You've hit a common and legitimate point of confusion with meter stacks and feeder busway installations, especially when dealing with large service sizes and multiple downstream branches. Here's a breakdown of the potential code compliance issues and possible explanations

The Problem:
368.17 Overcurrent Protection

This section generally requires the busway to be protected against overcurrent in accordance with its ampacity rating. Having a 2000A breaker feeding a 1200A busway clearly violates this on the surface.

Tap Rules (240.21)

You're right. Trying to apply tap rules here is problematic. The tap rules generally deal with conductors, and even then, your scenario appears to be tapping a tap, which is usually not allowed.

Possible Explanations & Considerations


Engineering Supervision and Calculated Load

This is the most common reason why these installations are allowed.



Detailed Load Calculations

A qualified engineer MUST perform very detailed load calculations for each apartment unit served by each meter, taking into account diversity factors and expected usage patterns.

Demand Factors

These calculations demonstrate that the actual maximum load on any 1200A or 800A section of busway will never exceed its ampacity, even under the worst-case scenario.

Supervision and Restriction

These are typically for larger multi-family installations that are under the supervision of qualified persons for proper maintenance and operation

Documentation

This detailed load study is crucial and MUST be submitted to the AHJ (Authority Having Jurisdiction) for approval. The manufacturer's "permissible configurations" are typically based on this type of engineering analysis.2.

Specific Busway Design and Listing



Listing

The busway assembly as a whole, including the main disconnect and the branch taps, might be specifically listed and labeled for this type of configuration. Look for markings that explicitly state the permissible configurations and the conditions under which they are allowed. The listing could incorporate specific OCPD requirements.

Manufacturer's Instructions

The busway manufacturer's instructions are critical. They may contain specific requirements for OCPD sizes and installation methods that are part of the product's listing.

High Fault Current Withstand

The busway sections are often designed and tested to withstand very high fault currents, even if the overcurrent protection doesn't trip immediately at 1200A or 800A. This withstand capability allows for selective coordination with downstream breakers.3.

Feeder Protection as a System (Selective Coordination)



Selective Coordination Study

A selective coordination study might be performed to demonstrate that downstream breakers will clear faults before the main breaker trips, thus protecting the smaller busway sections from prolonged overcurrents.

Purpose of Main OCPD

The main breaker's primary function in this case might be to protect the entire service entrance conductors and the main busing upstream of the feeder busways, rather than the individual feeder busway sections themselves.4.

AHJ Acceptance



Local Amendments

Your AHJ might have specific local amendments to the NEC that address meter stacks and feeder busways in multi-family dwellings.

Alternative Methods

The AHJ might accept an "alternative method" of compliance if you can demonstrate that the proposed installation is safe and meets the intent of the NEC. This requires detailed documentation and justification.

Important Considerations



"Per Manufacturer's Instructions" is Not a Blank Check

Simply saying "it's per the manufacturer's instructions" isn't enough. You need to understand what those instructions are based on (load calculations, listing requirements, etc.) and demonstrate compliance.

Load Growth

Consider potential future load growth in the apartments. Over-optimistic load calculations can lead to problems down the road.

Enforcement

AHJs have different levels of scrutiny. Some might simply accept the manufacturer's instructions without further investigation, while others will require detailed load studies and justification.

Action Steps


Obtain Detailed Manufacturer's Documentation

Get the complete installation instructions, listing information, and any engineering reports related to the busway assembly.2.

Perform (or Review) Load Calculations

Critically review the load calculations for each apartment unit. Ensure they are accurate, conservative, and account for diversity factors.3.

Consult with the Engineer

If you're not the engineer of record, discuss the design with them to understand the rationale behind the OCPD sizes.4.

Consult with the AHJ: Present your documentation to the AHJ before installation to get their feedback and approval. Be prepared to answer questions and provide additional information.In conclusion, while the scenario appears to violate 368.17 at first glance, it can be code-compliant if supported by detailed load calculations, specific busway listing and design, selective coordination studies, and AHJ approval. The key is to thoroughly understand the basis for the design and demonstrate that the installation is safe and meets the intent of the NEC. Flag for review