I have a 200A main panel installed in our garage on an exterior block wall (single family, residential, construction in 1980). A sub-panel was added next to the main panel supply via a 100A breaker (covers both columns of breakers).
There are three sub-panels on the exterior side of the same wall (two are essentially disconnects via a breaker). Each sub-panel is protected by a breaker at the origin of its feed.
There is only 1 ground rod, and the 3 external subpanels are 7 feet, 10 feet, and 22 feet respectively from the ground rod location.
I have read the several questions and their replies that are similar but not identical. From those it seems I should have at least 2 ground rods but have no requirement for ground rods for each sub-panel.
One external sub-panel is fed from the sub-panel inside, i.e., daisy chained (main to sub to sub sub-panel). Several responses have indicated a ground needs to go back to the main panel from each sub-panel.
Is this requirement satisfied if the sub sub-panel has a ground to the sub-panel which then has a ground to the main panel?
Given the proximity of the external sub-panels would it be inappropriate to also connect each to the nearby ground rod?
Lastly how is the resistance of the ground rod (to be <25 ohms) measured with an ohm meter? I.e., one probe on the rod and the other is placed where?
With 2 ground rods is the requirement meant to be two separate ground wires, or one to each rod, or simply attach the ground wire to one rod then tie each ground rod together with an appropriate gauge wire?