Okay, let's break this down. You're installing a DC PV system on a detached garage with a pre-existing 100A subpanel and a separate grounding electrode. You're asking about the necessity of a bond from the DC disconnect in the garage to the AC sub-panel grounding conductor, given you also have an EGC run back to the main house inverter.

Here's a breakdown of the situation and my interpretation based on the 2020 NEC



Key Concepts



Grounding Electrode System (GES)

The local grounding electrode (rod, plate, etc.) at the garage. Its purpose is to provide a path to ground for fault current and limit voltage on the structure.

Equipment Grounding Conductor (EGC)

The wire (usually bare or green) that connects non-current-carrying metal parts of electrical equipment (like enclosures, disconnects, module frames) back to the main service grounding point. This provides a low-impedance path for fault current to trip the overcurrent protection device (breaker).

Bonding

Connecting metal parts together to ensure they are at the same electrical potential. This is crucial for safety, preventing voltage differences that could create shock hazards.

Analysis Based on 2020 NEC and Your Situation


Garage Grounding Electrode System (GES)

You've confirmed the garage has its own grounding electrode system (690.47 & Art. 250). This is required.
2.

Equipment Grounding of PV System

You're also running an EGC from the PV system in the garage back to the inverter at the main house. This is

essential. All exposed non-current-carrying metal parts of the PV system in the garage must be connected to this EGC (690.43).
3. The Core Question: Bonding the DC Disconnect to the AC Sub-panel Ground



This is where it gets tricky and depends on your specific setup and interpretation. I will answer with the information provided. However, be sure to consult with your local inspector to ensure code compliance for your specific install. NEC Requirements and Interpretation

The goal is to create a robust and low impedance equipment grounding path. NEC 690.43 requires grounding of PV equipment. The DC disconnect needs to be effectively bonded to the EGC running to the house. This can be accomplished by the following. If the DC disconnect (assumed metal enclosure) is connected to the PV racking/module frames that are bonded/grounded AND connected to the EGC going back to the house, the additional bond to the AC subpanel ground may not be needed, and depending on the local inspector, may not be desired. If there is no bonding to PV racking/frames and the EGC going to the house, the bonding to the AC subpanel may be desired to meet 690.43.

Is this additional bonding recommended/needed

This is where local interpretations may come into play. However, I would recommend

not adding this bonding for the following reasons. You already have an EGC that ensures a low impedance path for fault current to trip back to the main panel. By bonding the DC disconnect to the AC subpanel ground, you are creating another parallel path to ground. However, it is not needed and may even create fault current issues if there were ever a problem.4. Important Considerations



Module Frame Grounding

Make absolutely sure the PV module frames are properly grounded to the EGC. Many modules require specific grounding methods (using listed grounding lugs).

Conductor Sizing

Ensure all EGCs are properly sized according to NEC Table 250.122. The size will be determined by the rating of the PV system's overcurrent protection.

Secure Connections

All grounding and bonding connections must be secure and made using listed connectors/lugs suitable for grounding.

In summary

You've covered the essential requirements (garage GES and EGC back to the house). My opinion is not to bond the DC disconnect to the AC subpanel grounding conductor.

Crucial Next Steps



Consult with Your Local Electrical Inspector

This is the most important step. Local interpretations of the NEC can vary, and they are the final authority. Show them your wiring diagram and explain your intended grounding and bonding approach.

Review Module Installation Manuals

The module manufacturer's instructions will specify how to properly ground the module frames.

Double-Check All Connections

Ensure all grounding and bonding connections are tight, corrosion-resistant, and properly sized.

Consider Professional Consultation

If you're still unsure, consider consulting with a qualified solar installer or electrical engineer who specializes in PV systems.

Disclaimer

I am an AI chatbot and cannot provide definitive electrical code advice. This information is for general guidance only and should not be substituted for professional advice or local code requirements.

Always consult with a qualified electrician and your local electrical inspector before installing any electrical system. Flag for review