You're on the right track, and your concern is valid. Let's break down the requirements

Here's a breakdown of the situation and the NEC (National Electrical Code) requirements:
Existing Service Entrance

Has a GEC connected to a water pipe that no longer qualifies as an electrode (due to plastic). This is a problem but is not being modified as part of the new project and needs to be considered.

New Transformers (Remote from Service)

You're installing transformers 300+ feet away from the service and need a GEC for these transformers.

NEC Requirements (Relevant Sections)



NEC 250.30(A)(6) Separate Building or Structure. This is the general concept of providing a grounding electrode system for separately derived systems (like your transformers) at a remote location. NEC 250.30(A)(3) Grounding Electrode Conductor Connection to Grounding Electrodes. States that all grounding electrodes required in 250.50 must be bonded together to form the grounding electrode system. NEC 250.50 Grounding Electrode System. This section states that if available a metal underground water pipe, the metal frame of the building (where effectivey grounded), and a concrete encased electrode should be part of the grounding electrode system. NEC 250.64(C) Continuity. Ensures the grounding electrode conductor is continuous. NEC 250.53(D)(2) Supplemental Electrode Required. Requires a supplemental grounding electrode if the sole grounding electrode is a rod, pipe, or plate.

Analysis and Solution


Water Pipe Bonding at Existing Service

The existing service still must bond to the accessible metallic water piping. Even though it's not a qualifying GEC electrode, it still needs to be bonded per NEC 250.104(A).
2.

Ground Rods at Transformers

Installing ground rods at the new transformers is absolutely the right approach for providing a grounding electrode system for that separately derived system.3.

Bonding the Two Grounding Electrode Systems



This is the critical part, and where your concern is valid. Yes, you must bond the new grounding electrode system (ground rods at the transformers) to the existing service grounding electrode system. This is to ensure that there's a low-impedance path for fault current to return to the source.

How to Bond the Two Systems



Equipment Grounding Conductor (EGC)

Run an equipment grounding conductor (sized per NEC 250.122) from the transformers back to the service entrance equipment grounding bus or neutral bus (depending on the transformer connection). This is the primary method for bonding the two systems.

Grounding Electrode Conductor (GEC) to Existing System

In some cases, a grounding electrode conductor can be used, but is less likely. Because the distance exceeds 20 feet, it would need to be protected from physical damage. This method is generally used when no other conductor is present to create the grounding electrode.

Important Considerations



Size of Bonding Conductor

The size of the bonding conductor (EGC) is critical and must be sized according to NEC 250.122. This depends on the overcurrent protection device protecting the transformer feeders.

Continuity

Ensure the bonding conductor (EGC) is continuous and properly terminated at both ends.

Documentation

Document everything thoroughly for future reference and inspection.

In Summary

Yes, you are correct to be concerned about simply installing ground rods at the new transformers and bonding to the water pipe. You

must bond the new grounding electrode system back to the service entrance grounding electrode system using an equipment grounding conductor (EGC) sized per NEC 250.122. Additionally, you still need to bond to any accessible metallic water piping even if the water piping system is not a qualifying grounding electrode.

Disclaimer: I am an AI chatbot and cannot provide electrical code interpretation. Always consult with a licensed and qualified electrician or electrical engineer for specific advice on your project. The NEC is a complex document, and proper interpretation and application are essential for safety and compliance. This information should be used as a starting point for discussion with a qualified professional. Flag for review