John, as most know Ive been retired from AC Power Distribution Engineering for yearsssssss and am rusty on the latest NEC so NO WARRANTY, but that being said here was my design approach back in my day:
1) I computed the maximum continuous load.
2) I sized the conductors to have a rated ampacity of at least 125% of the computed max continuous load.
3) I performed a voltage drop calculation based on amps and circuit length and if necessary, I upgraded the conductor size
4) I sized my Overcurrent Protection Device (fuse or breaker) to adequately protect the feed wires.
5) If I were feeding a sub panel (instead of say a single load) I sized the conductors with a matching minimum ampacity of the sub panels rating.
I always considered buried PVC as a Wet or Damp Location..........I had always used THHN for dry and THWN for wet or damp locations HOWEVER my good friend and retired electrician Dusty MI says they now have the same specifications
6) AS far as Voltage Drop, I would consider the total length (Main to sub PLUS sub to motor) and upgrade the size if necessary to reduce voltage drop. THE MAIN CALCULATION concerns what the final voltage will be at the load once all the runs (main to sub plus sub to load) and size the wires accordingly.
7) If in doubt or even close, to be safe rather then sorry and to avoid having to do it twice and allow for additions, I would up the wire size to the next higher ampacity.
8) If in doubt as to the environment and temperatures and associated ampacity ratings (like your charts with 3 different ampacities), I simply used the lower ampacity wire rating BUT THAT WAS JUST ME, IT MAY WELL HAVE BEEN (probably was) OVERKILL
John T Too long retired Electrical Engineer so see what the other experienced professional electricians and engineers have to say !!!!!!!!!!!!!!!! IM RUSTY ON THIS STUFF
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