In "PV Grounding and Bonding; Part 2", Ryan Mayfield discusses equipment-grounding conductors (EGCs) required in PV systems. Here, we show two PV system examples—a batteryless grid-tied and an off-grid system—for determining the wire gauge required for the EGCs of the DC circuits.
Consider a 4.8 kW PV array of twenty-four 200 W modules (Voc = 57.8, Isc = 4.69). The array has three, eight-module series strings (aka the “PV source circuits”) wired to a combiner box, which is integrated with the inverter and contains three series fuses each rated at 10 A. Because the combiner box is integrated, there is no “PV output” circuit wiring that needs to be sized.
NEC Table 250.122 tells us that our 10 A fuses require #14 AWG, the minimum size allowed for copper EGCs for our PV circuit (i.e., array to inverter). The PV module frames are bonded to a #6 AWG EGC per NEC 690.46.
Let’s say we have a 3.6 kW PV array of eighteen 200 W modules (Voc = 36.2, Isc = 7.67). The array has six three-module series strings wired to a combiner box containing six 15 A breakers. Again, the NEC table 250.122 dictates that the copper EGC for the PV source circuit be #14 AWG minimum.
Technically, the “PV output circuit” runs from the combiner box to the battery bank. But this installation uses a pre-assembled power panel that includes all required disconnect, overcurrent, and ground-fault protection between the combiner box and the battery, and thus where the “PV wiring” terminates. An 80 A breaker in the power panel protects the wiring between the combiner box and the charge controller, and thus dictates our EGC to be #8 AWG minimum between the combiner box and the power panel.
If the battery cables are routed through metallic conduit, a grounding/bonding bushing is commonly used to attach a short EGC to the ground bar in the power panel. Just like with PV circuits, the breaker protecting the wiring between the battery and the inverter (commonly rated for 125, 175, or 250 A) will dictate this EGC size, per table 250.122.