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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.

NEC 250.32 enter image description here

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)?

  • What size electrical service is being supplied to the garage? What size is the grounding conductor that's buried with the gas line? – Tester101 Aug 15 '16 at 20:39
  • The garage service is 100A Gas line conductor is probably 12 or 14 GA - will have to take a look when I get home – CoAstroGeek Aug 15 '16 at 21:15
  • 12 AWG is too small, you'd need at least 8 AWG copper to use it as the grounding conductor for the feeder. I think you're going to have to find a way to run an 8 AWG grounding conductor to the garage. – Tester101 Aug 15 '16 at 22:18
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Ok, just got off the phone with a guy from our regional building department. The conductor that is buried with the gas line is NOT meant to electrically connect the metal pipe in the garage with the main service line (& therefore house, which has metal pipe from meter). The conductor is a tracer and is only there to assist in locating the gas line. It should not be connected to the pipe at either end.

So with a plastic pipe on the gas, there is no alternate conductive path back to the house, so I'm good on Requirement 2. I just bond the neutral & ground at the sub-panel and put in two grounding rods (which should have been there for the old system as well).

  • I thought it was strange that the gas pipe was bonded, but figured maybe it was a local amendment. Glad it worked out favorably. – Tester101 Aug 16 '16 at 2:01

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