Here's the situation. I've added on to my detached shop/garage and need to upgrade the very small subpanel. The existing garage pre-dates the 2008 NEC by at least a decade, and is supplied by a 3 conductor (2 hot, 1 neutral) service. The old panel didn't even have separate ground and neutral bars.
I've found several threads that address this (Can I install a Subpanel..., How can I safely connect a subpanel..., How to properly ground a subpanel...), and the consensus seems to be that NEC 250.32B Exception 1 & 2 would apply and I should bond the ground & neutral at the subpanel in the new panel and install ground rods at the detached garage.
HOWEVER - I have also added gas service to the garage. Does this mean that I don't meet requirement 2 of Exception 1? The gas line is bonded to the equipment ground in the house and garage as required by NEC 250.104B and NFGC 7.13.1, The gas service to the garage was installed by a contractor using a plastic pipe, but I believe a conductor is buried with it to provide electrical continuity between the garage service and the main line.
So, if I don't meet requirement 2 of Exception 1, does that mean that I need to install a ground conductor from the subpanel back to the main panel. This would be a substantial PITA.
If I do have to install this ground conductor, how deep would it have to be buried? Same as the requirement for direct bury cable (24" I think)?