Calculating voltage drop on a 40 HP well pump.

[kledford]

I have to install a feeder to power a 40 HP. 3 phase, 480v well pump. Here are the specs.
1. Length of run 1300 ft.
2. Direct buried aluminum cable from service to well.
3. F.L.A =61. amps
4. Using a electronic soft-start.
5.S.F. 1.15
6. Pump is 460V. utility voltage is 480V.
I came up with #250MCM aluminum. Do I have to calculate the locked-rotor current when doing these calculations?
Thanks.

 

[ActionDave]

AL 250MCM sounds a bit large.
No you do not need to calculate locked rotor amps. You get that off the motor nameplate if it is on there.

 

[ptonsparky]

Using a cheat sheet that a mfg of Cablecon supplies, 2/0 is ample for a 40 HP with FLA of 61 amps.

My spreadsheet gives a result of 2/0 being a bit on the small side with over 5% VD.

My handy dandy app for Droid suggests 4/0 al for 3% VD, and 2/0 al for 5%.

 

[kwired]

You need to determine what % of drop you want to have, and at what point do you want it to apply. You likely want a value based on FLA, but also need to consider starting current, though a soft start control mostly negates the need to consider starting current.

On a long run like you have, power factor correction does make significant difference in running current/running voltage drop.

 

[kledford]

Thanks to everyone for your help.

Thanks to everyone for your help.

 

[TMMB]

TOM

can u provide us your sheets, regards

 

[ptonsparky]

TOM

can u provide us your sheets, regards

www.paigeelectric.com/specs/P7304D.pdf

I used the formula in a spread sheet so it isn't a clear cut cheat sheet, you will have to do a little math and work with Excel, or whatever SS you have.

You are calculating for VD only so the resulting wire size may not be the minimum required by the NEC

 

[weressl]

I have to install a feeder to power a 40 HP. 3 phase, 480v well pump. Here are the specs.
1. Length of run 1300 ft.
2. Direct buried aluminum cable from service to well.
3. F.L.A =61. amps
4. Using a electronic soft-start.
5.S.F. 1.15
6. Pump is 460V. utility voltage is 480V.
I came up with #250MCM aluminum. Do I have to calculate the locked-rotor current when doing these calculations?
Thanks.

Enter values in yellow boxes (only!) for AC voltage drop calculation Three Phase Three-Wire Circuits

Drop (30?C) = 2K x L x I X p.f./D =(42.4x1300x61X0.84)/211600=13.35 volts drop 2.78%
Enter12.9 for >50%A or 11 for <50%A for Copper
21.2 for Aluminum21.2
Enter wire size (with slash) (i.e.. 12/, 1/, 2/0, 500/ - see Table)4/0
D = Conductor cross section (Automatically inserted)211600cm
One way length of conductors in feet =1300ft
Total full load circuit current = 61A
Circuit Voltage =480V
Power Factor =0.84



fini

 

[Crohnos01]

Do I remember a NEC requirement on single motor circuits for a 125% amp multiplier or was that just for over current protection calcs? (I don't have the code in front of me at the moment)

 

[kwired]

Do I remember a NEC requirement on single motor circuits for a 125% amp multiplier or was that just for over current protection calcs? (I don't have the code in front of me at the moment)

125% multiplier is for minimum conductor ampacity and has nothing to do with voltage drop or overcurrent protection.

Motor overload protection is often 115% of nameplate full load current.

Short circuit and ground fault protection can be well above full load current depending on type of protective device used. Most common is 175% for most time delay fuses and 250% for typical inverse time circuit breakers.

 

[Crohnos01]

Ah, OK thanks. I couldn't remember the exact verbiage, I was thinking of the ampacity, apparently.