We are trying to reuse the existing conductors and are checking their ratings and voltage drop.

Regarding the voltage drop, a 30A feeder would have needed #8 with a #8 ground (aluminum). Upsizing the phase conductors to 1/0 would have required a #1/0 ground due to 250.122.B. Therefore, it appears the ground is undersized.

Possible solution:

Since we are putting in a new pump control panel, we have the ability to customize the panel. The plan would be to change the main feeder breaker to a 50A breaker (instead of 30A) and have a 50A breaker main disconnect in the pump panel. The 50A feed would initially need #4 phase and #10 ground (aluminum). With that being the starting point, looking at the load of 15 amps, the #4 would still need upsized. Therefore, they have been upsized from #4 to #1/0. Checking the ground, the new ground would only need to be a #4 which the installed #2 would just be an improvement.

Here is the math for 50A feeder: circular mills:

(new conductors/base conductors) x base ground.

(105600/41740) x 10380 = 26261 CM for new ground = installed #2 meets this.

Does this approach appear viable and I guess my confusion is why with the lower ampacity (30A) VD correction a larger ground is required, but if I increase the circuit ampacity (50A), the VD corrected ground is smaller?