Why replace the pump? So you can use 480? Do they make 480 volt single phase pumps? No? That means a conversion to three phase. Static, rotary or VFD. $s keep adding up.
It is all numbers.
If you go to a 480V three phase pump, and add a voltage doubling VFD, then you are paying more for the pump and controller. (much more when you consider that the existing pump is already paid for and thus 'free').
But a 480V three phase pump means far smaller/cheaper wire. At 1800 feet that difference could pay for the new pump.
It is stupid to throw $2000 at a pump and controller to save $500 in wire. But it is equally stupid to throw an extra $2000 in wire to save a $1600 pump.
As someone else stated: there are going to be lots of ways to skin this cat. Lots of trade-offs, eg. allowing higher voltage drop to save on wire costs.
I personally would not design for more than 10% voltage drop running, even with a boost transformer helping out. But I would certainly compare the wire cost vs cost of electricity 'lost' by having a high voltage drop. Using 6 AWG Al for this single phase pump is IMHO borderline, but probably good enough. And loads cheaper than the wire originally being discussed.
(Side note: voltage drop calculations often use the resistance of wire at 75C. But wire used for a long run is oversized vs ampacity, and will run cool. The resistance used for voltage drop should probably be about 20% lower, meaning that if the calculator says you get 10% drop, you probably only have 8% drop.)
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