Based on the description and assuming we're talking about a separately derived system (SDS) like a transformer, here's a breakdown of grounding requirements and whether you need a new grounding electrode

Understanding the Situation
120/208V 3-Phase Transformer

You have a transformer that's creating a new power system from the primary side (not shown).

100A Main Breaker

The panel you're adding is protected by a 100A circuit breaker.

A, B, C, N

You're bringing the three phases (A, B, C) and the neutral (N) from the transformer to the new panel.

Question

Can you use the existing ground rod for the new panel's grounding electrode system?

NEC (National Electrical Code) Requirements

The NEC governs grounding requirements for SDSs. Here's the gist:1. Separately Derived System (SDS) Grounding

When you have a transformer creating a new power system, you must ground that new system. This is different from the grounding of the service entrance.2.

Grounding Electrode System (GES)

The grounding electrode system is what connects the SDS to earth. It consists of:

One or More Grounding Electrodes

This is what physically connects to the earth (e.g., ground rods, ground rings, metal underground water pipe).

Grounding Electrode Conductor (GEC)

The wire that connects the SDS's neutral (specifically, the system bonding jumper point) to the grounding electrode(s).3.

Grounding Electrode Types



Acceptable Electrodes

The NEC specifies what can be used as grounding electrodes: Metal underground water pipe (with certain restrictions) Metal in-ground support structure (e.g., rebar encased in concrete foundation) Ground ring Rod and pipe electrodes Other listed electrodes4.

Multiple Electrodes

If you use rod, pipe, or plate electrodes, the NEC generally requires you to have at least two electrodes bonded together. This is often satisfied by having two ground rods spaced at least 6 feet apart.5.

Size of Grounding Electrode Conductor

The size of the GEC is based on the size of the derived phase conductors from the transformer but is typically not required to be larger than #3 copper (check NEC table 250.66)

Answering Your Question

Here's a breakdown of why the existing ground rod probably needs to be augmented: Requirement for Multiple Electrodes

The NEC typically requires multiple grounding electrodes (at least two ground rods). If the existing rod is the only grounding electrode, you'll likely need to add another one and bond them together.

Suitability of Existing Rod

You would need to confirm the size and material of the exisiting rod to ensure it meets NEC requirements.

Recommendations1. Add a Second Ground Rod

The simplest and most common solution is to drive a second ground rod at least 6 feet away from the existing one. Bond these rods together with a grounding electrode conductor (GEC).2.

Bond to Other Approved Electrodes

If available, see if you can bond to other acceptable grounding electrodes that already exist, such as a metal underground water pipe (with certain restrictions) or the building's rebar encased in the concrete foundation.3.

Proper Bonding

Ensure the grounding electrode conductor (GEC) is properly sized and connected from the neutral of the transformer to the grounding electrode(s).

Important Considerations Local Codes

Always consult your local electrical codes and inspectors. They may have more stringent requirements than the NEC.

Qualified Electrician

This work should be performed by a qualified and licensed electrician who understands grounding and bonding requirements.

Bonding Jumper

Ensure there is a proper system bonding jumper at the transformer. This jumper connects the neutral conductor to the transformer enclosure and the grounding electrode system. The size of this conductor is determined from NEC Table 250.102(C)(1).

Corrosion

Grounding electrodes and conductors can be subject to corrosion, especially in certain soil conditions. Use appropriate materials and installation techniques to minimize corrosion.

In conclusion, while you might be able to use the existing ground rod as part of the grounding electrode system, you will almost certainly need to add another grounding electrode and connect them properly to comply with the NEC. You need to have a complete grounding electrode system for the newly derived system.Disclaimer: This information is for general guidance only. It is essential to consult the NEC and local codes, and have a qualified electrician perform the installation. Flag for review