You're right to be concerned. Having multiple bonding points in this scenario creates the potential for circulating currents and can compromise the integrity of the ground fault current path. Let's break down the NEC requirements and best practices for fire pump circuits and emergency systems

Understanding the Issue
The Concern

The fundamental problem with having two bonding points (S1 and S2 in your scenario) is creating parallel paths for ground fault current. When a fault occurs, current will flow through both paths back to the source. If one path has significantly lower impedance than the other, most of the fault current will flow through that path. However, even with impedance, you may create circulating currents.

Why Circulating Currents are Bad

Circulating ground currents can lead to several problems:

Nuisance tripping

Ground fault protection devices can trip unnecessarily due to the circulating current.

Overheating

Conductors carrying the circulating current can overheat, potentially leading to insulation breakdown and fire hazards.

Compromised Protection

The effectiveness of the ground fault protection system can be diminished because the fault current is split between multiple paths.

Noise

Can cause havoc with control circuits.

Analysis of Your System1. Fire Pump Controller (S1) and Switchboard (S2)

You should NOT have bonding points at both S1 and S2, which would create circulating currents in the system.2.

Generator as a Non-Separately Derived System (Non-SDS)

In your setup, the generator is a Non-SDS, no bonding between neutral and ground. The generator is not a separately derived system because the neutral from the generator is connected through to the utility neutral.3.

ATS (3-Pole, Unswitched Neutral)

The ATS is a 3-pole unswitched neutral, in this case it is a common neutral system.

NEC Considerations and Requirements NEC 250.30 (SDS vs. Non-SDS)

This section is crucial. Since your generator is operating as a non-SDS, it

cannot be bonded at the generator itself. The neutral from the generator is connected through to the utility neutral. NEC 695 (Fire Pumps)

NEC 695 contains specific requirements for fire pump installations. It emphasizes the need for a reliable power source and fault current path.

NEC 700 (Emergency Systems)

This article covers emergency system wiring, including the need for proper grounding and bonding.

Best Practice and Recommended Configuration

Given your configuration, here's what is typically recommended:1. Bonding Point



Single Bonding Point

Establish the sole bonding point at the main switchboard (S2). This is the service equipment and the location where the neutral is bonded to ground.2.

Fire Pump Controller (S1) Grounding

Run a separate grounding conductor (equipment grounding conductor, EGC) from the main switchboard (S2) to the fire pump controller (S1). This EGC provides a low-impedance fault current path.3.

Generator Grounding

Run a grounding conductor from the generator frame to the main switchboard (S2). Since the generator neutral is not switched, it shares the grounded conductor connection at the main switchboard.4.

Ensuring Low Impedance

Use appropriately sized grounding conductors. Refer to NEC Table 250.122 for sizing equipment grounding conductors. Oversizing is often a good idea to minimize impedance.

Revised System Grounding and Bonding Plan Switchboard (S2)

This is where the neutral-to-ground bond will be located. This creates the System Grounding Electrode connection.

Fire Pump Controller (S1)

A separate EGC runs from S2 to S1. No neutral-to-ground bond at S1.

Generator

Frame of the generator must be grounded to the neutral bar at the switchboard (S2). This is the single point of grounding for the emergency system.

Important Considerations Coordination Study

A coordination study should be performed to ensure that the overcurrent protective devices (OCPDs) are properly coordinated, including ground fault protection.

Professional Consultation

Given the critical nature of fire pump and emergency systems, consult with a qualified electrical engineer or a licensed master electrician experienced in these types of installations. They can review your specific design and ensure compliance with all applicable codes and standards.

In Summary

You are correct to be concerned about having multiple bonding points. The NEC does not allow for multiple points of bonding, and this creates issues such as circulating currents. One single point of bonding is needed for the system, and the emergency system should be bonded back at the MDP/Switchboard. This solution will allow the system to comply with NEC code, while having proper grounding.Disclaimer: I am an AI chatbot and cannot provide professional engineering advice. Always consult with a qualified electrical engineer for your specific project. Flag for review