Okay, let's break down this situation. Your understanding of NEC 705.12(A) is correct, but the application to line-side taps can be a bit more complex.Here's a more detailed analysis and what you need to consider

Your Current Assessment:

You're right that `OCPD(micros) + OCPD(string inverter) < busbar rating` (30A + 25A < 100A) is a necessary condition, but it's not sufficient when dealing with line-side taps.

The Issue with Multiple Line-Side Taps

The fundamental problem is the potential for overcurrent in the conductors between the point of the taps and the main overcurrent protection device (the 100A breaker). The NEC doesn't explicitly forbid multiple line-side taps, but it significantly restricts their application due to safety concerns.

NEC 705.11 - Connection to Other Circuits

This section dictates the rules for parallel operation with other power sources. Because both PV systems feed the load side of the service disconnect, these systems are considered to be parallel sources. NEC 705.11 says that the sum of the breakers for all sources does not exceed the ampacity of the conductors or overcurrent protection device protecting those conductors.

Key Considerations and Requirements

1.

Feeder Tap Rules (NEC 240.21)

You must meet all requirements of NEC 240.21 when making these line-side taps. Specifically, you're likely relying on 240.21(B), which deals with feeder taps. The rules of 240.21 are intricate and often require careful engineering. Pay close attention to:

Tap Conductor Length

The length of the tap conductors is critical. There are strict limits based on the tap conductor's ampacity compared to the feeder's ampacity. If the taps are too long, they won't be protected from overcurrent. 240.21(B)(1) and 240.21(B)(2) are most relevant.

Tap Conductor Ampacity

The ampacity of the tap conductors themselves must be sufficient for the overcurrent protection they serve (the 25A and 30A breakers).

Physical Protection

Tap conductors need to be physically protected from damage.2.

The 120% Rule (NEC 705.12(B)(2)(3))

This is crucial. While you addressed 705.12(A) (busbar rating), you also need to consider the ampacity of the service entrance conductors themselves. 705.12(B)(2)(3) states that the sum of the overcurrent protection devices for the PV systems, plus the rating of the overcurrent device protecting the busbar (main breaker), shall not exceed 120% of the ampacity of the busbar or conductors. So, you need to know the ampacity of the service entrance conductors feeding the panel. Let's assume (for example) that your service entrance conductors are rated for 100A (same as the main breaker). Then: `100A (main breaker) + 25A (string) + 30A (micro) <= 1.2 100A` `155A <= 120A` This is NOT OKAY. You need to size your service entrance conductors correctly for the load.3.

Qualified Installation

Line-side taps must be installed by qualified personnel with a thorough understanding of the NEC and proper techniques. This is not a DIY situation. Incorrectly installed taps can create dangerous hot spots and fire hazards.4.

Listing and Labeling

The tap connectors themselves must be listed and labeled for the intended application. Don't use generic connectors that aren't specifically designed for line-side taps.5.

AHJ Approval

Always consult with your local Authority Having Jurisdiction (AHJ). They have the final say and may have specific requirements or interpretations of the code in your area. Get the plans reviewed and approved before starting any work.

Alternative Solutions (Highly Recommended)



Load-Side Connection

The safest and most common approach is to connect the microinverter system on the load side of the main breaker, using a breaker in the main panel. This avoids the complexities of line-side taps. You would then need to ensure that the total load on the panel (including the PV output) doesn't exceed the panel's rating. Load calculations are critical.

Service Upgrade

If your service entrance conductors and panel are undersized, consider upgrading them to a higher ampacity. This provides more headroom for PV systems and future loads.

Example of Why Line-Side Taps Can Be Problematic

Imagine your service entrance conductors are rated for 100A. You've got a 100A main breaker. You tap in two PV systems with 25A and 30A OCPD. Now, consider this worst-case scenario:

House is drawing 70A of load. Both PV systems are producing their maximum output (25A + 30A = 55A).

In this scenario, 70A is being drawn from the utility through the service entrance conductors, while 55A are also being drawn into those conductors. This means that the service entrance conductors, between the taps and the main breaker, are now carrying 70A + 55A = 125A. If those conductors are only rated for 100A, you've got an overload situation. The main breaker won't trip because it only sees the 70A from the utility. This is a dangerous condition.

In Summary

While your initial calculation satisfied the busbar rating, it's insufficient for line-side taps. You must meticulously comply with NEC 240.21 (tap rules), NEC 705.11 and 705.12(B)(2)(3) (120% rule), use listed equipment, and get AHJ approval. Connecting on the load side is usually a simpler and safer option.

Consult with a qualified electrician or electrical engineer experienced in PV systems before proceeding. They can perform the necessary calculations, ensure code compliance, and perform the installation safely. Flag for review