VFD breaker sizing and cable sizing

Status
Not open for further replies.
C

Cableman

Member
The VFD controls a 300HP motor, 480V, 3 phase. The VFD draws 385Amps. The motor FLA is 357 Amps The designer wishes to protect the VFD with a 400A 100 percent breaker, but he says that the feed from the vfd to the motor has to be 125% of the draw which will get it to 481 Amps and he would need at least 2 sets of 4/0 (and not with one set of 600KCM)

Would appreciate your comments regarding the breaker and cable sizes
 
See Art 430 Section X for VFD requirements. Assuming that the 385A is the input rating of the VFD then the minimum size rating for the disconnecting means is 115% which would mean if the breaker is used as the disconnecting mean you would need a breaker with a minimum 443A so I would say that the 400A is not enough even at 100% rating. As for the feeder to the motor I would personally go with smaller parallel cables for ease of installation. And if 125% if tge FLA is 481A then neither of the conductors specified will suffice. 2 EA 4/0 only gives you 460A and 600 MCM only gets you 420A. Unless you are using 90C terminations for the motor and the VFD is rated 90C- both of which I doubt.
 
The branch circuit or feeder conductors to the VFD have to be sized at 125% of the rated input of the VFD.

430.122 Conductors ? Minimum Size and Ampacity.
(A) Branch/Feeder Circuit Conductors. Circuit conduc-
tors supplying power conversion equipment included as
part of an adjustable-speed drive system shall have an am-
pacity not less than 125 percent of the rated input current to
the power conversion equipment
Click to expand...
 
petersonra said:
Not any real difference than any motor circuit other than using the actual FLA rather than from a chart in the code book.
Click to expand...

Wll there is a slight difference if the drive is oversized for use with a smaller motor.
 
Status
Not open for further replies.
Top