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:
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.