Do I need a ground rod for a detached garage panel?

Section 250.32(A) states that building or structure supplied by a feeder or branch circuit shall have a grounding electrode or grounding electrode system installed in accordance with Part III of Article 250. So yes, a detached garage with a subpanel needs its own electrode. The same rule applies to a shed, a barn, a pump house, and an outdoor hot tub panel mounted on a post.

There is one exception. Section 250.32(A) Exception says a grounding electrode is not required where only a single branch circuit, including a multiwire branch circuit, supplies the building or structure and the branch circuit includes an equipment grounding conductor. A single 20 ampere circuit to a shed for a light and a receptacle does not need a rod. A subpanel does.

Why two rods

Section 250.53(A)(2) requires a single rod, pipe, or plate electrode to be supplemented by an additional electrode unless the single rod has a resistance to earth of 25 ohms or less. Almost nobody tests, so almost everybody drives two. Section 250.53(A)(3) requires rods to be spaced not less than 6 feet apart, and the informational note recommends spacing equal to the rod length for better performance.

Rods must be at least 8 feet long per 250.52(A)(5), and 250.53(G) requires the upper end to be flush with or below grade unless the rod and its connection are protected. Section 250.66(A) says where the sole connection is to a rod, pipe, or plate electrode, the grounding electrode conductor does not need to be larger than 6 AWG copper.

If the garage slab has at least 20 feet of 1/2 inch or larger rebar or 4 AWG bare copper embedded in the footing, that concrete encased electrode under 250.52(A)(3) must be used where present and accessible during construction, and 250.66(B) limits the conductor to 4 AWG copper when it is the sole connection.

Four wire feeder, no bonding jumper

Section 250.32(B)(1) requires the equipment grounding conductor to be run with the supply conductors to the separate building and connected to the disconnecting means and to the grounding electrode. The grounded neutral conductor is not permitted to serve as the equipment grounding conductor at the separate structure in current Code. That means four wires to the garage: two hots, a neutral, and an equipment grounding conductor.

At the garage panel, keep the neutral bar isolated from the enclosure. Remove or leave out the main bonding jumper screw. Land the equipment grounding conductors and the grounding electrode conductor on a grounding bar bonded to the can. The old three wire feeder allowance was removed and is no longer permitted for new work.

Disconnect at the structure

Section 225.31 requires a disconnecting means for all ungrounded conductors supplying or passing through the building. Section 225.32 requires it to be installed at a readily accessible location either outside the building or inside nearest the point of entrance of the conductors. A main breaker subpanel satisfies both.

Field takeaway

Subpanel in a detached garage means four wire feeder, isolated neutral, bonded ground bar, a disconnect at the structure, and two 8 foot ground rods 6 feet apart tied together with 6 AWG copper. Use the concrete encased electrode instead if the footing rebar is available.