You've got some good thinking here, but let's break it down and address potential issues to ensure compliance and safety

Potential Problems and Considerations:

1. Load Calculation at the Main Service Panel (MSP)



125% Rule for PV

The 125% rule applies at the main service panel. You correctly identified that a 40A PV breaker contributes (40A 1.25 = 50A) to the service calculation. So, your main panel must be rated to handle the existing load plus that 50A.

Crucially, you MUST account for ALL loads at the MSP, not just the HVAC. Other loads like lighting, appliances, etc., need to be considered to ensure the 200A service is adequate. NEC 705.12(B)(2)(3)(a)

This section specifically addresses the 125% rule related to backfed breakers (like PV). It says the sum of the overcurrent devices supplying power to the busbar (including the PV breaker) CANNOT exceed the busbar rating plus 125% of the power source on the opposite end of the busbar (e.g., the utility feed). In your case, if your MSP busbar is rated for 200A, and the utility feed is 200A, then the sum of your breakers connected to it is: 200 (busbar) + (200 1.25) = 450a!2.

Subpanel Ampacity and Conductor Sizing



6 AWG and 60A

6 AWG copper conductors are typically rated for 55A in the 60°C column, 65A in the 75°C column, and 75A in the 90°C column. (NEC Table 310.16). The 75°C and 90°C columns are generally applicable, and will always be provided by NEC regulations for this application. It is critical that the termination point for the 6 AWG is listed for that temperature or higher, or the value in the 60°C is used.

Subpanel Overcurrent Protection

You're planning to feed the subpanel with a 60A breaker at the MSP. This protects the 6 AWG wire.

Subpanel Busbar Rating

Your 100A subpanel must have a busbar rating of at least 100A. This is typically standard for 100A subpanels.3.

Neutral and Grounding in the Subpanel



4-Wire Feed

You MUST run a 4-wire feed (L1, L2, Neutral, Ground) from the MSP to the subpanel. This is code and essential for safety.

Separate Neutrals and Grounds in the Subpanel

The neutral and ground buses in the subpanel MUST be kept separate. The neutral should be isolated from the enclosure. The ground bus should be bonded to the enclosure.4.

Rapid Shutdown for PV



NEC Requirements

Ensure your PV system complies with the latest NEC requirements for rapid shutdown (especially 690.12). This often involves having a disconnect switch readily accessible to firefighters. Your relocated HVAC disconnect box might not fulfill the PV rapid shutdown requirements. This should be a separate labeled disconnect.
5.

Local Codes and Permits



Permits and Inspections

This work requires permits and inspections from your local building department. Contact them before starting any work. They may have specific requirements or interpretations of the code.6.

Feeder Demand Factor



Applicable? If the 6 AWG feeding the subpanel also feeds other loads BESIDES the HVAC and the PV system, a feeder demand factor may apply that would lessen the required size of the wire. It doesn't sound like this is the case, but it's worth mentioning as a general principle.

Recommended Actions


Comprehensive Load Calculation

Perform a thorough load calculation for your entire MSP. Include EVERYTHING connected to it. This is the MOST important step.2.

Consult with a Qualified Electrician

Even with DIY knowledge, it's highly recommended to consult with a licensed electrician. They can review your plans, perform the load calculations, ensure code compliance, and handle the permitting process. This is especially crucial for PV systems and subpanel installations.3.

Verify Wire Ampacity and Termination Temperature

Check the wire type (THHN, THWN, etc.) and the temperature rating of all terminals. Ensure you are using the correct ampacity rating from NEC Table 310.16, based on the lowest-rated temperature for terminations.
4.

Choose Appropriate Equipment

Select a properly rated subpanel, breakers, and other components.5.

Plan for Rapid Shutdown

Design your PV system to meet the latest rapid shutdown requirements.

In Summary:

Your idea has potential, but it hinges on a proper load calculation at the MSP and adherence to all applicable NEC codes. The 6 AWG wire and the 60A breaker might be sufficient for the subpanel feed, but thorough verification is essential. The PV backfeed at the MSP needs to be carefully considered. Professional consultation is HIGHLY recommended. DO NOT proceed without it. Electrical work can be dangerous, and non-compliance can have serious consequences. Flag for review