How to determine correct wire size when using a phase convertor.

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kledford

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Here's the scenario,
We are providing power to run a irrigation pump with a phase convertor,
1. Utility is giving 480V. single phase at the road.
2. We must install underground cable 1650 ft. to phase -convertor/ pump location.We will be using aluminum direct buried cable. ( 60 -C.)
3. Phase convertor has a capacitor start. Starting amps. =55A.
4. Pump is located next to starter.
5. Pump is 480V. 3 PH. 20HP. FLA. 24.8. STARTING AMPS=119. ( Across the line start).
6. The convertor is designed to start 20 HP and run 40 HP.
7. I am having trouble calculating the voltage drop to size the underground cable, do I base it on the single phase load of 55 A. or the 3 phase load of 24.8 or a combination of the two?
We are doing more and more of these and do not want to undersize the feeder but also don't
want to be installing larger cable than needed. Thanks for any help available.:)
 
How is the single-phase starting load going to be 55A when the motor start is 99 kVA?
 
Thanks for the reply,
The phase convertor has a type of soft- start that lets it start at a lower current then it times out,
you start the convertor first then after it is up and running you start the pump which comes on at
at full load, with out the start assist on the convertor it would have a starting current of 160 amps.
Thanks.
 
Kledford...

Neglecting Converter losses, then the 1-ph kVAin must equal the converter's 3-ph kVAout.

If the converter's kVAout = Sqrt(3) x Vout x Aout, then, Vin x Ain = Sqrt(3) Vout x Aout! Since Vin=Vout, then Ain = Sqrt(3) x Aout.

Regards, Phil Corso
 
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