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I'm currently rewiring my new-to-me detached garage. Garage is from the 70's but has a newer Siemens panel (previous owners added a hot tub). The feed from my service panel is a sheathed 4 conductor cable in buried conduit, but both ground and neutral are bonded on the neutral bar. Hmm, ok well I lucked out and have the 4c feed, so I can just add ground bars and get the neutral and ground separated per code, right?

Here's where I'm a bit confused. I went to the service panel to just lay eyes on the same four conductors, but I only have the two hots and neutral there. The green ground wire is nowhere to be seen. Looks very much like the same cable, was looking to see if ground was snipped at the clamp but can't see any evidence of it.

I found this thread: Is my ground correct at my subpanel?

... which describes some techniques for checking out the ground on a subpanel. As described there, I disconnected the feed neutral and ground conductors in my garage and put my meter across them- no resistance. So the ground conductor in my garage has continuity back the neutral in the service panel, but I don't know how!

And now, my question: Beyond what I've done so far, how much should I do to verify this ground before separating neutrals and grounds in my subpanel? I'd think I'd like to know it's something substantial and not accidental. Is it possible it's tied to the conduit somewhere, and the conduit to the chassis of my main panel? I suppose I could tear out some drywall above the main panel to see what I can see.

Odds and ends:

-There's one access on the conduit where it enters the house- nothing interesting in there, just same sheathed cable.

-The subpanel and garage does have two GEC spiked into the earth behind the garage away from the house.

-Conduit leaving the garage is PVC, metal entering the house.

Thanks for any advice.

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  • Best option: get an electrician. Cheaper than some consequences of your mistakes. – Solar Mike May 3 '20 at 18:34
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Follow the conduit

I gather the conduit is metal leaving the main panel. And clearly at some point it transitions to PVC.

It's possible to do that at a coupler, but I rather suspect it's at an intermediate junction box. I bet the cable from the garage lands in this intermediate junction box, and the ground wire is tied to the metal box at that point. Then, the metal conduit carries it back to the panel.

You would want to find that box, and put the mark 1 eyeball on that setup and confirm it is so.

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  • Correct, metal leaving the house where it's buried, then PVC at the garage. I like the strategy, but unfortunately it would seem to involve digging out the conduit. No visible junction boxes. I would assume a buried junction box is highly unlikely. I will take another look and rack my brain for potential oddball places for a junction. – jagmandan May 4 '20 at 16:00
  • @jagmandan that's not allowed. Junction boxes must remain accessible. There must be an intermediate point. – Harper - Reinstate Monica May 4 '20 at 16:34
  • Update: I was able to get a friend to do a locate on the buried cable. We confirmed there are no outside junction boxes. Also, upon further inspection of the conduit transition into the house, I can see that the cable is marked same as the subpanel, so my mystery junction is inside somewhere. Next step is to pull the drywall between my main panel and the exterior cable entry to see what I can see. – jagmandan Jun 9 '20 at 13:54
  • @jagmandan I would carefully search any accessible junction boxes first, on the hopes they followed Code. Look for them in sneaky places, like behind false backs on built-in bookcases. Also see if your friend's tracing tech can follow the wire inside the house (turn off every other circuit). – Harper - Reinstate Monica Jun 9 '20 at 14:23
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The neutrals and grounds in a sub panel being required to be isolated is a change that happened in the 99 code so if the sub was installed prior to the adoption of the 99 code it was ok the same as a 3 wire and having the neutral and grounds tied together. After that date the ground was required to be isolated and a 4 wire with additional grounding electrodes on a detached building was required.

The ground is normally a bare conductor, with a rod pipe or plate electrode a #6 copper wire is all that is required. With the garage having the 2 electrodes just follow the wire back it is almost always bare solid copper, but can be insulated green and can be solid or stranded and may be a number 4 if it ties into your foundation rebar called a Ufer ground that is required to be at least #4 wire.

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  • Thanks, aware of the allowance for existing feeds to be 3 conductor. I have two concerns: (1) the inspector is going to see 4 feeds and tell me I need to use it and separate grounds/neutrals in the sub and (2) I'd really rather have them separated for the safety benefits. – jagmandan May 4 '20 at 16:01

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