Multi-Motor with VFD

Status
Not open for further replies.
JoeStillman

JoeStillman

Senior Member
Location
West Chester, PA
430.24 gives the rules for sizing conductors supplying several motors. Pretty much the same as it ever was (in a nutshell); 125% of the largest motor plus all the other motors. Fine. But what if the largest motor is on a VFD? If 430.122 requires us to size conductors for the rated input current of the power conversion device instead of the motor full-load amps, why isn't that stipulated in 430.24 as well?

For example, if I have a multi-motor feeder to (2) 10HP ATL starters and a 60HP-rated drive feeding a 50HP motor. Do I use 50HP as the largest motor or 60HP (the rating of the drive)?
 
JoeStillman said:
430.24 gives the rules for sizing conductors supplying several motors. Pretty much the same as it ever was (in a nutshell); 125% of the largest motor plus all the other motors. Fine. But what if the largest motor is on a VFD? If 430.122 requires us to size conductors for the rated input current of the power conversion device instead of the motor full-load amps, why isn't that stipulated in 430.24 as well?

For example, if I have a multi-motor feeder to (2) 10HP ATL starters and a 60HP-rated drive feeding a 50HP motor. Do I use 50HP as the largest motor or 60HP (the rating of the drive)?
Click to expand...

Because 430.122 applies to the INPUT conductors, not the motor leads. Read it again more carefully. On the load side of the VFD, all normal rules apply.
 
JoeStillman said:
430.24 gives the rules for sizing conductors supplying several motors. Pretty much the same as it ever was (in a nutshell); 125% of the largest motor plus all the other motors. Fine. But what if the largest motor is on a VFD? If 430.122 requires us to size conductors for the rated input current of the power conversion device instead of the motor full-load amps, why isn't that stipulated in 430.24 as well?

For example, if I have a multi-motor feeder to (2) 10HP ATL starters and a 60HP-rated drive feeding a 50HP motor. Do I use 50HP as the largest motor or 60HP (the rating of the drive)?
Click to expand...
You don't treat that drive as a 50 (or 60) hp motor, you treat it as whatever its input rating is. Though I don't like it and have tried to make my point against such things before - the way code is written you could only be driving a 20 hp motor with that drive but the conductors supplying is must still be sized to the drive input rating, not the output HP rating.

Personally if I were doing your install I would still use 50 hp motor values in place of this drive, unless some inspector was going to call me on it.
 
I wonder if the code rule takes into account the fact that some drives can go over 60 Hz. In that case the motor could exceed it's rated BHP.

I probably should keep my mouth shut, but it just seems to be inconsistantly applied, whatever the reason.
 
The conductors supplying the motors would be installed per 430.24. However, would not the conductors to the VFD have to
meet the requirements of 430.122? Good question.
 
Status
Not open for further replies.
Top