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The house we bought has a detached garage built from cinder blocks perhaps 50+ years ago (with no copper plumbing), and had only one 20A line installed underground from the home. I'd like to install a 100A subpanel in the garage so I'll be able to add additional circuits, including for EVs. What diameter & length grounding rod(s) for a 100A sub panel are required in Northern NJ?

If it matters, the garage is 70 ft from the home, and the home has a 200A panel. I'm planning to run about 111 ft of 3-3-3-5 SER copper underground cable (depth of 2 ft) from the home.

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    Why waste money on copper SER? Aluminum 1-1-1-3 is 1/3 the price, and 2-2-2-4 is even cheaper if you can live with 90A. If you heard bad things about aluminum, "that which you heard" never applied to heavy feeder like this. Heck, the wire is landing on lugs made of aluminum. However, you may want to consider conduit. Much easier to maintain. I know everyone's plan is to not have a problem, but rocks don't listen lol.
    – Harper - Reinstate Monica
    Commented Aug 16, 2022 at 4:12
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    Also, ordinary SER cable cannot be used underground, even inside a conduit. You need special dual-rated TC-ER/SER cable for that (Encore Wire's the only folks who make it, and it's not exactly trivial to find as a result). Of course, you could use a direct bury multiplex cable (USE-2 or MHF) for this job, instead....
    – ThreePhaseEel
    Commented Aug 16, 2022 at 5:18
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    Another question: is the existing 20A circuit to the garage run using direct bury cable, or individual wires in a conduit?
    – ThreePhaseEel
    Commented Aug 16, 2022 at 5:19
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    Agreed that AL is much more cost effective than copper, it works fine as long as you put the "goop" (NoAlox) compound on it. I also agree that putting it in conduit is a good idea, but have you priced conduit lately!? It's outrageous. A 10' stick of 1 1/2 is $27 That's almost $200 just for the conduit. Not sure of the size needed for the OP's project, maybe they could go down to 1 1/4". Get this 3" conduit is $86 for a 10' stick.
    – George Anderson
    Commented Aug 16, 2022 at 9:31
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    @GeorgeAnderson is NoAlox necessary for feeder cable when landing AL wire on AL lugs in a panel?
    – FreeMan
    Commented Aug 16, 2022 at 12:45

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To actually answer your question (lots of comments about the wire, but none about your question): I'm pretty sure a ground rod is a ground rod, is a ground rod. In just about all cases, for an outbuilding, you need 2 ground rods at least 8' long spaced at least 6' apart (further is better). There is a ton of info here on SE regarding sub-panels. Do a quick search and you'll find everything you need to know.

In doing some research, I ran across this article, which may help you: https://galvanelectrical.com/wp-content/uploads/2019/03/groundrodmarkingcompliance.pdf So I'm not entirely correct in saying there's no difference. Once again, as we say here often, the AHJ is the final say for where you live.

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