Will my plan work to temporarily power an irrigation pump?

Question

Parents are rebuilding a house and we need to keep running the irrigation pump to keep the 2 acres of St. Augustine grass alive.

The pump is 5HP single phase Starite DSeries DMG) and the manual says it pulls about 28 amps full load.

Originally the line was 2 gauge aluminum running from the panel inside the house down the 200 ft to the pump. We now need another 160 ft to reach the construction temporary panel setup.

We have 200 ft of an 8 gauge 3 conductor wire that was used to power an RV with 50 amp service,

Question is, I'm wondering if its safe enough "for now" to use that 8 gauge copper 3 conductor to reach the 2 gauge aluminum cut off point.

So basically I hook up the 8 gauge copper, running 160 ft, to the 2 gauge aluminum (using appropriate AlCu connectors)

Connection would be:

1. Construction service panel
2. 35 Amp double pole breaker
3. #8 Cu 2 conductor+G [160ft]
4. #2 Al 2 conductor+G [200ft]
5. 5hp 220V irrigation pump using 28 amps

This is only temporary, and will get a proper code compliant solution put in once final inspection is due.

Answer 1

This will be no problem with those wire sizes.

160’ of copper#8 with 28 amps is 2.58%

200’ of aluminum #2 will be 2.13%

So a total voltage drop of 4.71% this is below the NEC recommended 5%. Note the actual voltage drop at startup will be higher but a standard water pump should be fine with class 10 overloads (standard fuses and breakers are usually ~ class 10 or can handle motor starting currents 3-5x there rating for 10 seconds.

I would bet that pump will be to speed in 1 second with that wire possibly 2 seconds but well below the trip value. So it should work.