You're in a tricky situation and the AHJ's interpretation is debatable. Here's a breakdown of the relevant code sections and considerations

Key Code Sections:
NEC Article 314: Outlet, Device, Pull and Junction Boxes; Conduit Bodies; Fittings; and Handhole Enclosures

This is the article the AHJ is referencing. Its applicability hinges on whether the enclosure is considered a pull box or not.

NEC Article 376: Metal Wireways

This is the article you believe applies.

NEC 376.2: Definition of Wireway

Sheet metal troughs with hinged or removable covers for housing and protecting electric wires and cable and in which conductors are laid in place after the wireway has been installed as a complete system.
Arguments for it being a Wireway



"Laid in place"

The definition of a wireway emphasizes that the conductors are laid in place after the wireway is installed. Your description implies this is the case. The conduits enter the wireway and then the conductors are individually routed and terminated into the panelboards.

Function

Wireways are intended for continuous runs of conductors. If the primary function of the wireway is to support the conductors for the terminations at the panels and not to pull the conductors through to the end of the wireway, this supports your argument. You are not simply pulling the conductors straight through the enclosure as if it were a junction box. You are manipulating and terminating them within the wireway.

Arguments for it being a Pull Box



Conduit Runs

The AHJ is focusing on the 2.5" conduits entering and exiting (via nipples) the enclosure, implying a "through pull" scenario. They are applying the 8x conduit diameter rule for straight pulls in 314.28(A)(1).

Accessibility

If access to the conductors within the wireway is primarily for pulling conductors into the panelboards (rather than for maintenance or modifications along a continuous run), the AHJ might consider it a glorified pull box.

Why the AHJ Might be Wrong (Or Why Your Argument Could Win)



Intent of the Code

The 8x rule in 314.28(A)(1) is specifically for situations where conductors are being pulled straight through an enclosure. If the conductors are not being pulled directly through, but rather are being routed and terminated within the enclosure after entering through the conduit, the rule may not apply.

Practicality

A 20" high wireway would be significantly larger and potentially impractical in your installation.

What You Need to Do


Document Everything

Take very detailed pictures (when you are able to upload them), clearly showing the conduit runs, the wireway, and the panelboard connections. Make a sketch of the installation, noting the exact routing of the conductors.2.

Clearly Explain Your Intent

Write a detailed explanation to the AHJ, explicitly stating that the wireway is not being used as a simple pull box. Emphasize that conductors are being laid in place after the conduits enter the wireway and that they are being individually routed and terminated.3.

Reference the Code (Carefully)

Don't just say "it's a wireway." Explain why* it meets the definition of a wireway according to NEC 376.2 and why the 8x rule in 314.28(A)(1) is not applicable because it is not a straight through pull of conductors.
4.

Compromise (Maybe)

If the AHJ is still hesitant, consider if there's a way to slightly increase the wireway height (if space allows) while still arguing that it's a wireway. This might demonstrate a willingness to work with them.5.

Appeal (If Necessary)

If you truly believe your interpretation is correct and you've exhausted all other avenues, explore the possibility of appealing the AHJ's decision. This is a last resort, as it can be a time-consuming and expensive process.

In summary: You have a legitimate argument. Your success will depend on your ability to clearly and persuasively demonstrate that the wireway is being used in accordance with its intended purpose and that the pull box rules are not applicable. The key is to emphasize the "laid in place" aspect and the fact that conductors are not being pulled straight through the enclosure. Good luck! Flag for review