Motor specs.

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JdoubleU

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I am replacing a VFD for a 75 hp/208 volt motor. I noticed that way back when they pulled a 4/0 to feed it. If I am doing the calc. right I would need to pull 300 MCM or parallel 1/0. What are your thoughts? I know I would have to derate but can I pull parallel conductors in the same raceway?
 
Jakewhis said:
I am replacing a VFD for a 75 hp/208 volt motor. I noticed that way back when they pulled a 4/0 to feed it. If I am doing the calc. right I would need to pull 300 MCM or parallel 1/0. What are your thoughts? I know I would have to derate but can I pull parallel conductors in the same raceway?
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I think you are right.

No reason you can't pull all 6 conductors in the same conduit if you parallel them, but you end up needing (6) 2/0 to do so.

who knows how they came up with a 4/0. might have once been a smaller motor. 60HP would be 4/0 at 208V.

curious what size wires are running from the vfd to the motor.
 
The conductor ampacity for a VFD needs to be 125% of the drive current, not the motor {430.122}. What is the rated current of the drive?
 
infinity said:
The conductor ampacity for a VFD needs to be 125% of the drive current, not the motor {430.122}. What is the rated current of the drive?
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most times they are a hair under the charts in the NEC for the motor. but to be completely legal, it is the rated drive input current that determines the conductor amapacity required.
 
ok, so when I find out what size vfd needed, I then need to size the conductors to the vfd which is some cases a smaller conductor than the ones that go to the motor?
 
Jakewhis said:
ok, so when I find out what size vfd needed, I then need to size the conductors to the vfd which is some cases a smaller conductor than the ones that go to the motor?
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It is possible, but I don't recall it ever happening to me.

It is not unusual for it to go up one size though as sometimes people spec a VFD larger than the motor.
 
What I just found out is that the VFD had a rating of 221 amps. So can you pull a 4/0 to the VFD and then parallel 2/0 to the motor. My next question would be how do you size a VFD to a motor?
 
Jakewhis said:
What I just found out is that the VFD had a rating of 221 amps. So can you pull a 4/0 to the VFD and then parallel 2/0 to the motor. My next question would be how do you size a VFD to a motor?
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221A would require 277A worth of conductor or 300 kcm. 221A is the NEC FLC for a 75HP 208V motor so whatever you feed to the drive would also be appropriate for feeding the motor.

you could use (2) 1/0 if they were in seperate conduits, or (2) 2/0 if in the same conduit.
 
I've had 350's to a drive and 4/0's to the motor before. You definitely got to be paying attention. One wire size DOES NOT always fit all.
 
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