You are absolutely correct. Your interpretation of 250.64(E)(1) is spot on, especially considering the concentric knockouts and the plastic inverter enclosure. Here's a breakdown of the issue and recommended solutions

Understanding the Problem
Plastic Enclosure

Because the inverter enclosure is plastic, the grounding relies entirely on the grounding conductors within the raceway (FMC in this case). You can't depend on any incidental grounding through the plastic case.

FMC as a Grounding Path

While the green insulated equipment grounding conductor (EGC) inside the FMC does provide a grounding path, the code requires the metal conduit itself to also be grounded on both ends when used with a conductor inside acting as an EGC, as you mentioned.

Concentric Knockouts

Concentric knockouts are designed to be removed easily to accommodate different conduit sizes. When not properly bonded (and often even when they appear "tight"), they introduce significant impedance to ground fault current flow. This defeats the purpose of a low-impedance grounding path.

250.64(E)(1) Importance

This section of the code is crucial for safety. It ensures that the metal conduit is equipotential with the equipment it's connected to. In the event of a ground fault, it provides a low-impedance path back to the source (inverter), allowing the overcurrent device to trip quickly, preventing potential electrocution hazards and fire risks.

Why the Internal Ground Wire Alone Isn't Enough

The ground wire inside the FMC is essential but it cannot be the sole means of grounding the metal conduit. The metal conduit acts as an additional, parallel grounding path. It's a redundant safety measure. If the internal ground wire is damaged or becomes loose, the grounded conduit then helps carry the fault current.

Solutions (Prioritized for Best Practice)

Here's a breakdown of ways to address the issue, in order of preference:1. Best

Replace the FMC with EMT conduit (or rigid metal conduit, RMC) and use appropriate grounding bushings on both ends. This provides a robust, code-compliant grounding path. This is the most reliable and preferred solution.

Grounding bushings should be properly sized for the EMT and the ground wire. Tighten all connections securely.2.

Acceptable (But Less Ideal)

If you must keep the FMC, you must bond it to ground at both ends. Here are two ways to do that:

Use Approved Bonding Jumpers and Connectors

Install a grounding (bonding) jumper that runs from the inverter ground bus bar to a grounding lug attached to the FMC connector. Use a short, listed grounding jumper that matches the size of the EGC inside the FMC. At the combiner MLO, install a grounding lug on the FMC connector and bond it to the ground bus bar in the combiner MLO with a similar grounding jumper. The grounding lugs and bonding jumpers must be listed for the purpose and rated for the wire size.

Grounding-Type FMC Connectors

Remove the existing FMC connectors and replace them with grounding-type FMC connectors listed for grounding purposes. These connectors have a built-in grounding lug or a mechanism that ensures a direct, low-impedance connection between the FMC and the enclosure. Connect a grounding jumper from the grounding lug on the connector to the ground bus bar in the inverter (if it's accessible) and to the ground bus bar in the combiner MLO.

Important Considerations

Proper Grounding Jumper Size

The size of the grounding jumpers must be adequate for the circuit's overcurrent protection. Consult the NEC (National Electrical Code) table 250.122 to determine the correct size based on the circuit breaker rating.

Listed Equipment

All grounding components (bushings, connectors, lugs, jumpers) must be listed and labeled for grounding purposes.

Secure Connections

Ensure all connections are clean, free of corrosion, and properly tightened. Use an anti-oxidant compound on aluminum connections.

Inspection

After making the corrections, have the system inspected by a qualified electrical inspector to ensure compliance with all applicable codes and regulations.

Summary*You're correct about needing to ground the FMC on both ends. Prioritize replacing the FMC with EMT or RMC. If that's not feasible, use proper grounding bushings or connectors and bonding jumpers to create a solid, low-impedance grounding path. This ensures the safety and reliability of your solar system. Flag for review