Can a cord and plug attached to a wallbox be used as a transfer means for fixed wiring downstream of the box?
In my area the inspectors require 156% wire size (125% x 125%) if a flexible cord is used as a non permanent connection. Something like a portable generator.
The other requirement is it needs to be 30amp or less. With that said there is usually a transfer switch to the specific circuits being powered.
Larger supplies are required to be in 6’ or less liquid tight. All parts must be listed for the use or type of power generated (like Solar, motor driven generator).
Some inspectors are easy some are real buggers. I have set a couple dozen of these up over the years and the biggest hassle is proving the grounding to show if it is a separately derived system. (I mentioned Solar since the comment about DC). Just another note the 156% wire size is on fabricated cords not the ones that come with the transfer switch that are molded.