voltage drop

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sduckworth

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I have a situation where i need to run quad app 600 feet to feed a 15
hp water pump from a generator(60kw), if possible. This is neccesary because of the unique situation involving the customers equipment and backwater... which i won't bore you with. Motor voltage is 460, full load current is 21A. I have done the voltage drop calc's serveral different ways to try and size conductors. I am somewhat confused on the proper way to approach this situation, mainly what is the proper method to calculate voltage drop and what about initial starting current.
 
The formula is VD = 1.732KIL/cm

VD = Voltage Drop
K = Constant (circle mil ohms) for copper wire use 12.9 will be close enough
I = Amperage
L = Total length of circuit (If the pump is 600' from the generator the circuit length will be 1200')
CM = Circle mil of the wire that you attend to use.

The trick is what you should use for I. I assume (dangerous) that you would use FLA, but this may result in excessive drop during motor starting. If you used the locked rotor current and size the wire for minimal voltage drop, the conductors will need to be huge with the added expense that goes with it. Some value in the middle would probably be a good compromise. See 695.7 for the VD requirements for fire pumps. While your pump is not a fire pump, it may give you an idea of what would be reasonable.
 
One suggestion I have is to install a VFD to ramp the motor up slowly. If you do that you can forget about start up current and just use the FLA for I.
 
Iwire make a good point. If you do not use a starter you should check the name plate on the generator for Xd and Xd'.
Xd is the generator?s steady-state synchronous reactance, while Xd' is the generator?s direct-axis transient-reactance. Xd' is the starting reactance
for the generator and is used with the starting amps of the motor. So, you have the VD of the generator + the VD of the conductor.
 
VD Calc

VD Calc

I would use the Locked-rotor-amperage which would be on the order of 5-6 times the fla. That current times the single-length AC resistance of the cable will give the maximum voltage drop during startup.
The 695 fire pump section requires that you keep the VD during startup to a maximum of 15%, with the running VD at 115% of load to a maximum of 5%.

Specifically: starting with a #8 cable, NEC Chapter 9: 0.70 ohms/N per 1000 feet in steel conduit. For 600 feet that equates to 0.42 ohms.
Motor fla NEC 430.250 = 21 amps
Using 6X starting current as LRA = 126 amps (nameplate of motor will give NEMA design letter which will provide a more accurate lra)

VD = I * R = (126) * 0.42 = 52.92V line-to-neutral
L-L VD = 1.732 * 52.92 = 91.65V
As a percentage of 480 this equal 91.65/480 = 19.1 %
But the motor nameplate is likely 460, so the correct percentage is 91.65/460 = 19.9 %.

Run the calculation for the next wire size up, #6AWG and it probably meets the 15% fire pump criteria and you could be confortable that it will start on a Full Voltage starter.
This calculation is shown in the NEC handbook in Chap 9 after the AC resistance table.

If you use a VFD, size the wire for 5% VD at 115% of fla.

JM
 
mayanees said:
I would use the Locked-rotor-amperage which would be on the order of 5-6 times the fla. That current times the single-length AC resistance of the cable will give the maximum voltage drop during startup.
Click to expand...
I most cases you could say that, but since your distance is so long you need to take into account the generator VD. A typical Xd' = 0.15.
125 amps x .15 x 1.73 = 33 volts. Check the genny and get the correct value.
It may be a smaller value.
 
Gen inclusion

Gen inclusion

bob said:
I most cases you could say that, but since your distance is so long you need to take into account the generator VD. A typical Xd' = 0.15.
125 amps x .15 x 1.73 = 33 volts. Check the genny and get the correct value.
It may be a smaller value.
Click to expand...

Totally agree Bob. I kinda forgot the generator. Since the motor is pulling 5-6 * 15HP for the first few seconds, (~ that's 75-90 kW)and the generator can deliver a limited amount with a rating of 60 kW, there will be some additional generator voltage drop.
But I think if it's a newer generator with an electronic governor then 60 kW is a safe size.
John M
 
mayanees said:
But I think if it's a newer generator with an electronic governor then 60 kW is a safe size.
John M
Click to expand...
The Generator is fine. The OP was concerned about the VD. I just saying don't forget the generator
VD.
 
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I was under the impression that VD was a concern because at lower voltages more heat is generated with motors. Since the start up time is relatively short, is VD at start up even really that much of a concern?

If it is, what would be the allowable VD? 3%? 10%?

(of course there would have to be enough voltage to get the motor started)
 
,,, starting VD

,,, starting VD

Lou...
Starting VD... ...
The only concern is that the motor start, and the only model I have to go by is the fire pump criteria, which per NEC 695 is 15% max on startup.
So I think it's a good design practice to limit the starting VD to 15% for any motor.
And you'd have to figure it promotes longevity for the motor.
JM
 
I would say the importance of limiting VD during motor start up depends greatly on what the motor is driving.

Typical water pumps are easy starting and I would not worry much about VD at start up for this waste water pump.

If it was a hard starting load like a compressor I would be concerned.
 
Guys
As always when i read this forum, i am impressed with the amount of knowledge and the willingness to share. Please keep it up, you do an invaluable service to those trying to learn.

Thanks for all the replies
Steve
 
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