200HP motor @ 600' voltage drop calculations

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Ambient44

Member
Location
Oak Lawn, IL
I have 200Hp motor installation on a pump that will be 600' from the MCC the FLA is 230A @ 460V /EFF= 95%/ PF = 0.86 / No load current 60A/ in rush current X 6.3. with the voltage calculations for a 3-phase 3-wire motor load voltage drop I come up with the following; 0.158 (2 x res. of conductor) x 600(one way length of conductor) x 230 (current in conductor) x 0.866 divided by 179.4 (Cir. mills for 4/0 copper conductor) = 75 amps.
1. is this the correct calculation for a 3-phase/3-wire motor?
2. if this is correct what cir. mills should I use for the calculations, the normal conductor for the current ie 3/0?
3. If this is correct do I add the 75 to the FLA and size my conductors accordingly?
Additional information:
Locked rotor torque 230% / break down torque 240%/ Conductors are to be installed in GRC outdoors on a rack.
Thanks for your input.
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
1. No, your calculation is not correct. The calculation for voltage drop would be 2*R*L*I*0.866 (no division by circ. mils.) and would produce a result in volts, not amps.

2. The conductor for your motor circuit must have an ampacity of at least 240*1.25=300Amps, per 430.22. So you'd need to start you calculation with at least a 350mcm circuit conductor.

3. Using a 350mcm, you'd get 2*(0.0000367ohm/ft)*600ft*230A*0.866=8.77 Volt drop. 8.77 Volts is a 1.8% drop on a 480V system. You have to decide if this is an acceptable voltage drop.
 

dkarst

Senior Member
Location
Minnesota
1. No, your calculation is not correct. The calculation for voltage drop would be 2*R*L*I*0.866 (no division by circ. mils.) and would produce a result in volts, not amps.

2. The conductor for your motor circuit must have an ampacity of at least 240*1.25=300Amps, per 430.22. So you'd need to start you calculation with at least a 350mcm circuit conductor.

3. Using a 350mcm, you'd get 2*(0.0000367ohm/ft)*600ft*230A*0.866=8.77 Volt drop. 8.77 Volts is a 1.8% drop on a 480V system. You have to decide if this is an acceptable voltage drop.

Using a conductor that size you really should take inductive reactance into account and using that I calculated a 12.7 volt drop which is ~ 2.6%.

In my opinion you really need to be worried about starting a motor of this size at this distance. Depending on the starting power factor, starting load (you stated pump so maybe if it is centrifugal load you can make a reasonable guess), the starting voltage drop could be in neighborhood of 40 - 60 volts. If you have a stiff 480V at your panel, they motor starting torque would be reduced to ~ 85% of rated. More work is required here to determine whether the pump will start successfully.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
I am kind of surprised a motor of this size would be started across the line.

We have been running into a lot of places where the utility is not allowing this.
 

nollij

Member
Location
Washington
For my circular mil calculations I usually calculate <20% voltage drop during starting. I think SKM equipment sizing conductor defaults to <3% running (which would also be a good general basis).

Either way, the conductor has to be sized so much bigger because of the NEC requirements that voltage drop is not typically the driving sizing issue.

Also, remember that starting torque is proportional to the voltage squared so, at 0.8pu voltage the starting torque would be at 0.64pu. You might want to check the load torque characteristics just to verify that you can start your motor if you have a very long cable length and a decent expected voltage drop during starting.
 

Ambient44

Member
Location
Oak Lawn, IL
Thank you all for your responses to my inquiry. I will take into consideration the calculated voltage drop to be somewhere between 1.8% to 2.6% of the 480v present.

They are saying the pump would require a soft start to run the motor, I will get their specifications on the starter and evaluate the situation with the calculated voltage drop with the pump manufacturer and the starter manufacturer.

Are there any other suggestions for options on running a motor of that size that distance other then increasing the conductor size?

I know this can be a harsh environment in the winter here in the midwest and they are using the pump to move oil through a 8" line 1600'. So the calculations on the torgue during start-up should take those factors into consideration as well. I have some digging to do. I will keep this post going for updates. Thank you again.
 

GeorgeB

ElectroHydraulics engineer (retired)
Location
Greenville SC
Occupation
Retired
... and they are using the pump to move oil through a 8" line 1600'. So the calculations on the torque during start-up should take those factors into consideration as well.
OK ... I have a question for you. Torque with an impeller or centrifugal style pump varies with speed, so a soft start or inverter (both are $$$ at 200HP) will significantly reduce your starting current. A positive displacement pump, on the other hand, has torque which varies directly with pressure. My experience is with the positive displacement variety. Give us a few more details.
 
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