What section of the NEC covers wiring methods inside large, walk-in air handlers? 300.22 does not seem to apply (I don't have my codebook with me so I trying to do this from memory).
300.22A doesn't for obvious reasons.
300.22B doesn't seem to since the prewired motors inside are fed with nonmetallic sheathed metal flex (not allowed in a duct or plenum due to their potential as a product of combustion and possible contributor of smoke).
300.22C doesn't seem to since, according to the handbook notes, areas manufactured specifically for air handling don't apply.
Inspectors at numerous jobsites are allowing set-screw emt connectors and metal sealtight inside these large units. They also aren't requiring duct-seal or other type of air stop where conduits penetrate the unit (thus allowing air to blow out of these pressurized rooms). Every other seam or door is fully sealed.
The pre-wired motors don't have disconnects within site of them and (as stated previously) are fed with metal sealtight. Can the rest of the motors be wired the same? How can I get by with adding a product of combustion to a duct/plenum and not having a motor disconnect within site?
Can sealed, plastic cased, fluorescent lights be used inside or are "jelly-jars" on metal boxes needed? Are set-screw fittings ok? Don't the penetrations need to be sealed?
Nobody can tell me what section of the NEC says what can and can't be done inside an air handler? These things are EXTREMELY common...not one-off oddballs. I can't believe there isn't a specific article dedicated to them. What keeps me from packing an airhandler full of combustible materials? Perhaps these are UL listed units with specific instructions from the factory on what is allowed which would explain why the NEC is silent on the subject.
I remember now that disconnects are not needed within sight of the motors since they are fed from VFD's (430.102(B)FPN#1). 300.22(B) almost has to apply. It allows "enclosed gasketed-type luminaries" so the sealed plastic fluorescents must be ok. It obviously makes no sense that the code doesn't allow plastic coated metal flex due to combustability concerns but lets numerous plastic fixtures be installed even though readily available jelly jars would add no possibility of smoke. The factory wired motors obviously don't have to follow the NEC as long as their wiring methods get past the UL inspectors.
I don't really see how the NRTL listing has anything to do with alterations to that product after it is installed. I do agree the code seems fairly quiet if not silent but there are some articles you could apply.
300.22(B) is right on.
This might apply if there is a significant temperature delta
300.7 Raceways Exposed to Different Temperatures. (A) Sealing. Where portions of a cable raceway or sleeve are known to be subjected to different temperatures and where condensation is known to be a problem, as in cold storage areas of buildings or where passing from the interior to the exterior of a building, the raceway or sleeve shall be filled with an approved material to prevent the circulation of warm air to a colder section of the raceway or sleeve. An explosionproof seal shall not be required for this purpose.
You wouldn't want it raining in your boxes if this is A/C. If nothing else you don't want it sucking dirt in and packing the box. Sealing around penetrations will be important to the HVAC tech I imagine, if nothing else to keep it from whistling and causing a callback.
And I wouldn't be surprised if omitting a safety switch ends up being a callback to retrofit one.
FPN's aren't enforcable as Code...they're just notes.
The only thing that I've ever done differently with a VFD is using a safety switch that has secondary contacts that open a fraction of a second before the mains...to let the VFD know not to freak out when it suddenly loses its motor load.