Okay, let's break down your situation and address your questions regarding the bus duct and PV connection according to the 2017 NEC.

Understanding the NEC Requirements (705.12(B)(2)(3)(a))

You've correctly identified the relevant NEC section. It deals with the overcurrent protection requirements when connecting PV systems to the load side of service equipment. The calculation 1.25 x PV output current + existing overcurrent device rating < busbar rating is the key to determining if you can make the connection.

Analysis of Your Situation Bus Duct Rating

800A

Basement Breaker

600A

PV Output Current

120A

Calculation

(1.25 120A) + 600A = 150A + 600A = 750A.

Conclusion

750A < 800A. Based on the information you've provided, the calculation satisfies the NEC requirement of 705.12(B)(2)(3)(a).

Addressing Your Questions(1) Does the plug-in unit on the bus duct have OCPD?

This is the critical question and depends entirely on the specific plug-in unit you intend to use. Not all plug-in units have overcurrent protection.

Check the Plug-in Unit Specifications

You

MUST consult the manufacturer's specifications and documentation for the specific plug-in unit you plan to use. Look for terms like "fused plug-in unit," "circuit breaker plug-in unit," or something similar. If it doesn't explicitly state it has overcurrent protection (fuse or breaker), then it doesn't.

If No OCPD

If the plug-in unit does not have OCPD, you

cannot rely on it for protection of the conductors from the bus duct to the inverters. You will need to provide a separate overcurrent protective device (fuse or breaker) in an enclosure (disconnect). This will likely negate your space-saving strategy and bring you back to the problem of finding space for the disconnect with the required working clearances. This is a very common situation when working with bus ducts.(2) Is the plug-in unit good for backfeed?Again, this depends on the plug-in unit.

UL Listing and Backfeed Rating

The plug-in unit

must be listed for backfeed or reverse current flow. This will be indicated in the manufacturer's specifications. Typically you will need a breaker plug-in unit rather than a fused one.

UL 1741 and Inverters

Your inverters must be UL 1741 listed for grid interconnection. This is a requirement for connecting to the utility grid.

Check the Bus Duct Nameplate

Review the bus duct nameplate again, but you're primarily looking for information about the plug-in units. The bus duct itself is likely rated for reverse current flow, but the plug-in unit is the key component.

Important Considerations and Next Steps1.

Manufacturer Documentation

The most crucial step is to obtain the

exact model number and manufacturer of the plug-in unit you intend to use. Then, get the product specifications directly from the manufacturer's website or by contacting their technical support. Do not rely on assumptions or general information.2.

Working Space

You are right that 110.26 is applicable. Pay close attention to the required working clearance (depth, width, and height) in front of electrical equipment. This can be a significant constraint.
3.

Alternatives



Larger Plug-in Unit with OCPD

Consider if there's a physically larger plug-in unit that does include an appropriately sized breaker. This may still be more compact than a separate disconnect.

Basement Connection

If space is absolutely critical, re-evaluate the possibility of connecting the PV system on the load side of the 600A breaker in the basement. While this may involve longer conductor runs, it could be the only option if you cannot find a suitable plug-in unit and don't have space for a separate disconnect.

Dedicated Overcurrent Protection (OCP)

If there is no plug-in with overcurrent protection, you will need to install a fusible disconnect switch or circuit breaker to protect the conductor from the bus duct to the inverters. The overcurrent protection must be sized appropriately for the ampacity of the conductor and the rating of the inverters. The rating of the overcurrent protection should follow the rules in 690.9 for PV systems. This is separate from the 600 amp breaker that feeds the bus duct.
4.

AHJ Consultation

Discuss your proposed design with your local Authority Having Jurisdiction (AHJ) before starting the installation. They may have specific interpretations or requirements that you need to meet. Get their approval in writing.5.

Engineering Review

Have a qualified electrical engineer review your design and calculations to ensure they meet all applicable codes and standards.

In summary, your primary focus should be on the specific plug-in unit and verifying its OCPD and backfeed capabilities. Without that information, you cannot proceed safely or in compliance with the NEC. Flag for review