I tried reading through the Google newsgroup archives to see if maybe this
question had been asked and answered already, but as soon as I got to an old
post by Altavoz it became too painful to continue. So, here goes.

If you're running a motor load at the end of a length of wire, it seems easy
enough to use a table or a program to determine the appropriate gauge of
wire to use to keep the voltage drop within a desired range. But how do
you calculate what to use if the wire gauges change midway through the run?
Here's an example. Let's say you have a three horsepower, 240 volt, single
phase motor at the end of 300 feet of 10 gauge wire. From what I have come
up with when plugging those values into some of the online voltage drop
calculators, that's pretty much the maximum you'd want to do in order to keep
below a 5% voltage drop. But what if you have 200 feet of wire leading up
to the point where that 300 foot run of 10 gauge begins? It's easy enough
if you were using 10 gauge the whole way, but what if you had 200 feet of
2 gauge, then 300 feet of 10 gauge? How does that change things?

The above example is taken from a real-world installation of a submerged well
pump where the pump is 300 feet down, and there's a 200 foot run from the
meter to the junction box for the well pump. I'm wondering because not only
would it answer some questions about the efficiency of the well pump, but
also because in the future there would be maybe a 200 foot run from the
meter to a subpanel for a shop that would be running motors on lathes, mills,
etc. Granted, there wouldn't be a 300 foot run of cable from a lathe/mill
motor back to the subpanel, but I'm assuming the calculations would be done
the same way, just using different values for distances. Any bones that
could be thrown to a guy who was never very good at math would be much
appreciated.