I use the diameter of the conductor from page 685 table. By experience, 12" wide would be the smallets width.
The key word above is 'experience'. As to going to the 'Code' MINIMUM size calcs, in the real world that I work within, that does not happen. Six data centers, and the minimum width is 24" to 36" tray. Looks sort of 'dumb' sometimes with only two (2) fiber within inner ducts.
Also, I wonder if the person that created the question looked at any of the mfg. documentation, and saw that the smallest tray for power use is 12", based on weight of the 500KCMil.
