Okay, let's break this down step-by-step to ensure you understand the code requirements and the practical implementation of your rooftop generator setup. You're right to be careful; improper grounding and neutral handling in separately derived systems can be dangerous.

Understanding Separately Derived Systems (SDS) and Your Setup Separately Derived System (SDS)

An SDS, like your rooftop generator, is a power source that derives its voltage independent of any other power source. It is required to establish its own neutral reference (grounded conductor) and grounding system.

Your Setup

You're using a rooftop generator with a 4000A breaker. You're then using a switchboard with circuit breakers (not a traditional ATS) for automatic transfer between normal power and the generator's optional standby power.

Your Assessment of Grounding is Correct

Yes, your understanding of the bonding and grounding requirements is correct, in general. Here's the relevant NEC guidance to keep in mind: NEC Article 250.30 addresses grounding of separately derived AC systems. It's the core reference for your setup. System Bonding Jumper (SBJ)

This must be installed at the generator or at the first disconnecting means. It connects the generator's neutral terminal to the generator's frame. This is where the generator's neutral becomes a grounded conductor. The size of the SBJ must follow NEC 250.28.

Grounding Electrode Conductor (GEC)

This connects the generator's frame to the building's grounding electrode system (main building ground bar). The size of the GEC must follow NEC 250.66.

The Core Question: Neutral Switching/Disconnecting

You're right to be puzzled about the neutral disconnect. Here's the key point: In a separately derived system used for optional standby, you typically do not need to switch the neutral.

Let's analyze why and what the alternatives are based on the NEC. NEC 250.30(A)(1) Exception No. 1 states: "A grounded conductor shall not be installed for an optional standby system where conditions of maintenance and supervision ensure that only qualified persons will service the installation and the optional standby system is used only for a specific load, as permitted by 702.5. (600 kVA or less)" 702.5 Capacity and Permissible Load "Optional standby systems shall be permitted to supply any load."With the exception above, it may not be necessary to disconnect the neutral conductor using a four-pole breaker as long as you meet the requirements of the NEC for optional standby systems.

Reasons You Generally Don't Switch the Neutral (In Your Case)1.

Code Requirements

NEC generally doesn't require neutral switching in optional standby systems if the conditions of maintenance and supervision are met to ensure only qualified persons will service the installation.2.

Operational Simplicity

Switching the neutral introduces unnecessary complexity and potential for issues. A solid, continuous neutral connection is typically more reliable.3.

Common Practice

For optional standby systems, it's far more common not to switch the neutral.4.

Fault Current Path

The neutral-to-ground bond at the generator (SBJ) and the connection to the building's grounding electrode system (GEC) create a low-impedance path for fault current to return to the source (generator). This is critical for proper operation of overcurrent protective devices (your breakers). Breaking the neutral would interrupt this path during a fault.

How Your System Works With A Solid Neutral1.

Normal Power Operation

Utility power is feeding your switchboard. Your generator's 4000A breaker is likely open (off). The generator's neutral is bonded to its frame via the SBJ and connected to the building's ground via the GEC, but it's essentially floating relative to the utility's neutral because the generator is not producing power.2.

Power Outage

When the utility power fails, your transfer scheme (using your breakers and power logic) will: Open the main utility breaker(s) feeding the essential/standby loads. Start the generator. Close the generator's 4000A breaker, energizing the essential/standby loads from the generator.3.

Generator Supplying Load

The generator's neutral-to-ground bond provides the necessary grounding reference for the generator's output. Fault current now returns to the generator neutral via the continuous neutral conductor and the SBJ. The breakers in your switchboard protect against overcurrent conditions.

Important Considerations and Recommendations

Qualified Personnel

As stated in the NEC, make sure only qualified people will service the system, and keep detailed service records.

Coordination

Ensure the generator's output breaker (the 4000A one) is properly coordinated with the downstream breakers in your switchboard to ensure selective tripping (the closest breaker to the fault trips first).

Ground Fault Protection

Verify that your generator is equipped with ground fault protection, and that it's properly set up. Your 4000A breaker is likely LSIG (Long-time, Short-time, Instantaneous, Ground fault).

Voltage Drop

Size your conductors appropriately to minimize voltage drop, especially on long runs.

Documentation

Maintain comprehensive documentation of your system, including single-line diagrams, wiring diagrams, and settings for all protective devices.

Local AHJ

Always consult with your local Authority Having Jurisdiction (AHJ) to confirm that your design meets local code requirements and that they approve of your approach.

In SummaryYou likely don't need to switch the neutral in your optional standby generator system. Ensure you have a solid neutral connection from the generator to the switchboard, with the SBJ at the generator and the GEC connecting the generator frame to the building's ground. Follow NEC 250.30 closely and consult with your local AHJ to confirm code compliance. Flag for review