attaching a picture of the box with four wires coming into the box...one to the red wire on the dimmer, one to the black wire on the dimmer, and the two others. Please help, I don't want to burn down my house! Thank you CC25enter image description here

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I bet the two wires that are taped together are neutrals, not grounds. (BTW, that tape should be replaced with a wire nut.) There doesn't appear to be a ground wire in this box. The metal box itself may or may not be grounded by a metallic conduit or armor. You couldn't say for sure without a lot more thorough checking, beyond the scope of a StackExchange answer.

You definitely don't want to connect the ground wire to a neutral. The switch will work but it's not safe at all.

NEC 404.9(B) addresses grounding for switches.

(B) Grounding. Snap switches, including dimmer and similar control switches, shall be connected to an equipment grounding conductor and shall provide a means to connect metal faceplates to the equipment grounding conductor, whether or not a metal faceplate is installed. Snap switches shall be considered to be part of an effective ground-fault current path if either of the following conditions is met:

(1) The switch is mounted with metal screws to a metal box or metal cover that is connected to an equipment grounding conductor or to a nonmetallic box with integral means for connecting to an equipment grounding conductor.

(2) An equipment grounding conductor or equipment bonding jumper is connected to an equipment grounding termination of the snap switch.

Exception to (B): Where no means exists within the snapswitch enclosure for connecting to the equipment grounding conductor or where the wiring method does not include or provide an equipment grounding conductor a snap switch without a connection to an equipment grounding conductor shall be permitted for replacement purposes only. A snap switch wired under the provisions of this exception and located within reach of earth, grade, conducting floors, or other conducting surfaces shall be provided with a faceplate of nonconducting, noncombustible material or shall be protected by a ground-fault circuit interrupter.

So if the box itself is grounded, you can bond the ground wire on the switch to the box with pigtail and a ground screw or ground clip.

If there's no ground available, and you're replacing an existing switch, you can cap the ground and use a plastic faceplate. There are some locations in there where you're supposed to install GFCI protection if the circuit doesn't already have it when you replace the switch.

Even if the code doesn't require it with your old wiring, installing GFCI protection on circuits without a ground is worth looking into, it may not be too expensive and it is a big safety improvement.

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    That is knob & tube wiring, so no grounding means exists as far as I can see. – Speedy Petey Dec 20 '15 at 12:51
  • @Speedy Petey; just curious, are you basing your statement "that is knob & tube wiring" solely on the cloth insulated wiring? – Jimmy Fix-it Dec 20 '15 at 15:38
  • I think you can see the tubes if you blow up the photo. (Can't see them with a phone size screen) – batsplatsterson Dec 20 '15 at 16:03
  • Looks like 1940's cloth to me there is probably a compression connector under the tape, that was normal for the time. – Ed Beal Dec 20 '15 at 16:31
  • @JimmyFix-it, I am basing it on that fact that I know what K&T looks like. I can see the fabric tubes and individual wires entering each opening. – Speedy Petey Dec 20 '15 at 17:02
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Speedy is right -- this is knob and tube wiring not AC/MC or conduit, so there is no ground whatsoever. Cap the ground wire off with a wirenut, replace that electrical-taped mess with a wirenut as well as heaven help you when that tape falls off and the neutral shorts to the floating box, and connect the hot and switched hot in as the old switch was wired.

GFCI, or better yet DFCI (Dual Function Circuit Interruptor -- a GFCI and an AFCI in the same package) protection for the branch circuits would indeed be wise as well -- if this place still has fuses, the time's ripe to upgrade to a modern breaker panel.

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