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I am retired electrician. I have had contractors tell me to change ungrounded (2 prong) outlets to 3 prong and connect the ground to neutral (bootlegging). There was only 2 conductor/no ground cable going to the metal box. Grandfathered in was his answer.

Another contractor (similar...no ground to box) permitted GFCI replacement, but bootleg the ground...and no marking.

Both appear to be illegal. Is this unsafe?

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    So what is your question? If you want to bring them up to code you now can run a new ground and it is no longer required to be in the same cable/ conduit. Bootleg grounds are not safe. GFCI's can be used and labeled no equipment ground to provide a level of safety. – Ed Beal Aug 18 '17 at 12:53
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Yes you can replace two prong ungrounded receptacles with GFCIs. Since you say you are a retired electrician I'll be brief. 2014 NEC Article 406.4(D)(2) will explain how to do it.

No you should never bootleg a ground. If you want to install a ground, it can be added as outlined in 2014 NEC Article 250.130(C) and Exhibit 250.47. They can guide you through it.

I am mentioning the NEC year because these articles have moved around in different editions. Good luck explaining it to contractors. Hope this helps.

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You bet it's unsafe. If the neutral wire breaks, everything that's supposed to be grounded will now be lifted up to 120V and shock people, even if no ground fault is present.... and the GFCI will not know about this. And neutral wires do break, especially when servicing old MWBCs.

Never bootleg ground

The attraction of bootlegging ground to a GFCI is then, plug-in GFCI testers will work. They're not supposed to work on an ungrounded GFCI. He is doing that because he is a simpleton, or because he must impress an inspector who is a simpleton.

If the GFCI has no ground, it should be labeled No Equipment Ground. And if you stick a 3-light+GFCI tester in there, you should get 1 light. Pushing the test button should do nothing. If you get 2 lights, or a trip, violation.

Why shouldn't a GFCI tester work on a receptacle with only two wires going to it? Because a normal and healthy circuit needs 2 wires. A ground fault is by definition current taking a third path. If there are only 2 wires, the tester can't create a round fault, as no third path is available.

If you really, really want to make a GFCI tester work in an ungrounded GFCI, just get a 2/3 prong cheater with a little ground pigtail and use a partial reel of THHN to extend that down to the basement or anywhere you can get a real ground. That should give you the third path for fault/test current.

Before you retrofit grounds...

2014 Code revisions give you broad freedom to retrofit ground any way practical. Any circuits from the same subpanel can share ground paths as long as they're the required size. But before you order a bunch of drywall spackle... Remember the fundamentals: Current travels in loops, and grounds are concerned with two loops.

First service - hot wire - equipment - neutral - service. The GFCI provides flawless protection for that loop - better than you can have with a ground alone! Which is why they're requiring GFCI in places that already have grounds. So for electrocution protection, the GFCI has covered it - you don't really need a ground.

Second earth - sky - lightning - earth. GFCI won't do a darn thing about that, but that is also not a big issue in an indoor receptacle. It might be worth retrofitting ground if you are worried about earth - carpet - fingers - delicate equipment - earth loops.

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