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I'm wiring in a light switch and light into a closet on the other side of the wall of this outlet. I've run romex from the lightswitch hole through the wall and into the back of this metal box (secured with a clamp). The metal box receives power from some flexible metal conduit with a black and a white wire in it. I've attached the white and black wires from the romex to the open screws on the outlet. Now I'm wondering what the proper way to wire the ground is.

In the picture, you can see that the prior wiring had a small wire connecting the ground screw on the outlet to the neutral screw coming out of the wall. Perhaps the previous electrician used this to retrofit the old two prong outlet to a three prong one. The box doesn't have an obvious ground spot. Besides my new romex, there are no other wires.

The house was built in 1923 in Minnesota and has had various renovations over the years. The prior owner owned the house since 1947 and almost always had professionals do the work. The wall is plaster/lathe.

Edit:

I added some more photos of the old wiring. I found a spot in the box where it seems like I can thread a bolt, I'll try grounding it there.

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There is a good chance that the BX (flexible conduit) is being used as the ground back to the source or junction box. Which is fine. Just use the three wires that are shown and go to the switch. If you are going from the switch to a light, then you really do not need the ground anyway. I don't understand the loop from neutral to ground. I'd remove that and connect the ground wire. Obviously there is no GFCI in place.

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    We don't know if this is BX, FMC, or what...I'm actually suspecting this is FMC behind the scenes from the partial "TW" mark on the wire, but can't be sure.
    – ThreePhaseEel
    Oct 3, 2017 at 0:09
  • Or it could be NM cable with the ground wrapped around the cable under the clamp. You need to test the hot to the metal box and see if you get voltage.
    – ArchonOSX
    Oct 3, 2017 at 0:15
  • I added photos of the conduit and closer look at the wires. The conduit is pretty thick, seems thicker than BX. I guess I get to buy a multimeter. Would the cheap ones at Menard's be good enough?
    – Toby Kathan
    Oct 3, 2017 at 3:01
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Ground to the box

Your best bet here is to replace the receptacle with a self-grounding type, throwing away the improper neutral-ground "bootleg" jumper in the process, and then land the NM ground wire on a 10-32 screw into the ground screw hole in the back of the box. That way, everything should be grounded through the FMC, provided the FMC itself is grounded. (It likely is, but double checking to make sure you can read voltage from hot to ground with a meter doesn't hurt.)

(This is pretty clearly FMC -- the TW conductors without any visible assembly-paper wrapping are the tip-off here -- it also doesn't hurt that it's fatter than BX/AC, as a general rule.)

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