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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?

Main Panel

Interior sub panel next to main panel

Exterior sub panel for pool heater

Exterior sub panel from original construction

Exterior sub panel for pool equipment and outlet

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    Normally grounds run with the conductors, so feeding a subpanel from the parent subpanel is fine. All subpanels need 4-wire feeds unless they are grandfathered, in which case it's a good idea to retrofit, and the retrofit rules are looser.
    – Harper - Reinstate Monica
    Jun 12, 2022 at 21:04
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    Can you post photos of the insides of the various subpanels that clearly show where their feeds enter them from?
    – ThreePhaseEel
    Jun 13, 2022 at 1:00

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The requirements for 2 rods is a newer one.

a grounding electrode can even be a metal water pipe as long it is in contact with earth for 10’, used to not require more than 1.

Today it is code compliant to use 1 ground rod on new services if the grounding has an earth resistance of 25 ohms or less this measurement is made with a special meter (not a common ohm meter). If you do not have access to a earth ground meter just install 2 rods 6 or more feet apart further is better. But there is no requirement to update an existing system.

If you add a ground to a sub panel then the neutral will need to be isolated from the ground at the sub panel this and the 4 wire for sub panels have been code since 99. The only place neutral and ground are connected or bonded is in the main panel.

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