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I am not an electrician, so let's get that out of the way. However, in the past I have installed simple subpanels in detached buildings and had my electrical work inspected and passed first time, so I feel comfortable doing some basic, standard wiring.

Now the story, in 2014 my wife and I purchased and began renovating an older house. The house was built in the 30's and renovated the first time in the 80's and at the time upgraded with romex wiring. The house main service was a 200A fuse panel but we asked the sellers to have an electrician replace that with a new breaker panel, which they did and it was inspected. The house has a detached garage that was built in the 80's and it has a 100A subpanel installed in it, with just a few circuits for receptacles and lights.

I recently installed a shed out behind the garage and now to it I want to run a subpanel off of the garage panel. I plan to get the permits and have my work inspected. This weekend I did a review of my environment and found something I'm not sure about. When I pulled the cover off the subpanel in the garage I found that it's only fed with three wires, H-H-N and the ground and neutral bars are bonded. I was not expecting this.

I have not called my local officials yet (Virginia) because right now I've done no work or pulled any permit, I'm just feeling out my options. The shed is approx 70' away from the garage. A run back to the main house panel is impractical (paved driveway, concrete sidewalks) and even if I cut through those, would make it about a 175' run. I was planning to run 3 - #6 THHN and 1 - #8 THHN (ground) in conduit to the shed and put in two ground rods, and separate the neutral and ground in the shed sub. However, I'm concerned what an inspector may say about this setup seeing that I'd be feeding this subpanel from a subpanel fed with a 3-wire feeder configuration. My other option is just to run separate underground circuits for lights and receptacles to the shed and not do a sub, but that's not what I'd prefer to do.

I do believe the detached garage was built with older code that did not require 4 wires and grounding electrodes. But now, does that hurt me for pulling a subpanel off of that? Pending differences in local code, is this possible or do I need to make changes to the garage subpanel before I can do this?

Added to answer the question below: that is a good question, I did not crawl under the other end of the house to see if I had access to the point where it enters the conduit. I suppose you're thinking that if I did, I might be able to fish in a ground wire back to the main? However, I also posted a response below that I got from the county inspector today, which pending how difficult driving ground rods might get, sounds like a much more feasible solution. I was planning to rent a breaker hammer from HD to drive in the other rods anyways, adding two more while I have the rental doesn't sound terribly difficult.

  • Are you entirely sure that the house to garage connection isn't made with all-metal conduit? It could be that the connection is H-H-N-Conduit and they simply failed to remove the N-G bond. I have a whole factory of H-N-Conduit, and it's all state-of-the-art, not a green/bare wire to be found. – Harper Aug 13 '18 at 14:26
  • I am entirely sure there is no conduit. I can see it running under the crawl space in the house before it goes outside and under a sidewalk/patio. – Tom Glover Aug 13 '18 at 15:24
  • And I should also add that at some point it goes thru PVC conduit, that's what's coming into the garage cutoff. – Tom Glover Aug 13 '18 at 15:40
  • @TomGlover -- does the cable enter a conduit at the point it exits the crawlspace? – ThreePhaseEel Aug 13 '18 at 22:47
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    Hi, Tom. It looks like you've created a second "Tom Glover" account. You should follow the procedure to merge them. – Daniel Griscom Aug 14 '18 at 1:37
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I just looked and VA is on the 2011 NEC , I thought the change to 4 wire was back in the 99 code. So I would check with your local inspector to find out what the local requirements are (I would want 4 wire and ground rod). Very strange that a 2014 sub would be set up this way unless allowed by VA code modifications.

Added after op coment: you will need to update the garage to the 4 wire with ground rod to meet today's code if you want to pull a sub from the garage. I have used pipe and water hose to tunnel under driveways and sidewalks many times in the past so you don't have to cut them depending on your soil conditions its a bit messy but cheaper than cutting in most cases.

  • I'm sorry I don't think I was clear on that. We bought the house in 2014. I am assuming the subpanel was installed in the garage when it was built in the 80's. I do not know if that subpanel was inspected, I don't see a sticker anywhere on it. – Tom Glover Aug 13 '18 at 15:23
  • In the 80's this was a legal install, as I said above I think it was the 99 code cycle that required 4 wire. The safest solution would be to add a ground and isolate the neutral from ground in the garage. If In conduit pulling a single ground would be possible, if direct berial it would be tough and you may just want to pull the service to the new shed from the house. – Ed Beal Aug 13 '18 at 16:21
  • I would run separate underground circuits for receptacles and lights before I ran back to the house main service. It's hard to explain, but getting from the house to the shed is just really, really hard in my case. I emailed our local county building inspection office, I will see if they respond and post their answer if they do. – Tom Glover Aug 13 '18 at 18:19
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OK got this straight from the county inspector. Since the garage subpanel was installed with a 3-wire feed and no ground, the neutral and ground bars should be bonded, and they are. He said what I could do is put in one, preferably two, ground rods at the garage and run a #6 solid copper conductor to the bonded neutral bar. Then I could pull a normal 4-wire connection to the subpanel in the shed, where it has its own ground rods and unbonded ground/neutral. So that's a much easier solution for me to accomplish - which I should probably reserve judgement until I see how hard those ground rods go in.

  • You'll find an interesting article that discusses this situation at: selfhelpandmore.com/home-wiring-usa/… and a similar discussion to on the same question at: diychatroom.com/f18/detached-garage-sub-panel-grounding-q-14945 – Nicknamednick Aug 14 '18 at 2:20
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    Keep in mind that ground rods and ground wires do two very different things... – ThreePhaseEel Aug 14 '18 at 2:27
  • I do understand that, I was just restating what the inspector told me. Are you suggesting I should maybe do something different? – Tom Glover Aug 14 '18 at 11:59
  • @TomGlover -- I'm saying that you probably shouldn't treat what the inspector is saying here as gospel, and figure out what's actually going on with how that buried wiring is run and post it back to us, so we have an idea as to how practical adding a ground wire to the run is. – ThreePhaseEel Aug 16 '18 at 3:33

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