I have a main Square D panel with a post-installed Generac (Siemens) subpanel with automatic transfer switch to standby generator. I want to add a new 15A circuit to the subpanel so I can move a second refrigerator to a different room (no change in generator load, just shifting some to the new circuit). The subpanel has no ground bar. All the existing circuits in the subpanel are grounded through the main panel. How do I ground the new circuit properly?
Added 6/23/2020: All circuits in the transfer switch subpanel were previously in the main and "moved" to the subpanel. To be clear, I want to add a brand new circuit in the subpanel so the standby generator could power it as necessary. I would replace a single 15A breaker with a tandem 15A breaker to accommodate the new circuit. An example of a tandem breaker is in the lower left of the subpanel.
NOTE: I do not see any available knockouts in the subpanel nor do I see where a ground bus would be installed (e.g. predrilled mounting holes). So is this panel even designed for what I want to do? If not, it seems shortsighted on Generac's part.
I am uploading additional pictures as requested.
The aluminum cable is the ground for the generator. The is connection is highlighted. The green cable feeds back to the ground bus in the main. This connection appears to be mounted directly on the subpanel box.
Overview with subpanel on left transfer switch on top. The main is on right, including conduit connection
Subpanel detail showing nuetral bus
Subpanel detail showing where I would think ground bus would normally be installed
Added 06/24/2020: Subpanel labels