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I'm adding a circuit to a subpanel in my garage (built in the late 70s), and I've been reading a lot about subpanel ground / neutral bonding (how it's not supposed to be bonded at the subpanel, and that the ground / neutral need to be separate in the subpanel and you're sending the subpanel neutral back to the main panel neutral).

My subpanel is fed from a two-pole 60A breaker in the main panel, with a pretty heavy gauge 3 conductor wire, two hots going to the bus bars in the subpanel and one white going to the common. However, there's no dedicated ground wire coming from the main panel to the subpanel, and the subpanel's grounding bar goes directly to a ground rod outside the garage.

It seems to me that it's ok to have the subpanel grounded locally, and not run a fresh grounding conductor back to the main panel, but I wanted to double check that. Do I need to pull a new ground conductor from the subpanel back to the main panel? I think this would be difficult if not impossible, since it's about 80 feet away underground and I'm not sure about how good the conduit is.

Thanks!

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    I take it your garage is a separate (detached) structure? Also, can you get us what size and type (PVC, rigid metal, EMT) of conduit was used, as well as photos of the inside of the garage subpanel? Dec 10, 2021 at 0:36
  • @ThreePhaseEel yes it's detached. Conduit is maybe 2.5" PVC. I will post pics asap.
    – user101289
    Dec 10, 2021 at 1:00

2 Answers 2

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It seems to me that it's ok to have the subpanel grounded locally, and not run a fresh grounding conductor back to the main panel, but I wanted to double check that.

They outlawed it for a reason. But your installation is "grandfathered" so you're allowed to take your chances, just like you're allowed to continue in service legacy bathroom and kitchen receptacles that don't have GFCI protection.

There are several reasons, but the first one that comes to mind is if you lose your neutral, it will energize all your grounds. Same reason 3-prong dryers and ranges are bad. The ground rod will try to control it, but dirt is a poor conductor so it will more "bring the ground to phase potential" than the other way around, causing a voltage gradient across the ground. That can be serious business if you have a fence line or other metallic object at earth potential here but traveling elsewhere where earth is at different potential.

If you are able to run a ground wire and separate neutral and ground at the subpanel, I advise it. You could run a fishing tape down the conduit and see how far it can get.

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  • Thanks, that makes a lot of sense
    – user101289
    Dec 10, 2021 at 15:42
  • do you happen to know what size conductor I'd need to pull back to the main panel for a ground on a two-pole 60A service to the subpanel? Thanks
    – user101289
    Dec 10, 2021 at 17:27
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    That's tricky @user101280. There's actually no such thing as 60A wire. I doubt it is 55A wire since those are rarely seen in conduit, so I expect it's 65A wire, which requires #8Cu or #6AL ground. Copper can be bare. Dec 11, 2021 at 18:36
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Up until the mid to late 80's (maybe a bit later) the Code compliant method to feed a detached building was to run the conductors as a service with a single combined neutral/ground and bond the neutral bus to the ground at the first panel in the detached building.

I think it would be an improvement to separate the ground, but it is grandfathered and you are not obligated to unless performing some other upgrade to the feeder.

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