Besides an AHJ requiring 3rd party inspection and labeling are there any requirements when 3rd party inspection and labeling is required? For example say you have an existing rooftop unit with a UL listing from the manufacture. Say the scope of work is to replace the existing motor with an inverter duty rated motor connected to a new external VFD and also replacing the existing controls with new. My understanding of UL is that once you modify the internal workings you void the UL label. Is that correct? Is/are there any code(s) that states that the equipment has to be re inspected and labeled?
If you have to pull a permit for the work, it will be inspected and the inspector may require re-certification. The rules on when and when not a permit is required to retrofit existing equipment is totally local to your jurisdiction. If you ask them up front, they will almost invariably say yes. If you don't ask up front, and an inspector drives by and sees you working, he will likely interpret that you
did need a permit because you made him get out of the car (at least, that's my experience). If he can't see you working, it's the "honor system".
The other circumstance related to UL/NRTL listing has to do with insurance. UL stands for Underwriter's Laboratories, as in
Insurance Underwriters. Most industrial and commercial insurance policies will have some sort of clause buried in them that says, to the effect, if a fire or loss is traced back to a piece of unlisted equipment, the costs associated with the loss may not be covered. In other words, the insurance adjuster will be looking for a way out, making major modifications to something like an Air Handler without getting it inspected and approved is like handing him the golden ticket. But again, only if he can prove that THIS UNIT caused the damage. Lot's of people are willing to take this risk, especially when they see the quote from UL or some other NRTL to come out and re-inspect it.
