Okay, let's break down the considerations for connecting your PV array to the secondary side of the transformer, along with the potential need for a secondary bus enclosure. Your sketch is helpful in visualizing the concept.

Analysis of Connecting PV to the Transformer Secondary

While your idea has merit for simplifying the physical installation, several factors must be carefully evaluated for code compliance, safety, and utility requirements: NEC Compliance

This is the most important consideration. The NEC has specific articles addressing PV system interconnections, particularly Article 705 (Interconnected Electric Power Production Sources) and Article 690 (Solar Photovoltaic Systems).

705.12(D) Supply-Side Source Connection

This section of the NEC governs the interconnection of the PV system to the supply side of the service disconnecting means. This article discusses calculating the OCPD of the feeder conductors in order to determine the maximum amount of overcurrent protection of the supply side of the service disconnecting means. This is generally not considered the secondary side of the transformer.

705.12(B) Load-Side Source Connection

This section of the NEC governs the interconnection of the PV system to the load side of the service disconnecting means. This section will typically be used for this scenario.

705.12(B)(1) Ampere Rating of Disconnecting Means

You'll need to ensure that the total overcurrent protection on the load side of the utility transformer does not exceed the rating of the service disconnecting means.

705.12(B)(2) Ampere Rating of Equipment

You'll need to ensure that the sum of the overcurrent protection for the utility transformer and the inverter(s) does not exceed 120% of the ampacity of the busbar or conductor.

690.64 Point of Connection

This section states that the system must be connected in accordance with 705.

Utility Interconnection Agreement

The utility company will definitely have requirements for connecting a PV system to their grid. You MUST work closely with the utility from the very beginning of the design process. Their interconnection agreement will specify acceptable points of connection, metering requirements, protection requirements, and communication needs. They may not allow direct connection to the transformer secondary or may have specific requirements for doing so. They will likely require dedicated metering for the PV system to accurately measure energy generated and fed back to the grid. The utility may require a visible open isolating device to isolate the PV system from the grid for maintenance or emergencies.

Transformer Loading and Capacity

You need to calculate the existing load on the transformer and determine if the addition of the PV system's potential backfeed will overload it. Consider both the transformer's nameplate kVA rating and its actual operating conditions. Excessive backfeed could damage the transformer.

Voltage Rise

PV inverters inject current into the grid. If the impedance of the transformer and the distribution system is high, this can cause a voltage rise at the point of connection. The utility will likely have limits on voltage rise, and you may need to mitigate it with larger conductors or other measures.

Protection Coordination

The protection devices (fuses, circuit breakers) for the PV system, the transformer, the building service, and the fire pump must be carefully coordinated to ensure proper operation in the event of a fault. A short circuit on the building side should not trip the PV system breaker, and vice-versa. A qualified engineer should perform a protection coordination study.

Fire Pump

The fire pump circuit MUST be reliable and not affected by the PV system. NEC Article 695 covers fire pumps, and it emphasizes their independence and reliability. The connection of the PV system should not compromise the fire pump's power supply in any way. Depending on the situation, the fire pump might even need its own dedicated transformer, separate from the building service, to ensure its power is never interrupted.

Rapid Shutdown

NEC Article 690 requires rapid shutdown of PV systems for firefighter safety. You will need to incorporate a rapid shutdown system that de-energizes the PV array in the event of an emergency.

Secondary Bus EnclosureA secondary bus enclosure (also sometimes called a junction box or a distribution block) becomes necessary when:

Conductor Congestion

You have too many conductors to safely and neatly terminate directly at the transformer secondary terminals. The physical space inside the transformer's termination compartment becomes overcrowded, making it difficult to make secure connections and maintain proper clearances.

Tap Rules

If you are tapping conductors from the secondary conductors to feed the fire pump and PV system, the tap rules of the NEC (specifically Article 240.21) may require a fused disconnect for each tap, which is often more easily accommodated within a separate enclosure.

Ease of Installation and Maintenance

A separate enclosure can provide a more organized and accessible point for making connections and performing maintenance.

Higher Ampacity Requirements

If the sum of the ampacities of all the connected circuits exceeds the rating of the transformer secondary terminals, you'll need a bus enclosure with a higher ampacity rating.

When to Consider a Bus Enclosure

As a general rule, if you have more than 3 or 4 sets of conductors needing to be connected to the transformer secondary, it's probably time to consider a secondary bus enclosure. Also, if you're needing to tap the conductors to run the fire pump and PV system, a secondary bus enclosure is recommended.

Alternative Connection Points to Consider (Depending on Utility)

Separate Service

The utility might require a completely separate service for the PV system, with its own transformer and meter. This is often the simplest and cleanest approach, especially for larger PV systems.

Line-Side Tap

Some utilities will allow you to tap into the primary side of the transformer (before the transformer) using a dedicated transformer. This requires careful coordination with the utility.

Recommendation1.

Contact the Utility

This is your first step. Discuss your plans with them and get their specific requirements for interconnection.2.

Consult with a Qualified Electrical Engineer

An engineer experienced in PV system design and grid interconnection is essential. They can perform the necessary calculations, design the system to meet NEC requirements and utility specifications, and provide detailed drawings and specifications.3.

Consider a Separate Service

Explore the possibility of a separate service for the PV system. It may be more expensive upfront, but it could simplify the design, reduce potential conflicts, and improve overall system reliability.4.

Protection Coordination Study

Once you have a preliminary design, have a qualified engineer perform a protection coordination study to ensure that all the protective devices are properly sized and coordinated.5.

Detailed Drawings and Specifications: Develop detailed electrical drawings and specifications that clearly show all components, wiring, protection devices, and interconnection points.Connecting a PV system directly to the secondary side of a transformer is possible, but it requires careful planning and coordination with the utility and a qualified electrical engineer. Don't skip the necessary steps to ensure a safe and compliant installation. The fire pump requirements especially should be a major consideration in the design of the PV system. Flag for review