Okay, let's break down this scenario and see if it violates NEC 705.12 (2020). This is a common and tricky situation.

Understanding the Components MSP

Main Service Panel (200A/200A – assuming 200A busbar)

Existing PV

30A backfeed breaker in the MSP.

New PV (Load Side Tap/Feeder Tap)

30A backfeed from the load side of the 100A feeder breaker going to a subpanel.

Subpanel

Fed from a 100A breaker in the MSP.

The Key NEC Section: 705.12 (2020)

You're correct to focus on 705.12. It's all about safely handling the increased current potential from interconnected power sources. The relevant sections for this scenario are primarily: 705.12(B) Supply-Side Source Connections. 705.12(D) Load-Side Source Connections.

Because you're proposing a load-side tap, 705.12(D) is where we need to drill down. 705.12(D) has several requirements, including: 705.12(D)(2) Feeder Tap Connection

This section directly addresses your load-side tap situation. It dictates the conditions under which you can connect a power source to a feeder. It has two conditions to satisfy, either 705.12(D)(2)(3)(a) or 705.12(D)(2)(3)(b)

Analysis

Here's the breakdown of how the code applies to your situation:1. 705.12(D)(2): Feeder Tap Connection This is where you are landing the connection, at the load side of the 100A subpanel feeder. You must satisfy the two conditions of 705.12(D)(2)(3). This is where you must take an approach based off of either 705.12(D)(2)(3)(a) or 705.12(D)(2)(3)(b).
2. 705.12(D)(2)(3)(a): Feeder Ampacity The sum of the overcurrent devices supplying power to the busbar or conductor shall not exceed the ampacity of the busbar or conductor. So you must add the breaker that supplies the feeder with the new backfeed amperage. 100A (Feeder Breaker) + 30A (new PV backfeed breaker) = 130A Since this is under the ampacity of the feeder conductor, we can proceed.3. 705.12(D)(2)(3)(b): Overcurrent Device This is going to be your limiting factor, as the other condition will likely pass. This requires the overcurrent device to be rated for not less than 125% of the new systems backfeed. 30A 125% = 37.5A, which is not a standard breaker size, so you would round up to 40A for protection. The existing 100A sub panel feeder breaker is not rated for this 40A, therefore you cannot satisfy this condition, and a feeder tap is not allowed.4. MSP Breaker Sum Limitation (705.12(B)(2))

This section limits the total of all supply breakers (including PV) to not exceed 120% of the service rating. In your case: 200A (service rating) 1.20 = 240A You have: 30A (existing PV) + 200A (MSP) = 230A Adding the new system (30A) would bring the total to 260A.

Conclusion

Based on the information provided, this interconnection likely violates NEC 705.12 (2020) for the following reason: 705.12(B)(2)

The sum of overcurrent devices supplying power to the busbar exceeds 120% of the service rating. 260A > 240AThis situation would likely not pass inspection.

Recommendations1. Confirm Busbar Rating

Double-check that the MSP busbar is actually rated for 200A. Sometimes the breaker is 200A, but the busbar is lower.2.

Reduce Breaker Sizes

If possible, could the existing or new PV breaker sizes be reduced? This might bring the totals into compliance. However, be very careful not to undersize the OCPD for the inverters and wiring.3.

Supply-Side Connection

A supply-side connection (breaker in the MSP) might be possible, but only if you can comply with the 120% rule.4.

Line-Side Tap

Instead of tapping off a feeder, a line-side tap may be possible. This would require a suitable tap box and careful consideration of overcurrent protection and conductor sizing, and compliance with NEC 230.82 and other applicable sections. This is often a more expensive and complex solution.
5.

New Service

In some cases, upgrading the service is the only code-compliant option.6.

Load Shedding/Export Limiting

If you can implement certified load shedding or export limiting on one of the PV systems, you might be able to get this to pass. This requires specialized equipment and careful programming, and may not be allowed by the utility.

Important Notes AHJ Ruling

Ultimately, the Authority Having Jurisdiction (AHJ – local inspector) has the final say. Discuss this scenario with them before starting the work. They may have local interpretations or requirements.

Single-Line Diagram

Create a detailed single-line diagram showing all components, breaker sizes, wire sizes, conduit fill, and equipment ratings. This will be essential for the inspector.*

Engineering Stamp: Depending on the complexity and AHJ requirements, you might need a professional engineer (PE) to review and stamp the design.This is a complex area, and ensuring code compliance and safety is paramount. Don't proceed without a thorough understanding of the requirements and approval from the AHJ. I hope this helps you navigate this challenging interconnection! Flag for review