CMP-8 says it's all one wireway, or at least they rejected changing the Code to say otherwise
Well, making it so the 30 conductor limit applies to each compartment of a partitioned wireway was proposed as a Code change for the 2014 cycle and roundly rejected by CMP-8, so I think that's a definitive "one wireway" answer to my question, or at least the closest I'll get so far (quoting modulo markdown limitations from this NFPA doc):
8-134 Log #798 NEC-P08
Final Action: Reject
Submitter: Paul E. Guidry, Fluor Enterprises, Inc. / Rep. Associated Builders and Contractors
Recommendation: This is an updated proposal to one previously sent. Part of the text was left out. Please reject the previous proposal.
Add new text to read:
376.22 Number of Conductors and Ampacity. The number of conductors and their ampacity shall comply with
376.22(A) and (B).
(A) Cross-Sectional Areas of Wireway. The sum of the cross-sectional areas of all contained conductors at any cross
section of a wireway shall not exceed 20 percent of the interior cross-sectional area of the wireway.
(B) Adjustment Factors. The adjustment factors in 310.15(B)(3)(a) shall be applied only where the number of
current-carrying conductors, including neutral conductors classified as current-carrying under the provisions of
310.15(B)(5), exceeds 30. Conductors for signaling circuits or controller conductors between a motor and its starter and
used only for starting duty shall not be considered as current-carrying conductors.
For the purposes of this Section, wireways with metallic dividers installed constitutes separate wireways.
Substantiation: Wireways are often being used with dividers that are a listed part of the equipment. Often these divided
sections contain up to 30 current-carrying conductors in each divided section. Where dividers are used, this proposal
would allow each divided section of the wireway to be counted as individual wireways and allow up to 30
current-carrying conductors and 20% fill without derating of ampacities.
Panel Meeting Action: Reject
Panel Statement: CMP-8 acknowledges that the use of internal barriers in wireway is not prohibited. The submitter
has not provided adequate technical substantiation to permit increasing the number of conductors in a divided metal
wireway above 30 without applying the provisions of Section 310.15(B)(5).