430.130 - Breaker/Fuse for Drives

[jes25]

I'm trying to reconcile this section with the manufacturers instructions for an Allen Bradley Drive. The manufacturer instructions say a drive for a 10HP 480V motor needs a 25 to 40 amp fuse. The trouble is 430.130(A)(1) basically says to size the fuse per 430.52(C)(1)......which gives me a 14*1.75=24.5. So I don't see how Allen Bradley can recommend a 40 amp fuse here. (If you have a look a the exception the input current of the drive is 18.4 so still no 40 amp fuse allowed per the NEC)

I can't imagine AB's recommended fuses are a code violation so I'm wondering if anyone has any insight into what I'm doing wrong.

 

[jim dungar]

Ignore the motor horsepower, it is irrelevant when selecting protection on the input to a VFD.
You must use the drive input current of 18.4 amps. You also need to consider the manufacturer's instruction and Listing requirements for fuse protection. Finally you need to consider the NEC branch circuit requirements.

I have seen branch circuit protection that is actually smaller than the fuses for the drive. This allows the branch OCPD to clear all of the typical faults while the fuses protect against high level faults interna, to the drive. The Listed drive protects the motor.

 

[jes25]

Ignore the motor horsepower, it is irrelevant when selecting protection on the input to a VFD.
You must use the drive input current of 18.4 amps.

Thanks. I guess I still don't get how AB can say to use a 40amp fuse for an input of 18.4 amps in light of 430.130(A)(1). The exception allows this but it says,

The rating and type of protection shall be permitted to be determined by Table 430.52(C)(1) using the power conversion equipment’s rated input current

18.4x1.75 = 32.2 = 35A fuse

I see nothing here allowing us to fuse larger than Table 430.52(C)(1)

I have seen branch circuit protection that is actually smaller than the fuses for the drive. This allows the branch OCPD to clear all of the typical faults while the fuses protect against high level faults interna, to the drive. The Listed drive protects the motor.

I'm not clear what you mean by "branch circuit protection", I though the branch circuit protection would be the breaker/fuse on the input of the drive?