I have to install a 1 1/2 HP sprinkler pump motor, 9.9 FLA @220V 350 feet from electrical panel.?

Question

What size conductor must I use. It needs to be UF underground feeder.

Answer 1

If I were you I rather to be electrically safe than trying to risk the use of inappropriate electrical conductors. The ampacity of #14AWG, is roughly 15-Amps and If your sprinkler pump motor started to run it draws more current which is beyond 15-Amps.

Just use bigger size of wire, #12AWG,  copper, THHN is the smallest possible conductor size.

But you need to consider several factors; like length of wires, location, ambient temperature etc.

Answer 2

I wouldn't use anything smaller than #12 THHN, especially if you're going to run it through conduit. You can consult the NFPA 70 electrical codes at this website:

http://www.nfpa.org/aboutthecodes/AboutT...

Answer 3

The NEC requires either a 3% or 5% maximum voltage drop for circuits depending on the application.  The VD is either calculated or measured.  A #8 copper UF wire will have a 2.3% drop at that distance and load and a #10 UF will have a 3.7% drop. The # 10 cu should not be a problem.  You can use aluminum direct burial wire to save money but you would need a #8 AL wire to have a 3.9% VD and connectors must be listed for use with aluminum.  A #6 al wire would give you a 2.4% VD which is what I would use if I were doing it.  The higher the voltage drop the shorter your motor life.

Answer 4

Once you know the current, you use the wire tables and ohms law to ensure that the voltage drop is less than 5%, 11 volts in this case. Don't forget you get drop in each wire, so individually that's less than 5.5 volts per wire.

Eg, if the motor draws 10 amps,

#12 is 1.6 ohms/1000', which translates to 0.56 ohms for 350', which is E=IR = 10*0.56 = 5.6, close enough

1.5 Hp is 1.1 kW, and at 220 volts, that is 5 amps, but you have to allow for start up current which can be a lot higher.