I've previously mentioned in other questions my plans to upgrade two old and insufficient subpanels which serve my garage and its upstairs apartment. Note that the garage is attached to the house; it was an addition (in the mid '60s) but it is not a separate structure. The two panels are fed by a common feeder which is 3 x #4 copper in 1" EMT; the apartment panel is downstream of the garage panel and fed via a breaker in it. There is not a separate ground wire in the feeder although the EMT provides some ground continuity. In addition, since my service entrance panel is more than maxed out (the previous homeowner double-wired some circuits as the neutral bar was filled up, I'm planning to install a smaller sub-panel at the back of the house in an office/bath area I'm renovating and offload some circuits from the main panel. (See photo attached). The feed I'm planning for this new subpanel is 3 x #4 plus a #6 ground in 1" PVC, but I haven't cut any holes or bought any materials except for the panels themselves so far so I'm able to modify my plans if desirable.
I'm asking for recommendations for wiring and upgrading the grounds at all four of these panels. Currently the service entrance panel has a single ground rod driven I-don't-have-any-clue how long ago, and it looks badly corroded. At the back of the house, about 12 feet from where I'm putting in the new subpanel, there's a 1/2" ground rod which I drove myself a few years back when I found that an outlet where I wanted to place a computer did not have a functioning ground. The corner of the garage where the two old subpanels are has no ground and has a concrete walkway all around, but that's not a showstopper as I have a hammer drill. The two subpanels are within three feet of each other, so they ought to be able to share a ground connection. I'm not planning to upgrade the feeder in 1" EMT at this time as my eventual plan is to have a new, larger service entrance installed behind the home.
As the old saying goes, "I'd rather do it right than do it over." I definitely want a new ground rod for the garage panels, and I'm thinking that this is an opportune time to drive one or two new ground rods for the existing service entrance panel to replace the one which is corroded. For the new panel at the back of the house I could either drive a new ground rod, run wire about 20 feet around the perimeter of the room to the 1/2" rod I drove a few years back, or simply ground it back to the existing service entrance with the ground wire in conduit.
Since some of you will be curious, here are the projected loads for the various panels:
Garage Panel: Lighting (fluorescent, 8 single-tube fixtures), convenience outlets for power tools. Planning to add a 50A circuit for a welding machine (infrequent use) and to offload the laundry equipment (gas dryer [with provisions for electric], washing machine) onto this panel once it's changed out. Once service entrance is upgraded may add some more heavy-duty power tools on this panel, possibly a feed to a future swimming pool.
Apartment Panel: Small appliance & lighting circuits (gas range); 220V window AC/heat pump (20A circuit, downrated from original 30A as new A/C was more efficient). Eventually want to upgrade appliances to modern standards (dishwasher, disposal, installed microwave, etc.) but this can wait until I upgrade the service entrance.
Office Panel: Will be rewiring the back corner of the house which is the last vestige of fabric-covered ungrounded Romex which I'm aware of. Two circuits for computers and lighting, two circuits for a network closet and central sound system, offload HVAC (gas furnace, outdoor compressor, with provisions for eventual installation of ground-source heat pump and electronic air cleaner). Bathroom circuit. AFCI/GFCI protection as appropriate.