Search a voltage drop calculator (google it) and plug in the numbers. Check what the specs are on the guzzler, not just the generic voltage. Stay at least 2V above what it will run at. To check it, use a multi meter to know what your starting voltage is, don't assume it is 110 or 120. My voltage dances back and forth from about 121 to 125. Check at several times of day and even days of the week. Whatever the lowest voltage reading is will be your starting voltage in the formula for voltage drop. In the pump literature check what the voltage minimum is that it will run. If the starting V lowest supply reading is let's say 119 and the pump needs at least 108, I suggest you use 110 in the formula (108+2V). Thus if your supply is 122 you can use a longer length than if it is 118. Realize also that every time you add a length using a plug in or even wire nuts, you lose a little in the connection. You will be best off using one long wire for your supply rather than joining 2 or 3. If you do need to join any, I strongly suggest you use some dielectric grease on the connection for a better transfer. The one most utilities and solar engineers use is NO-OX-ID A SPECIAL made by Sanchem Inc. I use it on all electrical connections on my solar and for more demanding loads.
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.