Electrical – How to ground a 3 prong outlet in plastic box no green ground wire

There are only 2 correct and safe ways to add a 3-prong outlet where formerly there was only a 2-prong one*:

• EDIT: now allowed in Code since this answer was written: Retrofit just a ground wire, using proper wiring methods. There is no need for the ground wire to follow the same route as the power cable. Grounds can be shared: You don't need to homerun back to the panel if you can reach another grounded circuit with the same size or larger ground wire also coming out of that panel.
• Safe way #1: run a new cable with an appropriately sized ground all the way back to the breaker panel.
• Safe way #2: Use a GFCI outlet (with the "test" and "reset" buttons), which will cut off power to the outlet if it senses an imbalance of current. (EDIT: As @EricLippert correctly points out in the comments, you can also replace the circuit breaker with a GFCI breaker instead of using a GFCI outlet.)

There are many wrong ways to add a 3-prong outlet, most of which are dangerous and some of which are hard to spot or test for:

• Wrong way #1: Using a 2-prong to 3-prong adapter without properly grounding the tab (common but obvious).
• Wrong way #2: Replace the 2-prong outlet with a 3-prong one, but only hook up the black and white wires (this is hard to see unless you open up the outlet box, but an outlet tester with reveal the problem).
• Wrong way #3: Replace the 2-prong outlet with a 3-prong one and short together the neutral and ground wires (this is probably the sneakiest because it will test as OK with an outlet tester but is just as unsafe as #1 and #2).
• Wrong way #4: Replace the outlet with a 3-prong one and connect the ground to a random piece of metal (the outlet box, a pipe, metal cladding on an armored cable). This too may check out as OK with an outlet tester but is not safe.

*There are actually a few other ways to ground an outlet safely, e.g. with armored cable with a bonding strip or a grounded box plus an outlet that allows grounding through the strap. But those are both pretty uncommon.

You should be able to find ground screws at any home improvement store. Most boxes have a tapped hole for this purpose. If yours does not, you should be able to find grounding clips at the home improvement store as well.

Keep in mind, however, that continuity does not equal an adequate ground. Just because you have continuity between two of the boxes, does not mean you have an adequate ground all the way back to the grounding electrode. For example, the boxes might have armored cable running between them. Armored cable is only rated as a grounding conductor, if there's an internal grounding strip (which is not always the case). Armored cable could/should connect the boxes in such a way, that a continuity test would pass but does not provide a proper ground.

If you don't have an adequate ground; or you're not sure if you do or not, the best bet is to not connect the grounding terminal of the GFCI to the box. You should also place the "No Equipment Ground" sticker on the receptacle, but this is often neglected in residential situations.