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I am trying to add an exterior outlet to the rear of my house using an existing circuit that was once wired to a floodlight, and want to make sure I am grounding the circuit properly with my modification, or if it is even necessary. The circuit was cut-off at the exit from the house (prior to my ownership), probably to make room for an AC unit installation (I'll try to add a few pics). At the exterior of the home there is a metal "elbow" that is fed by metal conduit through the exterior wall/siding from the interior. My plan is to run new metal conduit up from the elbow into a new metal gang box with an outdoor in-use GFCI. I have read that you don't need to have a ground wire in a metal conduit, but the metal conduit is not in any way grounded to the main panel, so I am concerned I need to run new wiring that includes a ground wire from the main to the junction box that feeds through to the exterior.

The existing circuit is wired from the main panel into a metal junction box using 2-strand ROMEX (without ground wire), the hot wire is tied to a ROMEX feed that exits the junction to a switch in another room, then the return from the switch is wired to THHN wire that feeds through the metal conduit to the elbow at the exterior of the home (one feed, one return). I'm wondering if I need to rewire to the junction from the main panel with 12/2 ROMEX to properly ground the junction/conduit?

The three pictures (if I figure out how to add them) show 1) the termination of the circuit outside, 2) the wiring of the junction box, and 3) the conduit running from the junction box through to the exterior wall/joist.

I am a mechanical engineer with extensive home/appliance repair experience, understand the basics of home wiring, and have added a new circuit to a home once, but otherwise this is new territory and I haven't been able to find answers to my specific question anywhere so far. Any help is appreciated!

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  • Are there other paths you can use to get a grounding wire back to the panel, to another 20A or better branch circuit, or to your Grounding Electrode Conductor for that matter?
    – ThreePhaseEel
    Commented Oct 7, 2023 at 23:03
  • It is assumed if metal conduit is used, then it will be complete to the panel. It seems in your case it is not complete so you need a grounding wire or to complete it to the panel. I think you cannot complete to the panel without pulling all the wire/cable again, to be in code.
    – crip659
    Commented Oct 7, 2023 at 23:19
  • Thanks crip659! That's exactly what my intuition was saying. Fortunately the junction is right there at the panel, so it shouldn't be too much hassle. It's only a 15 AMP circuit, so I guess I can upgrade to a 20A fuse while I'm rewiring everything anyway.
    – John Doe
    Commented Oct 7, 2023 at 23:32
  • A 20 amp breaker needs all the wires on that circuit to be a minimum of 12 gauge. No 14 gauge(15amp) wire allowed. Romex is not allowed outside, in conduit or not. While you can put romex in conduit, THHN/THWN wire is better to pull. You need the wet rated wire outside anyway.
    – crip659
    Commented Oct 7, 2023 at 23:50
  • See also diy.stackexchange.com/q/36350/18078 (not quite a duplicate as this question is specifically about grounding conduit that's isolated from the panel, but highly related as to how retrofit grounds are done.) Which changed in 2014 code.
    – Ecnerwal
    Commented Oct 8, 2023 at 1:05

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Unless your area hasn't adopted NEC2014 or later, if there's no other defect to be corrected except a missing ground wire, you can simply run a 12AWG ground wire (green or bare, or green with yellow stripe) from the box (connected to the metal box with the box ground screw) to any of:

  • Ground at the panel feeding the box.
  • Ground of another circuit with 12AWG or larger ground, fed from the same panel.
  • The Grounding Electrode Conductor connecting the panel to the ground electrodes

You should verify that all the connections are properly tight on the conduit fittings; you may opt to simply run a ground wire in the conduit as well as grounding the conduit. Grounding the conduit is not optional, running an additional ground wire inside it is optional.

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