You're on the right track, and your understanding of 250.30(A)(6) regarding shared grounding electrode systems is accurate. Here's a breakdown of the situation, things to consider, and potential issues

Understanding the Code (NEC 250.30(A)(6))

This section allows multiple separately derived systems (like your two condo units each with its own service) to be connected to the same grounding electrode system (GES) if certain conditions are met. The primary condition is that the GES must meet the requirements for the largest separately derived system. Since you have two 200A services, this should be straightforward.

Steps and Considerations1. Verify the Existing Grounding Electrode System



Is it Code-Compliant? Just because the neighboring unit has new ground rods doesn't automatically mean they meet code. You need to verify they are properly installed, sized, and of an acceptable type. Look for: Proper number of ground rods (at least two, separated by at least 6 feet). Proper rod length (minimum 8 feet, driven into the ground). Proper rod material (usually copper or copper-clad steel). Correct size grounding electrode conductor (GEC) connecting the rods to the main panel. Refer to NEC Table 250.66 based on the size of the service entrance conductors in the largest service (likely your 200A service). Confirm there's a main bonding jumper (MBJ) properly installed in the neighboring panel, bonding the neutral bus to the enclosure and grounding electrode conductor. Resistance to Ground

Ideally, you'd measure the resistance of the existing GES to ground. If it's above 25 ohms, you'll need to supplement it with additional electrodes (e.g., more ground rods) to achieve the required resistance. While not always practical, this is the best way to ensure effectiveness.2.

Connection Method



Physical Connection

The GEC from your panel needs to connect to the existing GEC of the neighboring unit at a point that is accessible. This point should ideally be: At the main panel of the neighboring unit (at the grounding bus). At a point where the existing GEC is unspliced and continuous. If you are near the ground rods, and the connection point on the existing rod system is easily accessible, then connecting to the ground rod system itself is an acceptable approach.

Approved Connectors

Use only UL-listed connectors suitable for grounding and bonding. These are typically heavy-duty pressure connectors rated for direct burial if the connection is underground. Avoid using wire nuts or other general-purpose connectors.3.

Sizing Your Grounding Electrode Conductor (GEC)

Use NEC Table 250.66 based on the size of your service entrance conductors to determine the minimum size of the GEC from your panel to the shared grounding electrode system. Even if the neighboring unit has a larger GEC, your GEC must still meet the requirements for your service.4.

Bonding Jumper Sizing Ensure that your main bonding jumper in your service panel is properly sized per Table 250.102 (C) (1).
5. Inspection

Call your local electrical inspector before starting the work. They can offer guidance specific to your jurisdiction and may have preferences for how the connection is made. The work will almost certainly require inspection to ensure it meets code and is safely installed.

Potential Issues and Concerns Corrosion

Dissimilar metals (e.g., copper and aluminum) can corrode when in direct contact, especially in a buried environment. Use compatible connectors and consider using a corrosion-inhibiting compound.

Accessibility

The connection point to the existing GES must be accessible for inspection and maintenance. Burying a connection without proper access is a code violation and can create future problems.

Bonding Concerns

Ensure that all metallic water pipes, gas pipes, and other potentially conductive materials are properly bonded within both condo units. This is essential for safety. (NEC Article 250)

Voltage Drop

A long GEC run from your panel to the neighboring unit's grounding electrode system can potentially increase voltage drop. While this is less of a concern for grounding conductors compared to circuit conductors, it's worth considering if the distance is excessive.

Changes to the Neighboring Unit

If the neighboring unit ever removes or modifies its grounding electrode system, it could affect the grounding in your unit. Ideally, you'd have a written agreement with the neighbor about maintaining the shared system.

Future Expansion

Consider if either unit is likely to increase its electrical load in the future. This might necessitate a larger grounding electrode system.

Galvanic Corrosion

Connecting grounding systems of different materials (e.g., copper and galvanized steel) can lead to galvanic corrosion, especially in wet environments. Use appropriate connectors and protective measures if this is a concern.

Recommendation1. Consult with a Qualified Electrician

Given the complexity of grounding systems and the potential safety hazards, it's highly recommended* that you hire a licensed electrician to perform this work. They have the knowledge, experience, and tools to ensure the installation is done safely and correctly, and they can handle the inspection process.2.

Document Everything

Keep detailed records of the work performed, including the size and type of conductors, the connectors used, and photos of the installation. This will be helpful for future maintenance or troubleshooting.3.

Contact Local Authority

Check with your local building department and/or electrical inspector for any local amendments to the NEC or specific requirements in your area.

In summary, sharing a grounding electrode system between your condo units is permissible under the NEC, but it requires careful planning, execution, and verification that all code requirements are met. Safety should be your top priority, and hiring a qualified electrician is the best way to ensure that the work is done correctly. Flag for review