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.