…assuming that it does not have a separate service.
Maybe on a two-wire system with plastic conduit?
I guess another way of asking is: under what conditions would you want to see a panel for a separate structure (supplied by the house service) have its own GEC and electrode?
It looks to me:
If plastic conduit is used between the house and the separate structure:
If the panel in the separate structure has only one branch circuit, with no overcorrect protection, then that panel is treated as a typical distribution panel, the grounding an neutral bus bars are isolated from each other, and no GEC and electrode are needed.
If the panel in the separate structure has 2- 120V supply bus bars (multiple circuits), supplied by a 4-wire feeder (1 blk, 1 red, 1white, and one green), but has overcorrect protection installed then that panel is still treated as a distribution panel with the grounding and neutral bus bars isolated from each other, but the grounding bus bar should be connected to a GEC and electrode serving the separate panel.
The images are from the 2014 NEC NFPA Handbook. I think I’be interpreting this correctly, although I might not be describing it using accurate/the best terms.
A feeder feeding a panel would require the separate structure to have a GES (grounding electrode system). A branch circuit (single or multi-wire) would not require the GES. Determine if you have a feeder or not and go from there.
I guess the main question is under what circumstances would you want to see a GES installed at a separate structure without it’s own service, as in one of the illustrations shown above.
You’re missing the distinction of what is feeding the garage, if it’s a feeder then it requires a GES if it’s a branch circuit then it does not. The feeder would have additional OCPD’s at the garage (usually a panel) a branch circuit would not.
A branch circuit that is simply extended to supply a separate structure with Power needs no GES.
A panel in a separate structure that contains multiple branch circuits protected by ocpd’s should have its own GES. I really don’t understand. It’s just another distribution panel. The ones in the house don’t each have their own GES. Why would one in a separate structure need one?
1- Yes that’s correct, even an individual branch circuit to a separate structure would not trigger the requirement to install a GES.
2- Good question and I’m not sure why other than to say that the NEC wants ALL separate structures to have their own GES with an exception for those fed with a single or multi-wire branch circuit.
Mike Holt’s notes in the link in post #16 would indicate that is true:
Still doesn’t explain why if the separate structure has a one, two, or three circuit MWBC feeding it that it would be less susceptible to the aforementioned induced voltage from nearby lighting strikes.