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When extending an existing electrical circuit in an older home (comprised of 2 wire service (common and hot with no bare wire ground)) should one use 2 wire with a bare wire ground for the extended portion, connecting the bare wire to the common wire of the existing circuit?

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No, you should not do this. Grounding wires must be kept separate from conducting wires in all cases.

First, check if the circuit you are trying to extend is truly ungrounded. Does it use metal-clad cable and metal electrical boxes like this?

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If it does, the metal jacket may provide grounding for the circuit. You can try using a multimeter to measure AC voltage between the hot wire and the metal jacket. If you get about 120 volts, this is good evidence (but by no means conclusive proof) that you have a grounding system in place. You can then use two-wire-with-ground cable (e.g. 14/2 NM cable (or "romex")) to extend the circuit by bonding the bare ground conductor to the metal junction box with a green grounding screw.

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If you don't have a ground at all, then the only safe way to extend this circuit is to add a GFCI receptacle and connect the extension to the "load" terminals of the GFCI.

This will allow you to install three-prong outlets or grounded fixtures safely on the extended circuit. These devices must not have a ground wire connected, and must be marked with a sticker that says "NO EQUIPMENT GROUND." The upstream GFCI will protect those devices in the case of a dangerous condition. Here's a diagram:

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  • Excellent....very precise and clear answer Friedo. Thank you very much. – Fred Loveland Aug 13 '15 at 3:30
  • If your existing cable is very old armored cable (BX), the metal sheath probably isn't very good for grounding. Newer armored cable has a continuous grounding strip running the entire length of the coiled armor to ensure a solid ground. Another option, if you have access, is to run a lone ground wire (bare or green-insulated) from the panel to the location you're extending the circuit from and ground all of the new outlets. And you could still use a GFCI receptacle to protect all of the old ungrounded receptacles. – Craig Aug 13 '15 at 5:52
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    It may also become a toaster. – Mazura Aug 13 '15 at 10:27
  • If your area has adopted NEC 2014, you may be able to borrow a ground from another circuit. See this answer for more details. – Tester101 Aug 13 '15 at 11:19
  • FYI, current electrical code requires AFCI protection on certain circuit extensions. The areas which require AFCI protection are kitchens, family rooms, dining rooms, living rooms, parlors, libraries, dens, bedrooms, sunrooms, recreation rooms, closets, hallways, laundry or similar areas. But not bathrooms, unfinished basements or outdoor circuits. 210.12 They do make combination AFCI/GFCI breakers too. ;-) – Kris Aug 13 '15 at 23:33

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