The NFPA and IEEE recommend a ground resistance value of 5 ohms or less while the NEC has stated to “Make sure that system impedance to ground is less than 5 ohms specified in NEC 50.56. In facilities with sensitive equipment it should be 5ohms or less”.
[pdf] The recommended approach is to use a separate DC grounding electrode for PV arrays and frames, as this enhances protection against lightning and transient voltage. For lightning protection associated with grounding systems, refer to NFPA 780 and NEC 250.106.
[pdf] For standard PV power stations, grounding resistance should be below 4 ohms; for large-scale PV power stations or areas with frequent lightning, it should be below 1 ohm. In high-resistivity soil, flexible grounding, additional grounding electrodes, or deep-well grounding can help reduce resistance.
[pdf] Install copper-clad ground rods at least 8 feet deep and connect them to your solar array frames, inverter, and battery bank using 6 AWG copper wire. Grounding off-grid solar systems also requires bonding all metal components together, including module frames, mounting racks, and combiner boxes.
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