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I have a new 24'-24' wood structure detached garage on a concrete slab. I installed a 30 amp sub panel with 4 - #10 gauge THHN wire through conduit. Two hots, a neutral and ground going back to the Main panel. The neutral and ground are separated at the sub panel, do I need a grounding rod even though I am grounding back at the Main panel? If so am I supposed to attach the ground copper wire to the very small ground buss bar inside that sub-panel, and what size wire?

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...And you didn't put a concrete-encased conductor (UFER ground) in the slab? Sigh. That's the easy way out and the most effective ground you can make in most places.

Yes, you need a grounding electrode (ground rod) local to the detached building. Connecting to reinforcing steel in the slab would have provided a very good grounding electrode, but if it's already poured, that ship has sailed.

6 AWG solid is the default minimum size for a connector to a grounding electrode. 8 AWG might be acceptable given that the feed size is 10 AWG, but I'm having trouble tracking down the exact language where that is mentioned. It's one of the messier (or more confusing) sections of the code (IMHO.)

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    @Mike -- it indeed attaches directly to the ground busbar, but it does not need to be in conduit unless its subject to physical damage – ThreePhaseEel Oct 1 '17 at 22:51
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    Does the rod need to be 8' down in the ground . – Mike Oct 1 '17 at 22:55
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    I am correct in the fact that the neutral and ground buss bars are not to be connected in a Sub panel? – Mike Oct 1 '17 at 23:01
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    Yes, ground and neutral are isolated in sub panels, and the grounding electrode connects to the ground buss. – Ecnerwal Oct 1 '17 at 23:42
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    The reason most electricians use #6, even with a 100 amp service, is #6 AWG does not require protection from damage if it is attached to framing members of the building. 2014 NEC 250.64(B) Otherwise you can use #8 up to 100 amps if you want to protect it with a raceway. The remote building is treated as a service from Table 250.66 – ArchonOSX Oct 1 '17 at 23:56

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