You've hit on a common point of confusion in PV system grounding, particularly with supply-side connections. Let's break it down

The Core Issue: Separately Derived Systems vs. Not Separately Derived Systems

This boils down to whether the PV system is considered a "separately derived system" (SDS) or not. A separately derived system is a premises wiring system whose power is derived from a source other than a service. PV systems connected on the supply side are generally not considered separately derived systems.

Why Not a Separately Derived System? No Isolation

In a supply-side connection, the PV system's conductors are directly connected to the service conductors before the main service disconnect. There's no transformer or other isolation device between the PV system and the utility grid. This is crucial. A separately derived system like a generator or a transformer isolates the source with a winding isolation.

Why Your Observation is Correct

Because the PV system on a supply-side connection is not an SDS, you're absolutely right to question the need for a separate grounding electrode conductor (GEC) from the new PV disconnect back to the existing grounding electrode system.

The NEC Logic

Here's how the NEC supports your view: 250.24(A) Grounding Service-Supplied Alternating-Current Systems. This section requires the system grounding electrode conductor connection at the service equipment. Since your PV system connects before the service equipment, you're essentially extending the service. 250.30 Grounding Separately Derived Alternating-Current Systems. This section is not applicable to your situation because your PV system is not separately derived. It applies to things like generators that have isolation through induction. 250.12 Supply-Side Bonding Jumper. You correctly identified this as the likely appropriate conductor. The Supply-Side Bonding Jumper (SSBJ) is used to connect the equipment grounding conductors (EGCs) to the service neutral at the service equipment. Since your PV system is connected on the supply side of the service disconnect, the PV disconnect should be connected to the service neutral at the service equipment.

What to Do: The Recommended Approach1. Install a Supply-Side Bonding Jumper

Run a Supply-Side Bonding Jumper (SSBJ) from the ground bar in the PV disconnect to the neutral/ground bus in the main service panel. The size of this jumper should be determined by NEC Table 250.102(C)(1) based on the size of the largest ungrounded supply conductor.
2.

Equipment Grounding Conductor (EGC)

Ensure you have an EGC running with the PV system conductors from the PV array/inverter to the PV disconnect. This EGC is sized according to NEC 250.122.

Why the Misunderstanding Persists Habit/Assumption

Many installers are accustomed to treating all PV systems as if they need a GEC. They may apply the rules for SDSs out of habit or misunderstanding.

Over-Grounding

There's a (false) sense that "more grounding is better." However, unnecessary GECs can create ground loops and contribute to unwanted current flow, potentially creating safety issues.

Inspector Interpretations

Local interpretations of the NEC can vary. Some inspectors might incorrectly require a GEC even when not technically necessary.

In Summary

For a supply-side PV connection, you are correct in questioning the need for a separate GEC running from the PV disconnect back to the grounding electrode system. A Supply-Side Bonding Jumper (SSBJ) is typically the correct approach, connecting the PV disconnect's ground bar to the neutral/ground bus in the main service panel. This avoids creating a separately derived system where one doesn't exist and potentially introducing unnecessary grounding paths.

Important Caveats and Recommendations Local AHJ is King

Always, always, always consult with your local Authority Having Jurisdiction (AHJ) (i.e., the electrical inspector) to confirm their specific requirements. Their interpretation of the code is what matters in your locality. Get it in writing if possible.

Engineering Stamp

For complex installations, or when facing pushback from the AHJ, consider having a licensed electrical engineer review the design and provide documentation supporting your approach.

Documentation: Keep detailed records of your grounding system design, conductor sizes, and the rationale behind your choices. This will be helpful for inspections and future maintenance.By understanding the fundamental principles of grounding and the NEC's requirements for supply-side PV connections, you can ensure a safe and compliant installation. Good luck! Flag for review