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I am installing a 40 amp sub panel in my detached garage. I understand I need two ground rods for the sub panel in the detached garage. Since I am feeding the sub panel with a ground wire from the properly grounded main panel, can I consider the main panel grounding as one of the two ground rods for the sub panel? Or do I need to install a second ground rod for the detached garage?

Furthermore, if I have two ground rods can they be connected in series, or is parallel required.

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The general interpretation is you will need two at the detached building, and you can run them in series.

The NEC is subject to interpretation by the local Authority Having Jurisdiction, not everything is gets interpreted exactly the same. Section 250 on grounding is particularly confusing because it seems most paragraphs reference other paragraphs with other references.

When the NEC requires "supplemental" electrodes for ground rods the code allows connection of the supplemental electrode at the service. So some people argued that if a supplemental electrode was attached at the service it satisfied that requirement even if the service was in a different building. I don't know if wording changed but I haven't heard anyone even try to use that argument in probably 15 years, but you may find that argument on the internet. It's a bit of a risky argument because there seems to be two types of inspectors, those that like to argue and those that don't. And his could trigger them both.

You can run the rods in series, that is normal. When doing it that way it is best to run a single unspliced conductor through the first ground rod connector and continue to the second rod. You are allowed to connect the wire that serves the supplemental rod to the primary rod or it's conductor by Listed irreversible compression connector or exothermic welding, but easier just to make it continuous.

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You only need one ground rod, actually.

However, the one ground rod needs to pass an expensive ground-impedance test for which only professionals have the gear. This test costs more than a second ground rod. If it doesn't pass, or you aren't willing to do the test... then NEC allows a second ground rod as an alternative, on the assumption that if two won't do it, then you're just spraying and praying :)

So the ground wire back to the panel, while mandatory, does not count as a second ground rod. It has a completely different job, it's not just done as blind ritual.

If you need a ground rod because it's an outbuilding, then you need full monty (test or second rod or Ufer).

Note that "detached" does not mean "must go outside to get into garage". Detached means an actual air-gap between structures, not even a breezeway.

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  • So my detached garage with only the heavenly skies over the 6’ gap between house and garage needs a tin sheet cover or it’s truly detached? Right?? Jan 28, 2022 at 1:00
  • @Russ yeah, you have to weigh the cost of ground rods, disconnect/Rule of Six, etc. vs the cost of a breezeway :) Jan 28, 2022 at 1:50

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