Correct grounding of a sub panel.

[fredhargis]

I'll be having (or doing myself) a subpanel installed in my new shop. This will be a 100 amp panel off the main, and the building is 100 feet from the house. Now, as silly and simple as this question is, I ask because I'm out in the township where few rules apply to those who are "electricians". I'm certain that if I have it done, there will be 2 hots and a neutral ran to the shop (that's the way my last one was wired but the PO) with no ground. If I protest, they will probably hammer a grounding rod in and use that. So, my question: according to NEC, what is the proper ground in this case; back to the main panel?

[blackhat]

Yes, the sub must have a properly sized ground back to the main and the bond between neutral and ground lugs removed.

[Rick_B]

Ijust had a 100 amp sub panel  installed - it was only 1 foot from the main.  after a lot of reading/questions I came to the same conclusion that blackhat posted.  I had an electrician do the work - he used 4 wires - two hots from the breaker in the main panel, a neautral and a ground.  The sub panel had a neutral bar and a separate ground bar.  the neutral was not bonded to the panel enclosure and the ground was bonded tpo the enclosure.  From what I discovered the wire size typically recommended was 3-3-3-5.  I'm not sure if that would be correct 100 feet from the main?

Rick

[Roly]

Like most code questions there are some if's and exceptions.       If there are no other conductive paths between the buildings such as a water line, phone wire alarm systems etc  then it can be fed 3 wire to the sub panel in garage along with a ground rod and the neutral bonded in the garage.  HOWEVER it is not recommended because if you ever add a conductive path it will add a parallel path for neutral current.
  Suggest doing as blackhat said running 4 wires without rebonding the neutral at the garage but it also will require a ground rod at the garage connected to the ground bus only which also has the ground from the house on it.
 The separate buildings require grounds at their locations,  not required if in same building.    Roly

[fredhargis]

Thanks, guys. Grounded back to the main panel it is, whether I do it or have it done.

[kencombs]

Yep, 4 wire to the main panel, ground and neutral isolated at the new panel, and a ground rod in addition to the ground to the hose panel.

The rod doesn't provide a fault clearing path in the event of a short to ground, that's the job of the 4th conductor, the rod is for lightning protection.

[TDKPE]

Like most code questions there are some if's and exceptions.       If there are no other conductive paths between the buildings such as a water line, phone wire alarm systems etc  then it can be fed 3 wire to the sub panel in garage along with a ground rod and the neutral bonded in the garage.  HOWEVER it is not recommended because if you ever add a conductive path it will add a parallel path for neutral current.

3-wire feeders to an outbuilding are no longer permitted, as of (I believe) the 1996 code cycle.  4-wire, plus grounding electrode system, which usually is two ground rods.  Neutral isolated from the equipment ground, all the way back to the service equipment (usually that's the main panel, but not always).

As the poster above stated, the grounding electrode system is NOT there to clear ground faults - you could wire a 15A circuit directly to a ground rod or rods, and it will only pass a few amps, not remotely close to what's needed to operate the breaker to clear the fault.

[meackerman]

When I built my shop, back in 2013, I already has the wiring from the house to it from the PO....3 wires not 4. Inspector let us use it because it was existing, but we had to drive two ground rods.