Under what conditions would a separate structure have it's own grounding electrode...

…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:

  1. 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.

  2. 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.

NEC 250.32

I do not think that is allowed in Canada ,

First you must determine what is feeding the structure . A single circuit or a MWBC would not need a grounding system .

What is an MWBC? Multi wire branch circuit?

What would? (see image)

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.

This seems to conflict with post #3. I don’t understand.

No it actually expands on what Jim said. He stated:

A single circuit or a MWBC would a branch circuit as I stated so no GES required.

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.

What else could possibly feed a structure except for a single circuit or a MWBC?

The illustration I posted with the GES appears to me to show a MWBC providing power to a 120/240V panel. No?

All panels in all detached structures require a GES. Any feeder besides a single circuit, with an MWBC counting as a single circuit, require a panel.

A feeder for a panel.

A 14-2 cable to a garage for a receptacle circuit doesn’t need a panel or a GES.

A 14-3 cable to a garage for a receptacle circuit and a lighting circuit doesn’t need a panel or a GES.

A 10-3 cable to a garage for receptacles, lights, and an electric heat circuit needs a panel and a GES.

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.

This may help from Article 100 of the NEC:


  1. A branch circuit that is simply extended to supply a separate structure with Power needs no GES.
  2. 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.

Does this clarify a bit? http://www.ecmweb.com/qampa/code-qa-grounding-and-bonding-remote-building

I think once you process the exemptions and understand how they play then the whole picture is clear. At least that’s how I look at things like this.

I would suspect it might have something to do with the possibility of a lightning event, but have no hard data.

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.