Voltage drop question

[Johnablessed]

I have a run of conduit 172ft from a 12x12 box which branches out to parking equipment with runs about 50ft from the box. My voltage drop calc requires me to run #10s to the 12x12 box. My question is from that box can I size down to the respected size of wire my machines need (#14awg) to branch out to the equipment? The runs from the box to the machines is max 50ft. Also another question I have is when derating for conduit fill as per table 5c, do u derate after your voltage drop calc or before it. My thought process is you get the newly acquired amapcity of your wire from your voltage drop calc and then you proceed to take that ampacity and derate it the percentage you get from table 5c. Am I correct on this? Thanks