Okay, let's break down these scenarios and address your concerns about grounding and bonding in off-grid systems. You're right to be questioning these installations; multiple N-G bonds in parallel systems can definitely lead to problems.

Scenario 1: Parallel Generators (190kW & 275kW) - No Utility Service

You're correct. Bonding the neutral to ground in both generators when they are paralleled creates a parallel path for neutral current, potentially leading to: Objectionable Current Flow

Neutral current will split between the EGCs and the neutral conductor, potentially overloading the EGCs, creating noise, and interfering with sensitive electronic equipment.

Harmonic Circulation

Harmonics generated by one generator can circulate through the ground system to the other generator and connected loads.

Increased Risk of Electrical Shock

If there's a fault on one generator, the redundant ground paths increase the chance that a person could come into contact with a voltage potential.

NEC Considerations



Article 250 (Grounding and Bonding)

This is the key article. You need to adhere to the general requirements for grounding and bonding, but specifically the requirements for separately derived systems (SDS) even though there's no utility service.

250.20(B)

This is the key section that requires grounding of AC systems. Although there is no utility service, the NEC still dictates that your AC system must be grounded. Since you have a neutral present, the grounded conductor (neutral) must be bonded.

250.30 (Grounding Separately Derived AC Systems)

This is the crucial section. Even without a utility service, the NEC treats each generator as a separately derived system (SDS).

However, parallel operation complicates this. Since you have two generators feeding into a single distribution system, the SDS rules need careful application. 250.30(A)(1) and (2)

Discuss that the single MBJ should be made at the source of the SDS.

250.30(A)(3) and (4)

Provide that if the source of the SDS and the first disconnecting means are in separate enclosures, then specific rules must be followed in choosing the size of your GEC.

250.36 (High-Impedance Grounded Neutral Systems)

This is not applicable in your case since you are not using a high-impedance grounding method.

Here's the correct approach (based on understanding your description)


Identify a Single SDS Point

The key is to designate

ONE location within the system to make the neutral-to-ground bond. The best place for this is typically at the first disconnecting means or overcurrent device.2. Service Entrance Rating is Preferred

While not explicitly required because there's no "service," it's highly recommended to use service-rated equipment (like your UL891 switchboard) for the main distribution point. This ensures it's designed to handle the fault currents and grounding requirements.3.

Neutral-Ground Bond at the Designated SDS Point

Install a Main Bonding Jumper (MBJ) only in that single location (e.g., in your switchboard) connecting the neutral bus to the ground bus.4.

Remove Neutral-Ground Bonds in the Generators



Crucially, remove the neutral-ground bond in both generator breaker panels. The neutral conductors from the generators should land on the neutral bus in the switchboard without being bonded to ground at the generators.5. GEC from Switchboard to Grounding Electrode

Run a Grounding Electrode Conductor (GEC) from the ground bus in your switchboard to the grounding electrode system (ground rods, etc.). Size this GEC according to NEC Table 250.66, based on the size of the ungrounded conductors feeding the switchboard.
6.

EGCs from Generators to Switchboard

Each generator needs an Equipment Grounding Conductor (EGC) run with the circuit conductors to the switchboard. This EGC connects to the generator frame and to the ground bus in the switchboard.7.

Size EGC

The EGC is sized per 250.122 based on the OCPD of the circuit.

Argument Against a Non-Service-Rated Switchboard



Fault Current Handling

Service-rated equipment is designed and tested to handle the high fault currents that can occur in an electrical system. If you use non-service-rated equipment, it may not be able to withstand a ground fault, potentially leading to equipment damage, fire, or electric shock. While your UL891 switchboard might be fine, it's always best to follow best practices.

Inspectors

Many inspectors will expect to see service-rated equipment at the main distribution point, even in off-grid systems. It simplifies the inspection process and provides confidence in the safety of the installation.

In summary, remove the N-G bonds in the generators, install a single N-G bond in the switchboard (preferably a service-rated one), and ensure proper EGC and GEC connections.Scenario 2: Hybrid Microgrids (Inverter/Charger + Generator)

This is even trickier because of the inverter/charger combination. You're right to be concerned about multiple N-G bonds.

The Problem

The AHJs requiring N-G bonds at both the inverter and the generator are likely oversimplifying the application of SDS rules. If the inverter/charger does not isolate its input (generator side) and output (load side), then it's not truly creating a new, separately derived system on its output. It's essentially passing the neutral through.

NEC Considerations



SDS Definition

Carefully consider if the inverter/charger truly meets the definition of an SDS. If it doesn't isolate the neutral, it's more like a transformer-less inverter, and you shouldn't treat it as an SDS for grounding purposes.

250.20(B)

This is a vital article that requires all AC systems to be grounded.

250.30 (Grounding Separately Derived AC Systems)

Again, if the inverter is an SDS, this section applies.

Parallel Operation Issues

The core problem remains: multiple N-G bonds create parallel neutral paths.

Here's the recommended approach (assuming the inverter/charger does not isolate)


Designate a Single SDS Point

Because the inverter does not isolate, the main electrical panel becomes your SDS.2.

Bonding the Neutral

You will land the neutral wire from both your generator and inverter at the neutral bus, and create an MBJ to the ground bus.3.

Remove Neutral-Ground Bonds in the Generators



Crucially, remove the neutral-ground bond in the generator breaker panel.4. GEC from Switchboard to Grounding Electrode

Run a Grounding Electrode Conductor (GEC) from the ground bus in your switchboard to the grounding electrode system (ground rods, etc.). Size this GEC according to NEC Table 250.66, based on the size of the ungrounded conductors feeding the switchboard.
5.

EGCs from Generators and Inverter to Switchboard

Each generator and inverter needs an Equipment Grounding Conductor (EGC) run with the circuit conductors to the switchboard. This EGC connects to the generator frame and to the ground bus in the switchboard.

Dealing with Inspectors

This is the tricky part. Here's a strategy:

Documentation

Prepare a clear, well-documented grounding and bonding plan that outlines your reasoning and references the relevant NEC sections. Include a single-line diagram showing all grounding and bonding connections.

Communication

Before the installation, schedule a meeting with the AHJ to discuss your plan. Explain why multiple N-G bonds are problematic and how your proposed system meets the intent of the NEC while avoiding those issues. Be respectful, but stand your ground with your NEC knowledge.

Manufacturer Support

If possible, get a letter from the inverter manufacturer stating that their equipment does not require an N-G bond at the unit itself when used in this type of parallel system.

Alternative Argument

If the inspector insists on an N-G bond at the inverter, as a last resort, you could use a neutral-blocking device (a device that prevents DC current flow in the neutral conductor). This is not ideal, but it might satisfy the inspector while mitigating some of the objectionable current flow. However, neutral blocking devices are not universally accepted and may have their own drawbacks.

Important Considerations



Grounding Electrode System

Ensure a robust grounding electrode system is in place (ground rods, ground ring, etc.). This is essential for safety.

Equipment Listing and Labeling

Make sure all equipment is properly listed and labeled for its intended use.

Load Calculations

Perform thorough load calculations to ensure the generators and inverters are adequately sized for the expected loads.

Coordination

Coordinate with the generator and inverter manufacturers to ensure proper operation in a parallel configuration.

In conclusion, you are absolutely right to question the multiple N-G bonds. It is a common mistake that can lead to significant problems. By following the principles of SDS grounding, designating a single point for the N-G bond, and communicating effectively with the AHJ, you can ensure a safe and reliable off-grid power system.Disclaimer: This information is for general guidance only and should not be considered a substitute for professional electrical engineering advice. Always consult with a qualified electrical engineer and follow all applicable codes and regulations.* Flag for review