Okay, let's break down 705.12(B)(2)(3)(c) and how it interacts with your scenario in the 2017 NEC, along with the impact of monitoring and surge arrestors.

Understanding 705.12(B)(2)(3)(c) (2017 NEC)

This section of the NEC addresses the situation where you're connecting PV output circuits to a distribution equipment busbar that is not marked for backfeed. The key limitation is that the sum of all overcurrent devices (OCPDs) supplying power to the busbar (both utility and PV) cannot exceed the busbar's ampacity.

Your Base Calculation You have a 400A busbar. You plan to use 20A OCPDs for each PV circuit. Without anything else connected, 20 circuits 20A/circuit = 400A. This appears to reach the limit of the bus bar.

Impact of Monitoring and Surge Arrestors

Here's how the monitoring and surge arrestor circuits factor in, considering the code and best practices: Monitoring System

A monitoring system, while not directly contributing power to the bus bar, is considered a load and must be accounted for in your load calculation. The dedicated 20A breaker supplying this monitoring circuit does count against your available busbar capacity according to NEC. This is because the monitoring system is drawing power from the bus bar to function. Therefore, you're correct; you'd likely need to reduce one of your PV circuits (20A) to accommodate the 20A monitoring breaker.

Surge Arrestor (SA)

This is where it gets a bit more nuanced.

NEC Perspective (2017 NEC)

Generally, a surge arrestor is not considered a continuous load in the same way as a monitoring system. It's a protective device that primarily draws current during a surge event. While it does draw a very small amount of leakage current continuously, this is typically negligible and not a factor in this 705.12(B)(2)(3)(c) calculation. The NEC isn't concerned with the very small leakage current.

Practical Considerations

Even though the NEC probably doesn't mandate counting the SA breaker against your available PV circuits, a couple of best practices should influence your decision:1.

Derating

Consider the derating factors of the equipment. If you are approaching the max busbar rating you can derate to 80% to get the rating as well.2.

Future Expansion

If you anticipate any potential future load increases or additions, it's prudent to leave some extra capacity.3.

Code Interpretation

While the leakage current is negligible, an inspector could argue (though unlikely in most jurisdictions) that since the SA is connected, the breaker providing its power technically contributes to the total connected OCPD capacity. This is a very conservative interpretation.

Recommended Approach

Given your situation, I recommend the following:1. Consult with the AHJ (Authority Having Jurisdiction)

This is the most important step. Different jurisdictions have different interpretations. Ask them specifically if they require surge arrestor breakers to be counted in the 705.12(B)(2)(3)(c) calculation. Get their answer in writing, if possible.
2.

Assume the Most Conservative Scenario (Unless Told Otherwise)

To be safe and avoid potential issues during inspection, initially assume that the SA breaker does count against your available PV capacity. In your scenario, this means: 400A Busbar 20A Monitoring System Breaker 20A Surge Arrestor Breaker Leaves you with 360A for PV circuits. 360A / 20A per PV circuit = 18 PV circuits maximum.3.

Consider a Slightly Larger Busbar (If Possible)

If the cost difference isn't prohibitive, consider upgrading to a slightly larger busbar (e.g., 450A or 500A). This gives you more headroom and flexibility for future additions without needing to sacrifice PV capacity. Make sure to also use a distribution block to distribute power correctly.4.

Document Everything

Keep detailed records of your calculations, load analysis, and any communication with the AHJ.

Summary

While the 2017 NEC doesn't explicitly require counting the surge arrestor breaker against your PV capacity in this calculation (due to negligible leakage current), it's best to err on the side of caution and confirm with your local AHJ. Including it in your initial calculation provides a safety margin and reduces the risk of rejection during inspection. The monitoring system breaker definitely* counts. Finally, consider upgrading the busbar size if feasible to provide greater flexibility. Flag for review