I replaced a 2-prong outlet with a GFCI, wired as shown (connected ground wire coming into the box to the metal box as well as the ground screw of the GFCI, black wire to brass terminal & white to silver). The box has no separate grounding wire and wires are only in Romex, not metal conduit. Multimeter shows hot and neutral are correct. Later, I realized I never checked whether the ground wire was actually grounded at the service box. To be code compliant, do I need to apply the "no equipment ground" sticker regardless, or only if the bare copper wire coming in is not grounded at the service box?
The NEC in 250.130(C) now requires extending an Equipment Ground Conductor to the grounding system when making an extension of a two-wire circuit that does not have an EGC.
If your circuit was extended before the current requirement or was extended by local exception then the receptacle needs to be marked " No Equipment Ground" and the ground should be left disconnected.
406.4 General Installation Requirements (D) Replacements
406.4(D)(2) Non–Grounding-Type Receptacles. Where attachment to an equipment grounding conductor does not exist in the receptacle enclosure, the installation shall comply with (D)(2)(a), (D)(2)(b), or (D)(2)(c).
(a) A non–grounding-type receptacle(s) shall be permitted to be replaced with another non–grounding-type receptacle(s).
(b) A non–grounding-type receptacle(s) shall be permitted to be replaced with a ground-fault circuit interrupter-type of receptacle(s). These receptacles or their cover plates shall be marked “No Equipment Ground.” An equipment grounding conductor shall not be connected from the ground-fault circuit-interrupter-type receptacle to any outlet supplied from the ground-fault circuit-interrupter receptacle.
(c) A non–grounding-type receptacle(s) shall be permitted to be replaced with a grounding-type receptacle(s) where supplied through a ground-fault circuit interrupter. Where grounding-type receptacles are supplied through the ground-fault circuit interrupter, grounding-type receptacles or their cover plates shall be marked “GFCI Protected” and “No Equipment Ground,” visible after installation. An equipment grounding conductor shall not be connected between the grounding-type receptacles.
Informational Note No. 1: Some equipment or appliance manufacturers require that the branch circuit to the equipment or appliance includes an equipment grounding conductor.
Informational Note No. 2: See 250.114 for a list of a cord-and-plug- connected equipment or appliances that require an equipment grounding conductor.