Separate building question

[Jim Port]

Under the older code that allowed a 3 wire feeder, the neutral was bonded to ground like a service panel.

[Erby]

Ran into this today.

Detached garage with a 30 amp feed from the house panel.

Four wire and a grounding electrode with neutrals and grounds separated with the ground bar bonded to the panel.

Is it right?

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[Marc]

Yes.

A good way to check if it's right is to ask yourself two questions: is there an alternate path for neutral currents (there shouldn't be any); can a ground fault in the garage find it's way to the neutral in the main panel (it should).

Neither your ground bus nor the panel enclosure in the garage is connected to the neutral bus in the garage so there's no alternate path for the garage neutral currents. That answers question #1. As for question #2, you have four wire service to the garage so the ground bus in the garage is connected through the ground wire in the feeder to the neutral in the main panel making that an affirmative for question #2. You're good to go.

Marc

[mjr6550]

I'm saving this post. Mr. Katen's reply with the #1, 2, and 3 does a good job explaining this. This has always confused me because I can't recall seeing a detached garage with a separate ground rod in the past 30 years (I probably have seen a few, but they are rare).

Today I did see one in a 2003 garage. Four wires leaving the main panel, three showing up at the garage panel. Neutrals and grounds on same terminal. Assuming the conduit is plastic and the ground wire does not continue to the garage panel, it appears that this is ok?

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[plummen]

. . .

If there's a continuous metallic path between house and separate building (such as a ground wire or metallic conduit) then don't join the neutral bar to the ground bar in the panel of the separate building because that might result in neutral currents in the metallic path. Ground rod connects only to ground bus.

But in that case, you need to have an equipment grounding conductor in the feeder. Otherwise you have no low-impedance grounding path back to the service panel. The pre-2008 rule was that if there was a continuous metallic path, you needed a 4-wire feeder.

If there's no continuous metallic path between house and separate building, join the neutral bar to ground bar, just like you would on the main panel in the house.

That was acceptable pre-2008. After 2008, you always need a 4-wire feeder and ground/neutral separation regardless of metallic paths.

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[Jim Katen]

. . . Today I did see one in a 2003 garage. Four wires leaving the main panel, three showing up at the garage panel. Neutrals and grounds on same terminal. Assuming the conduit is plastic and the ground wire does not continue to the garage panel, it appears that this is ok?

If the junction box (the one with the foam in it) is in the garage, I'd call it wrong and tell them that they have a four-wire feeder and they have to separate the grounds & neutrals.

If the junction box is in the house, then I'd say that the installation is ok, per the pre-2008 NEC. (But still dumb.)

The foam is wrong no matter what.

[mjr6550]

The,junction box was in the house, so I assumed it was ok. Because of the foam I could not see where the ground wire terminated.