Detached garage subpanel w/ 3 wire feed and bonded neutrals/grounds

n3ntj

Member
I have a question about a subpanel setup that I don't see often. Home had a 200 Amp service with a detached garage approx. 100' away. There was no plumbing or other pipes or wiring b/w the home and the garage. The line to the detached garage is in plastic conduit.
The garage was built in 2010. The garage's subpanel was fed with 3 conductors (100 Amp) and had a separate ground rod driven just outside the subpanel's location. The neutral and grounding conductors were connected together at the terminal bars in the subpanel (the neutrals were not isolated).

I see one issue being that the detached garage's subpanel wasn't fed with 4 conductors (2 hots, neutral, and ground) but since there is only 1 non-hot conductor back to the main panel (and not a neutral AND ground), isn't the issue of objectionable currents a moot point (since there's only one non-hot path back to the main panel)?

Should the neutrals and grounding conductors still be isolated in this detached garage subpanel? If that's the case, there would be no grounding of this subpanel to the home's service... only the grounding conductors and panel enclosure connected to the ground rod.
 

suemarkp

Senior Member
What code cycle is that area on and do they have any local amendments regarding this issue? The 3 wire feeder to a detached structure was allowed until the 2008 code. As long as there were no grounded metal paths between the Service building and the detached building, it was OK.

If this was done under unmodified 2008 code, then it is not compliant. The reason they want 4 wires doesn't matter, it is what the code requires. But you could ask what is the hazard if this building had been built a few years earlier.

You ask "Should the neutrals and grounding conductors still be isolated in this detached garage subpanel?" Earlier, you said the neutral and ground were connected at terminal bars in the panel. That is how this needs to be with a 3-wire feeder -- neutral/ground is one common bar. It attaches to the incoming grounded conductor, the local ground electrode conductor, and all down stream grounded and grounding conductors. Never separate the neutral and ground bars unless you have a wire in the feeder for each bar.
 

n3ntj

Member
So, you're saying that my example may be perfectly OK? (depending upon what NEC was in force when the detached garage was built)? In other words, if the detached garage was built in 2005, my example would be OK? (bonded neutrals and grounding conductors are OK since there's only 2 hots and a neutral back to the main panel???)

If the detached garage was built in 2013, the subpanel would have been required to have (1.) a 4 wire feeder, (2.) neutrals and grounding conductors would need to be isolated, and (3.) a separate ground rod would be required?
 

GoldDigger

Moderator
Staff member
So, you're saying that my example may be perfectly OK? (depending upon what NEC was in force when the detached garage was built)? In other words, if the detached garage was built in 2005, my example would be OK? (bonded neutrals and grounding conductors are OK since there's only 2 hots and a neutral back to the main panel???)

If the detached garage was built in 2013, the subpanel would have been required to have (1.) a 4 wire feeder, (2.) neutrals and grounding conductors would need to be isolated, and (3.) a separate ground rod would be required?
A separate ground rod would only help for lightning protection, possibly not required by NEC? But if used would still have to be bonded back to the rest of the system via the EGC/GEC.
 

n3ntj

Member
I am getting confused now. In my example above, the grounding and neutral conductors can be bonded together in the garage subpanel since it is fed with 3 conductor feeder or it may not be?
 

jumper

Senior Member
I am getting confused now. In my example above, the grounding and neutral conductors can be bonded together in the garage subpanel since it is fed with 3 conductor feeder or it may not be?
If your 3W feed is legal, than grounds, neutrals, and ground wires to the rods will be all tied together in a common buss arrangement.

A 4W feed would separate and isolate the neutrals.
 

jumper

Senior Member
Section please?
Which part? Required GES or most likely 2 rods?

250.32 Buildings or Structures Supplied by a Feeder(s)
or Branch Circuit(s).
(A) Grounding Electrode. Building(s) or structure(s) supplied
by feeder(s) or branch circuit(s) shall have a grounding
electrode or grounding electrode system installed in
accordance with Part III of Article 250. The grounding
electrode conductor(s) shall be connected in accordance
with 250.32(B) or (C). Where there is no existing grounding
electrode, the grounding electrode(s) required in 250.50
shall be installed.

250.53 Grounding Electrode System Installation.
(A)(2) Supplemental Electrode Required. A single rod, pipe,
or plate electrode shall be supplemented by an additional
electrode of a type specified in 250.52(A)(2) through
(A)(8). The supplemental electrode shall be permitted to be
bonded to one of the following:
(1) Rod, pipe, or plate electrode
(2) Grounding electrode conductor
(3) Grounded service-entrance conductor
(4) Nonflexible grounded service raceway
(5) Any grounded service enclosure
Exception: If a single rod, pipe, or plate grounding electrode
has a resistance to earth of 25 ohms or less, the
supplemental electrode shall not be required.
 

n3ntj

Member
If it's fed with 3 wire feeder, than the neutrals and grounding conductors can be bonded together in a detached garage subpanel? I guess that would make sense since there'd only be one non-hot conductor path back to the main panel (the neutral).
 
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infinity

Moderator
Staff member
Prior to the 2008 NEC a 3 wire feeder was permitted, and the GES requirement (2 grounds rods, or 1 @ 25 ohms or less) was still applicable. Fast forward to 2008 then a 4 wire feeder and GES would be required.
 

ceb58

Senior Member
I am getting confused now. In my example above, the grounding and neutral conductors can be bonded together in the garage subpanel since it is fed with 3 conductor feeder or it may not be?
If the garage was built under the 05 code or before then the 3 wire with a ground rod was OK. Neutral and grounding electrode conductor bonded in the panel ( as long as there were no other metallic items). Now if it were built under the 08 or 11 code then the install would be a violation. It should have been 4 wire ( 2 hot 1 neutral and 1 EGC) ran to the panel with the neutral and EGC separated and isolated from each other.
 
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