You are absolutely correct in your thinking. Even though the PV backfeed is connected to the sub-panel, it does affect the main panel's load.Here's a breakdown of why your reasoning is correct and how the NEC addresses this

Key NEC Rule: 705.12(B)(2)(3)

This section covers the "120% rule" for backfed breakers in panels. The critical part relevant to your situation is that the busbar of the panel where the PV breaker is located cannot be loaded beyond 120% of its rating after accounting for the backfed power.

Applying the Rule to Your Scenario

1.

Main Panel



Main Busbar Rating

100A

Main Breaker Rating

100A

Maximum Backfeed Allowed (with 120% rule)

(120% of 100A) - 100A = 20A2.

Subpanel

Busbar rating: 125A PV breaker: 30A3.

Overload on Main

Your understanding is correct. Even though the PV connects to the subpanel, when the subpanel doesn't use all the power generated by the PV, the excess flows "upstream" to the main panel. The main panel's busbar can only handle 20A of backfeed, and you are feeding 30A, so the main panel is overloaded by 10A.

What to do about it?

Here are some possible solutions to resolve the situation:1. Reduce PV Breaker Size

This is the simplest fix. Reduce the PV breaker size in the sub-panel such that the backfeed doesn't exceed 20A. Note: The inverter has to be sized to limit the backfeed breaker size. For example a 30A breaker needs to be fed by an inverter that can generate at least 30A x 1.25 = 37.5 amps.
2.

Line Side Tap

A line side tap will not add to the load on the Main panel bus.3.

Upgrade the Main Panel

This is the most expensive option, but it can future-proof your system. Upgrading to a 200A main panel with a 200A main breaker and appropriate busbar rating will give you much more flexibility.

Important Considerations



AHJ (Authority Having Jurisdiction)

Always consult with your local AHJ (e.g., building inspector) before making any changes to your electrical system. They have the final say on what's acceptable.

Qualified Electrician

Hire a qualified electrician experienced with PV installations. Electrical work can be dangerous, and proper installation is crucial for safety and code compliance.

Load Calculations

A thorough load calculation for both panels is necessary to ensure the system is properly sized and safe.*

NEC: 705.12(A) This section describes the requirements for the sum of all overcurrent devices (OCD) on the busbar not to exceed the ampacity of the busbar.

In summary, you are correct that a 30A PV backfeed into a sub-panel ultimately connected to a 100A main panel exceeds the allowable backfeed limit for the main panel busbar, thus creating an overload. Flag for review