You've highlighted a common point of confusion and a potential conflict within the NEC regarding PV systems and equipment grounding conductors (EGCs). Here's a breakdown of the situation and how to interpret the relevant code sections

Understanding the Conflict
250.122(B)

Generally requires increasing the EGC proportionally when circuit conductors are increased for any reason other than ambient temperature or conduit fill derating. This is to ensure a low enough impedance path for fault current, allowing overcurrent devices to trip quickly.

690.45

Specifically allows EGCs to not be increased when circuit conductors are increased solely for voltage drop in PV system circuits. This provision exists because voltage drop, while impacting efficiency and performance, generally doesn't compromise safety related to fault clearing.

Is an Inverter Output Circuit a "PV System Circuit"?

This is the crucial question. Here's a nuanced look: Strict Interpretation

The term "PV System Circuit" isn't explicitly defined in Article 690. However, considering the overall context of Article 690, which deals exclusively with PV systems, a reasonable interpretation is that any circuit integral to the operation and safety of the PV system falls under this umbrella. This would arguably include the inverter output circuit.

Arguments for Inclusion

The inverter is an essential component of the PV system, converting DC power into AC power for grid interconnection. The inverter output circuit is specifically designed and installed as part of the PV system. Ignoring 690.45 for the AC side creates an inconsistency with the intent of allowing EGC reduction when upsizing conductors for VD, without compromising safety.

Arguments Against Inclusion (More conservative approach)

The AC output of the inverter interfaces directly with the building's or utility's AC system. This "interface" could be argued to make it more like a regular AC circuit and thus not subject to 690.45. Article 690 heavily focuses on the DC side of PV systems, and the lack of explicit mention of AC circuits when talking about this section suggests that 690.45 isn't intended to encompass all aspects of the AC side.

Resolution and Recommendations

Given the ambiguity, here's a recommended approach:1. AHJ Consultation

This is the most important step. Discuss the situation with your local Authority Having Jurisdiction (AHJ) before final design. Explain your rationale and the potential conflict in the code. The AHJ's interpretation is what ultimately matters.2.

Document Your Decision

Regardless of which path you take, thoroughly document your reasoning and the code sections you relied upon. If you deviate from a stricter interpretation of 250.122(B), make a clear case for how the safety requirements are still met.
3.

Conservative Design (If Uncertain)

If the AHJ doesn't provide clarity or if you are risk-averse, it's safer to follow the stricter interpretation of 250.122(B) and increase the EGC size proportionally with the circuit conductors. While it adds cost, it ensures compliance and likely exceeds safety requirements.
4.

Engineering Judgement and Fault Current Calculation

Calculate the available fault current at the point of interconnection. Consider the impedance of the entire circuit, including the conductors, the inverter, and any transformers. Ensure that the EGC, even if not upsized, is still adequately sized to safely carry the available fault current and allow the overcurrent device to trip within the required timeframe, per 250.4(A)(5). This is absolutely critical.

Why 690.45 Exists (The Rationale)

The reason 690.45 was included in the code is to balance safety with the practical realities of PV system design. Voltage drop is a common issue in PV arrays, especially when distances are significant. Increasing conductor sizes for voltage drop can be very costly. The code-making panel recognized that increasing the EGC solely for voltage drop, without a demonstrable impact on fault clearing safety, was an unnecessary burden. The implicit assumption is that the base EGC size, per 250.122(A), is sufficient to handle the fault current, even when the circuit conductors are significantly larger.

In Summary

The code is unclear on whether 690.45 applies to inverter output circuits. The most prudent approach is to consult with the AHJ and document your design decisions. Always prioritize safety and ensure that the EGC is adequately sized to handle fault current, regardless of whether it's upsized proportionally with the circuit conductors. If fault current calculations raise concerns, then 250.122(B) should be strictly followed. Flag for review