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Based on this question on EGCs in conduit (in the context of branch circuits) and this question on conduit as a sole ground source, I would like to know whether two subpanels, located ~100ft from the main breaker panel, can be grounded to the main panel through the buried EMT connecting them.

This is how my system (kinda) looks:

Depiction of Main Panel and (One) Subpanel

I have two 100A subpanels fed from one 100A main breaker. Eventually, I plan to upgrade to 200A service and each subpanel will have its own 100A breaker (still in the main breaker panel).

This will mean two sets of hots and neutrals (four ungrounded conductors and two grounded conductors) with each panel receiving one set. I am trying to determine if I need to run a separate EGC for each subpanel (ex. two 8AWG Cu wires), a single EGC to both subpanels (ex. one 8AWG Cu wire, as intimated by the first linked question), or if the EMT connecting the main panel to the subpanels serves as an NEC-approved ground.

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    The additional cost for raintigh couplers ( and enamel coated EMT if required) make PVC with a ground wire way less expensive in some areas. – Ed Beal Mar 13 '17 at 22:26
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    Are you sure it's EMT you buried? I'm not sure EMT is a valid grounding path outdoors. Also, are the two subpanels in the same occupancy or different rentable housing units? There may be an issue running 2 identical circuits alongside each other. – Harper Mar 13 '17 at 22:28
  • So the fun bit is that the EMT is not yet buried, but needs to be eventually to meet local code. That said, it is EMT (stamped right on the side) with raintight couplers already installed. The segments that do contact the ground right now are PVC. For some reason, the guy who installed it only used EMT where it didn't contact the ground... – Hari Ganti Mar 14 '17 at 4:22
  • @Harper Same occupancy unit. I believe, actually, we've discussed this in a few other questions of mine. Aside from accounting for derating (which forces 2AWG over 3AWG), what other problems do you suspect? – Hari Ganti Mar 14 '17 at 4:24
  • There's a rule against parallel branch circuits going from the same panel to the same destination. There are a number of exceptipns (switched, different voltages). Not sure if this is applicable to feeders to subpanels. – Harper Mar 14 '17 at 8:11
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After calling my city's building inspectors, I was told the buried EMT can serve as the grounding conductor between a main breaker panel and a subpanel if the conduit also uses grounding strap fittings to bond the conduit directly to a busbar. The panel knockout to conduit/locknut physical contact is not sufficient.

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