You're right to question that single-line diagram. It's highly unusual and potentially problematic to have two 4000A services configured that way. Here's a breakdown of your concerns and how to approach this situation, along with considerations for PV integration

Understanding the Single-Line Diagram Anomaly
Redundancy vs. Parallel Operation

Typically, dual services like this are for redundancy. The interlock ensures only one service is active at a time. This prevents paralleling sources, which, without sophisticated synchronization and protection, can lead to circulating currents, voltage imbalances, and equipment damage.

The Correct Functionality

The interlock's purpose is to prevent both breakers from being closed simultaneously. When one breaker is closed (feeding the load), the interlock mechanically or electrically prevents the other breaker from being closed.

Why the Diagram is Suspect

The single-line implies both services can be simultaneously connected, which defeats the purpose of the interlock in a standard redundant system. If this were intended for paralleling, it would require much more sophisticated switchgear with synchronizing relays, paralleling controls, and advanced protection features (like reverse power protection). The diagram shows none of that. Simply paralleling two unsynchronized services can be extremely dangerous and damaging.

Action Steps1. Clarification is Critical

Before proceeding any further, you must confirm the intended operating scheme with the facility's electrical engineer, the utility company, and/or the switchgear manufacturer. You cannot assume the diagram is correct. Ask these questions: "Is the intent to have both services connected simultaneously, or is this a redundant system where only one service is active at any given time?" "If the intent is for parallel operation, what synchronization and protection systems are in place?" "Can we get the switchgear's detailed control schematics?"2.

Assume Redundancy (Until Proven Otherwise)

Until you receive definitive information to the contrary, assume the system is intended for redundant operation (one service active at a time). This is the most common and safest configuration.

PV System Integration

Assuming a redundant service scenario, you have several options for integrating the PV system. However, the 12 parallel sets of conductors make it a bit more complex, and the large service size necessitates careful coordination with the utility.

General Considerations



NEC Compliance

Ensure all aspects of the PV system design and installation comply with the latest version of the National Electrical Code (NEC), particularly Article 690 (Solar Photovoltaic (PV) Systems) and Article 705 (Interconnected Electric Power Production Sources).

Utility Interconnection Agreement

You'll need a formal interconnection agreement with both utilities providing the services. This will dictate the allowed interconnection methods, protection requirements, and any metering requirements. Don't start design work without understanding the utility's requirements.

Point of Interconnection (POI)

The POI will be a critical decision. It determines where the PV output is connected to the facility's electrical system.

Overcurrent Protection

The overcurrent protection (breakers, fuses) for the PV system must be carefully sized and coordinated with the existing service equipment.

Interconnection Options (Assuming Redundant Services)


Load-Side Interconnection (Common Bus)



Description

This is the most common approach. The PV system's output is connected to the main switchboard bus after the main service breakers.

Considerations



12 Parallel Sets

This is the big challenge. You can't simply connect a single PV conductor to one of the 12 conductors. You need to maintain the integrity of the paralleled conductors.

Tap Box/Termination Cabinet

The most practical solution is to install a tap box or termination cabinet near the main switchboard. This box will: Provide a safe and organized way to terminate the 12 sets of conductors for each phase (A, B, C, and Neutral). Allow you to connect the PV conductors (appropriately sized for the PV system's output) to this termination box. Ensure proper current sharing among the parallel conductors.

NEC 705.12(D) - Supply Side Source Connection

Depending on the size of your PV system, this might require a service upgrade (adding a tap box ahead of the main breaker). The NEC rules prevent the sum of the main breaker's ampacity plus the PV interconnection circuit's ampacity from exceeding the bus bar rating.

Current Transformer (CT) Considerations

If you're adding CTs for monitoring or control (e.g., for export limiting), carefully consider where to place them to accurately measure the total load including the PV contribution.

Anti-Islanding Protection

Your PV inverters must have anti-islanding protection that will disconnect the PV system if either service is lost. This prevents the PV system from energizing the utility grid in an unsafe manner.2.

Supply-Side Interconnection (Before Service Breakers)



Description

The PV system is connected to the utility side of the main service disconnects (either before or at the line side of the existing service disconnects).

Considerations



Less Common

This approach is less common for large systems because it involves working directly with the utility's equipment.

Extensive Coordination

Requires very close coordination with the utility company.

Potential for Service Upgrade

Often requires a complete service upgrade (new switchgear) to accommodate the PV interconnection.

Higher Costs

Generally more expensive and complex than load-side interconnection.

Possible benefits

Can allow for export of excess power to the grid more easily (depending on utility policies).3.

Interconnection to a Dedicated Subpanel (Not Recommended in this case)



Description

Installing a separate subpanel downstream of the main switchboard and connecting the PV system to that subpanel.

Why it's likely not ideal



Not Optimized for Large PV System

If you're putting in a large PV array, this won't be the best approach for maximizing the offset of the total facility load.

Complicates Load Management

Makes it harder to track and manage the overall power flow and PV contribution to the facility's load.

Key Considerations for Paralleled Conductors (400 MCM Sets) Consistent Lengths

All parallel conductors must be the same length. This ensures equal impedance and current sharing.

Proper Termination

Use appropriate lugs and termination techniques for large conductors. Torque connections properly and inspect them regularly.

Avoid Mixing Conductor Types

All parallel conductors should be of the same material (e.g., all copper).

Derating

Consider conductor derating factors based on ambient temperature, conduit fill, and proximity to other conductors (NEC Table 310.15(B)(3)(a)). The large number of conductors in parallel might require significant derating.

Recommendations1. Prioritize Clarification

Contact the facility and the utility immediately to clarify the service configuration. Do not proceed without this information.2.

Hire a Qualified Electrical Engineer

A qualified electrical engineer with experience in PV system design, large commercial installations, and utility interconnection is essential for this project. They can: Perform a detailed load analysis of the facility. Determine the optimal PV system size and interconnection point. Develop detailed single-line diagrams, wiring diagrams, and protection coordination studies. Interface with the utility company.3.

Engage the Utility Early

Get the utility involved as soon as possible. Understand their interconnection requirements, metering requirements, and any limitations they may have.4.

Consider Monitoring and Control

Implement a comprehensive monitoring and control system to track PV production, facility load, and power flow. This can help optimize energy usage and ensure the system is operating safely and efficiently.5.

Thorough Documentation: Maintain meticulous documentation of the entire PV system design, installation, and commissioning. This will be invaluable for future maintenance and troubleshooting.This is a complex project, and a cautious, well-planned approach is essential for a safe and successful PV system integration. Good luck! Flag for review