The second option is given in 690.47(C)(2) and includes a new dedicated GEC that is run from the inverter and is bonded to the existing AC grounding electrode system. For this option, there is no dedicated DC grounding electrode—the installation only has the AC grounding electrode. The size of the new DC GEC that is bonded to the existing AC grounding electrode is based on 250.166, which permits the DC GEC to be bonded to the AC GEC but only if the AC grounding electrode is not accessible. In both 690.47(C)(1) and (2), the new DC GEC does not replace the required AC EGC that bonds the metallic equipment together.
For options one and two, the DC GEC is sized as specified in 250.166(A) and (B), except as allowed per (C) through (E). Section 250.166(B) establishes that the GEC shall not be smaller than the largest conductor supplied by the system and not smaller than 8 AWG copper. For the “largest conductor supplied by the system,” you should consider the conductors within the PV system—you do not need to consider the existing AC service conductors to determine the new GEC size. This section is focused on the DC system and the size of the conductors used to wire the array to the inverter. Sections (C) though (E) list the types of grounding electrodes that can be connected to the new GEC and indicate the maximum size of the DC GEC: 6 AWG for connections to rod, pipe, or plate electrodes; 4 AWG for connections to concrete-encased electrodes. For a connection to a ground ring, the DC GEC shall not be required to be larger than the conductor used to make the ground ring.
The third option and the most used method for establishing the DC GEC—a combined DC GEC and AC EGC—is covered in Section 690.47(C)(3). It allows a single conductor to serve the function of both conductors, as long as this one conductor is sized to meet the more stringent requirement, i.e. that conductor must be the larger of either the DC GEC, as described in 250.166, or the AC EGC, as specified in 250.122. Once this size has been established, the conductor must be unspliced or irreversibly spliced and run from the inverter’s marked grounding location to the grounding busbar in the associated AC equipment. This means running an 8 AWG (minimum) copper conductor with the AC conductors from the inverter and though any additional equipment, such as meters and disconnects, into the utility point of connection. This method is popular due to the relative ease of running the conductors, although the requirement for an unspliced or irreversibly spliced conductor can complicate the installation.
Meeting the requirements for DC system grounding and the grounding electrode system can be difficult and time-consuming, not only during installation but during the system design. This is likely the most commonly discussed subject for installers and inspectors alike. Take time to clearly document your system with electrical drawings that methodically reference the various NEC articles one by one. Of course, if you have any questions, consult your local authority having jurisdiction. They will undoubtedly have some important opinions on the matter.
Ryan Mayfield is the principal at a renewable energy design, consulting, and educational firm in Corvallis, Oregon. He trains PV installers and code officials and always finds the grounding and bonding portions of class the most entertaining and educational.