Okay, let's break down the grounding requirements for this scenario. Yes, you will likely need to connect the grounding systems of the two buildings with a grounding electrode conductor (GEC), typically sized at a minimum of 3/0 AWG copper for this application if a larger service entrance conductor size. Here's the rationale and the relevant NEC (National Electrical Code) references

Why a Connection is Likely Required

The primary reason for connecting the grounding systems is to create a common ground reference plane between the two buildings. This is critical for: Safety

To ensure that fault currents have a low-impedance path back to the source (generator) and that protective devices (circuit breakers, fuses) operate quickly and effectively to clear faults. A common ground prevents dangerous voltage differences between the two buildings, which could pose a shock hazard to personnel and damage equipment.

Equipment Protection

Grounding minimizes the risk of voltage surges and transients damaging sensitive electronic equipment.

Code Compliance

The NEC mandates that separately derived systems (like your generator) have a grounding electrode system.

NEC References

Here are some relevant sections of the NEC (2023 Edition is assumed; check the edition adopted in your jurisdiction): NEC 250.32 Buildings or Structures Supplied by Feeder(s) or Branch Circuit(s). This section specifically addresses the grounding requirements for buildings or structures supplied by a feeder (which is what the connection from the generator switchgear would be considered for the emergency system in your new building). 250.32(A) Grounding Electrode System. "Building(s) or structure(s) supplied by feeder(s) or branch circuit(s) shall have a grounding electrode system installed in accordance with 250.50" This means that the new building with the generator switchgear MUST have its own grounding electrode system (e.g., ground rods, a concrete-encased electrode, or a metallic water pipe). 250.32(B) Grounded Systems. "For a grounded system at the separate building or structure, the supply-side bonding jumper shall connect the grounded conductor to the grounding electrodes as specified in 250.32(A)." (This is the connection within the generator switchgear itself.) 250.32(C) Ungrounded Systems. For an ungrounded separately derived system, such as emergency generator, a grounding electrode conductor shall be installed and shall connect the equipment grounding conductors of the derived system to the grounding electrode as specified in 250.32(A). NEC 250.66 Sizing Grounding Electrode Conductor to Grounding Electrodes. This section provides the rules for determining the size of the grounding electrode conductor based on the size of the service entrance conductors. Since you mentioned 3/0 AWG, it's likely that the service-entrance conductors are sized such that 3/0 AWG is the minimum required. The generator switchgear likely has a higher ampacity that what is shown. NEC 250.104 Bonding of Piping Systems and Exposed Structural Metal. This section requires bonding of metallic water piping and structural steel.

How to Make the Connection1. Install a Grounding Electrode System at the New Building

This is the foundation of the grounding system for the new building. Common options include:

Ground Rods

Drive ground rods (typically 8 feet long) into the earth. Use at least two, spaced at least 6 feet apart.

Concrete-Encased Electrode (Ufer Ground)

If available, connect to the building's rebar that is encased in concrete foundation.

Metallic Water Pipe

If a metallic water pipe enters the building and is in direct contact with the earth for at least 10 feet, it can be used as a grounding electrode (supplemented by another electrode).2.

Connect the Grounding Electrode System to the Main Ground Bar in the Switchgear

Run a grounding electrode conductor (GEC) from the grounding electrode system to the main ground bar within the generator switchgear. This GEC should be sized according to NEC 250.66.

Important Considerations Separately Derived System

The generator is considered a separately derived system. This means it has its own neutral and grounding connections completely separate from the utility service's neutral and grounding. The neutral from the generator should not be connected to the building's neutral except at the generator switchgear.

Overcurrent Protection

Ensure all conductors are properly protected by overcurrent devices (circuit breakers or fuses).

Local Amendments

Always check with your local electrical inspector for any local amendments to the NEC.

In summary, it is likely necessary to connect the grounding systems of the two buildings, and a 3/0 AWG copper conductor is a common minimum size for this connection. Consult with a qualified electrical engineer or licensed electrician to ensure compliance with all applicable codes and regulations. They can assess the specific details of your installation and provide the most appropriate solution. Flag for review