You've hit a common point of confusion and a situation not explicitly covered in the NEC. Here's a breakdown of how I would approach sizing the EGC for your lithium battery installations, considering the NEC and best practices.

Understanding the Purpose of the EGC

The Equipment Grounding Conductor (EGC) is not primarily designed for normal current flow. Its primary purpose is to provide a low-impedance fault current path back to the source (inverter or charging source). This allows overcurrent protection devices (OCPDs – breakers or fuses) to quickly trip and clear a ground fault, minimizing the risk of shock and fire.

Key Considerations from the NEC Article 250: Grounding and Bonding

This is the core of grounding requirements.

Article 706: Energy Storage Systems

Specific rules for ESS installations.

Table 250.122

This table is the typical go-to for sizing EGCs based on the rating of the overcurrent protective device protecting the circuit.

The Challenge in Your Situation

The challenge is the bidirectional power flow and the multiple overcurrent devices involved. You have individual battery breakers (100A), the inverter breaker (175A or 250A), and the potential for current flow in either direction.

Recommended Approach and Justification

Here's my recommended approach, with reasoning based on the NEC and safety principles:1. Size Based on the Largest Overcurrent Device Protecting the Circuit Feeding the Rack/Cabinet

This is the most conservative and safest approach. In your case, it's likely the 175A or 250A inverter breaker. Reasoning: This breaker is the primary OCPD that will trip in the event of a ground fault within the battery cabinet. A ground fault within the cabinet (e.g., a battery shorting to the metal case) will create a fault current that flows back to the source, and the inverter breaker is what's intended to interrupt that fault.

Example

If your inverter breaker is 250A, consult Table 250.122. For a 250A OCPD, the minimum copper EGC size is typically #6 AWG. Always check the latest version of the NEC for the current values.2.

Bonding Jumpers Between Batteries and Cabinet

Use conductors that are also compliant with Table 250.122 for each battery's 100A OCPD but no smaller than #6 AWG. This is for additional fault protection from each of the batteries to the metal cabinet.
3.

Bond the Metal Rack to the Building Grounding Electrode System (GES)

This is a crucial step. The metal rack must be bonded to the building's grounding system. This ensures that the rack itself is at ground potential and provides a supplemental path for fault current. Use a conductor sized according to Table 250.66 (for grounding electrode conductors), taking into account the size of your service entrance conductors.
4.

Consider Local Requirements

Your AHJ (Authority Having Jurisdiction – the local electrical inspector) has the final say. They may have specific interpretations or requirements for ESS grounding. It's always best to discuss your installation plan with them before starting the work.5.

Documentation

Keep detailed records of your grounding and bonding methods, conductor sizes, and connections. This will be helpful for inspections and future maintenance.

Why Not Size Based on the 4/0 Conductor?

While you're using 4/0 conductors for the DC power circuit, the EGC isn't sized based on the conductor ampacity alone. It's sized based on the overcurrent protection of the circuit. The 4/0 conductors are sized to carry the normal load current, but the EGC is there for fault conditions.

Why Not Size Based on the Sum of the Battery Breakers?

This would be overly conservative and unnecessary. The likelihood of all batteries simultaneously experiencing a ground fault is extremely low. The primary concern is a single fault that trips the inverter breaker.

Important Considerations



Low-Impedance Path

Ensure your grounding connections are tight and corrosion-free. The goal is to create the lowest possible impedance fault current path.

Materials

Use appropriate grounding connectors and conductors suitable for the environment (e.g., copper lugs and copper wire).

Torque

Torque all grounding connections to the manufacturer's specifications.

Equipment Listing/Instructions

Always follow the manufacturer's instructions for grounding and bonding the battery system and inverter. They may have specific requirements.

Professional Consultation

If you are unsure about any aspect of the grounding and bonding, consult with a qualified electrical engineer or a master electrician with experience in ESS installations.

In Summary

Prioritize safety and compliance with the NEC. Size the EGC based on the largest* overcurrent device protecting the circuit feeding the rack/cabinet (likely the inverter breaker). Bond the metal rack to the building's grounding electrode system. And, most importantly, consult with your local AHJ to ensure your installation meets their specific requirements. Flag for review