VFD OCPD and Feeder Size

[arc_flashed]

I am looking for some help on sizing the cable between my motor controller, a VFD in this case, and the branch OCPD feeding it. Came across a situation at a customer site where they have a 250HP, 480vac motor fed by a VFD. The VFD is fed by a single set of 500mcm cables which meets the 125% of drive rated current (NEC 430 part X). The issue is the branch OCPD is 800amp fuses which I believe violates table 430.52(C)(1) which should be a max of 175% with time delay fuses (approx 525amps). They said they aren't going to pull more cable to the VFD but may look at changing the fuse to a 600amp.
My question is how do you properly size the conductors between the OCPD and the motor starter? Is it based on the 125% of the VFD rated input current or the OCPD? I understand thermal protection is provided by the VFD/starter but it feels wrong to have a single set of 500's on a 600amp OCPD.
The second question is the max rating of fuse. NEC seems to limit it to the 525amp mentioned above, but an Allen Bradley VFD lists the max dual element fuse for a 250HP as 675amps.

 

[david luchini]

My question is how do you properly size the conductors between the OCPD and the motor starter? Is it based on the 125% of the VFD rated input current...

Yes. (430.122(A))

I understand thermal protection is provided by the VFD/starter but it feels wrong to have a single set of 500's on a 600amp OCPD.
The second question is the max rating of fuse.

600A fuse would be correct per 430.52(C)(1) and 430.130(A).

 

[petersonra]

In most cases you are required to size the ocpd for the input of a vfd by using 430.130(A)(2) which refers you to the manufacturers instructions.

The minimum ampacity of the wires feeding VFD is 125% of the rated input current of the drive. 430.122(A)

 

[Jraef]

The issue is the branch OCPD is 800amp fuses which I believe violates table 430.52(C)(1) which should be a max of 175% with time delay fuses (approx 525amps). They said they aren't going to pull more cable to the VFD but may look at changing the fuse to a 600amp.
My question is how do you properly size the conductors between the OCPD and the motor starter? Is it based on the 125% of the VFD rated input current or the OCPD? I understand thermal protection is provided by the VFD/starter but it feels wrong to have a single set of 500's on a 600amp OCPD.

800A fuses would indeed be a 430.52(C)(1) Code violation for a 250HP 460V drive and motor, unless there is something smaller downstream. Pulling more cable might solve the cable rating vs fuse size issue, but is NOT going to solve the Article 430.52 violation.

600A would be acceptable for the fuse size. The cable size is going to SEEM incorrect from an Article 240 standpoint, but that's why MOTOR circuits have their own section. The idea behind this is that Article 430 also has other requirements for protecting the motor, which has the EFFECT of also protecting the branch circuit conductors. So for the most part, the breaker / fuse is primarily proving short circuit protection because of those added requirements that are part of "motor controllers" such as starters and drives.

The second question is the max rating of fuse. NEC seems to limit it to the 525amp mentioned above, but an Allen Bradley VFD lists the max dual element fuse for a 250HP as 675amps.

These are just maximums according to the UL listing process for the VFD. Nothing says you HAVE TO use a 675A fuse, you are still required to follow the NEC (or CEC) rules for the installation. So you can use a 600A fuse, based on the "next standard size up" provision. Where you MIGHT see an oddball size fuse like 650A or maybe 675A, especially moving into the future, is when the drive mfr must start putting in special Semiconductor rated fuses now because of changes to the UL listing requirements for SCCR.