You've raised a good point about applying 220.5(B) to microinverter output circuit calculations, and it's a source of some debate. Here's a breakdown of the considerations and my interpretation

The Short Answer:

While you present a logical argument, applying 220.5(B) to these calculations is generally NOT permitted by the NEC. Most inspectors will require you to use the unrounded values.

Here's why



Specificity of 705.28

Article 705 is very specific about how to calculate the ampacity of conductors and the OCPD size. The language in 705.28(A) and (B) that says "Where not elsewhere required or permitted in this code" is intended to prevent double counting or conflicting rules. It's generally not intended to allow you to override the specific calculations within 705 with a more general rounding rule. Think of 705 as a self-contained calculation method for PV system circuits.

220.5 Applicability

Article 220 focuses primarily on branch circuits, feeders, and service calculations for general electrical loads. While the math is similar, the intent is different. Article 705 is specifically designed to account for the unique characteristics and safety considerations of PV systems.

Safety Margin

The 1.25 multiplier in 705.28(B) and the requirements for OCPD sizing are intended to provide a safety margin. Reducing the current before applying the 1.25 multiplier erodes that safety margin.

Installer Responsiblity to do it Right

As you demonstrated in your write up, with just a few additional micro inverters, the calculated current increases and thus the conductor ampacity increases to a higher standard conductor size. As such, it is your responsibility to install a safe and code compliant electrical system, and its not difficult to add one additional microinverter and move up to the larger conductor and breaker.

Why your logic is appealing (but ultimately likely rejected by inspectors)



Consistency

Your argument for applying 220.5 to promote consistency across the code is understandable.

Cost Savings

You're right, the difference in equipment cost can be significant.

Common Interpretation and What to Expect



Inspectors will likely require you to use the unrounded values from 705.28(A) and (B). This means, in your 18 microinverter example: Calculate the total current from 705.28(A) as 24.3A. Calculate the conductor ampacity from 705.28(B) as 30.38A. Therefore, you need a conductor rated for at least 30.38A. This usually requires a #10 AWG conductor with a 75°C ampacity rating (typically rated at 30A, but you need to ensure it's rated for at least 30.38A). The OCPD (backfed breaker) must be rated at least 24.3A, but not more than the ampacity of the conductors. You would likely use a 35A breaker.

Key Considerations and Recommendations



Local AHJ



The final authority is your local Authority Having Jurisdiction (AHJ). Call them and ask for their specific interpretation before you design the system. Get their ruling in writing if possible. Manufacturer's Instructions

Always follow the manufacturer's instructions for the microinverters and the overall system. They may have specific requirements that supersede general code rules.

Conservative Design

When in doubt, err on the side of a more conservative design (larger conductors, higher-rated OCPD). This ensures safety and code compliance.

In summary, while the argument for applying 220.5(B) has some merit, the general consensus and most common enforcement practice is to apply the specific calculations in Article 705 without rounding down initial currents, as doing so can be interpreted as compromising the safety margins built into the PV system design requirements. Always consult with your AHJ for their specific interpretation. Flag for review