Okay, this is a common and complex issue in backfed power systems, especially when dealing with multiple sources (monitored and unmonitored). Let's break down the code, the reasoning, and a possible solution.

Understanding the Code (NEC 2023, based on your citations)



705.12 (Supply-Side Source Connections)

This section generally deals with the 120% rule. It essentially says the sum of all overcurrent protection devices (OCPDs) supplying a busbar, including the main breaker, cannot exceed 120% of the busbar's rating. This is your baseline protection.

705.13 (Load-Side Source Connections)

This section addresses the concerns specific to backfed sources on the load side of the service disconnect.

705.13(A) Any busbar or conductor on the load side of the service disconnecting means that is not monitored by the PCS shall comply with 705.12: This reiterates that the 120% rule still applies to unmonitored systems. 705.13(B) Where the PCS is connected to an overcurrent device protecting any busbar or conductor not monitored by the PCS, the setting of the PCS controller shall be set within the ratings of that overcurrent device: This is where your engineering team is likely drawing their reasoning, although indirectly. It's more focused on limiting the PCS output if it's feeding into a bus/conductor not directly monitored. The intent is to avoid overloading something the PCS isn't aware of.

Analysis of Your Situation


The 80% Derating of the Bus

The engineering team's 80% bus derating is based on common industry practice to de-rate a busbar for continuous duty. This is a common practice, especially with inverter fed systems. This is not defined in the code and is a design choice made by the company.2.

The Unmonitored 20A Backfeed

This is the crux of the issue. The code is clear that unmonitored sources must adhere to the 120% rule (705.13(A)). However, it doesn't explicitly say you must subtract this capacity from the allowable PCS output.3.

705.13(B) Interpretation

The engineering team is using a conservative interpretation of 705.13(B), even though it's not a direct fit. Their thinking is: "We don't fully know what the 20A unmonitored source is doing at all times. Therefore, let's be extra cautious and reduce the PCS output to compensate."4.

Bus Monitoring Completeness

You're correct to question whether the bus is "fully monitored" without sensing the output of the 20A backfeed. The intent of the code is to ensure the entire system is operating within safe limits. If you don't know what the 20A source is contributing, you lack complete knowledge.

Arguments for 80A PCS Limit (Your Position)



Literal Code Interpretation

You can argue that the code doesn't explicitly require subtracting the unmonitored source from the PCS output if the 120% rule is followed. The PW3 system is monitored and controlled, and its operation will be affected by the current going into the busbar. As long as the the existing solar system's breaker is factored into the 120% rule (which it should be as 20A is less than 120A 1.2 - 100A) it is covered.

PW3 System Capabilities

Modern PCS systems like PW3 are sophisticated. They can monitor voltage and current at the bus, and adjust their output accordingly. Theoretically, the PW3 should be able to detect an overvoltage or overcurrent condition and throttle back its output.

Arguments for 60A PCS Limit (Engineering Team's Position)



Conservative Design

They're prioritizing safety and avoiding any potential for overloading the bus. By reducing the PCS output, they create a safety margin.

Lack of Full Monitoring

They're rightly concerned about the unmonitored source. They don't have real-time data on its output, so they're taking a precautionary approach.

Potential for Unexpected Scenarios

The 20A system might be malfunctioning, producing more power than rated, or behaving erratically. The PCS cannot compensate for what it doesn't measure.

How To Interpret and Proceed


Engage the AHJ (Authority Having Jurisdiction)

This is the most important step. Present both arguments to your local electrical inspector or AHJ and ask for their interpretation. They have the final say. Document their decision in writing.2.

Document Assumptions and Calculations

Clearly document all assumptions, calculations, and code references. This will be essential for inspections and future troubleshooting.3.

Consider System Upgrades (Best Solution)



Monitor the 20A System

The ideal solution is to install monitoring equipment on the 20A backfed system. This could be as simple as a revenue grade meter that communicates its output to the PW3 or a separate monitoring system. This provides complete visibility and allows for more precise control. This way you can be certain your system is truly 705.12/705.13 compliant.

Replace the Existing System

If the 20A system is old or unreliable, consider replacing it with a newer system that is compatible with the PW3 and can be fully integrated.4.

If AHJ Approves 80A, Implement Robust Monitoring

If the AHJ allows the 80A setting, ensure the PW3 is configured to actively monitor the bus voltage and frequency. This will help it detect and respond to potential overloads or abnormal conditions.

Summary and Recommendation

While a strict reading of the NEC might support the 80A PCS limit if all other code requirements are met, the engineering team's conservative approach is understandable, especially given the presence of the unmonitored backfeed.

My Recommendation (Without Knowing AHJ)



Prioritize safety and code compliance. Consult with the AHJ and follow their guidance. If possible, implement a monitoring solution for the 20A system. This is the most technically sound approach and provides the greatest degree of safety and control. If monitoring isn't feasible, err on the side of caution and accept the 60A limit, or negotiate a compromise with the AHJ based on specific conditions (e.g., verified performance data of the 20A system).

Remember that electrical codes are minimum standards. Engineers often design beyond those standards for enhanced safety and reliability. Open communication, documentation, and the AHJ's approval are crucial. Good luck! Flag for review