Individual Motor Short/Ground Fault Protection (art 430.52) on VFD Driven Motors

The minimum rating should be enough to allow for the (brief) starting current of the motor without nuisance tripping of the protective devices. For this you need to compare the motor starting current curve with the overcurrent-versus-time tripping curve of protective devices (overload relays, fuses, circuit breakers).

VFDs have a separate rule, the circuit feeding the VFD must be 125% of the VFDs max amp rating, then the SCPD is sized accordingly. There is no starting current issue to contend with on a VFD driven motor. On the motor side, the VFD is supposed to provide the SCPD protection. Most of the major brands do if they are UL listed as motor controllers, but some of the cheap ones get their UL listing as "power converters" so they do not provide motor protection and they don't make that abundantly clear. Then you have to add something on the output side which can be a problem for the VFD if it trips, so be aware of that.

So what you are saying is that SCPD for a branch circuit feeding a VFD driven motor is sized to 125% of the VFD max amp rating? That has been my approach and it has worked without a problem in our process.

The reason i inquired is becuase there is no direct statement on the code for that. There is an indirect statement in table 430.52 as it states MAXIMUM.

I just wanted to make sure that i was not in violation of a VFD specific rule.

So to sum up: 125% of VFD max amps for inverse time OCPD rating is acceptable just as 130% or 150% since table 430.52 gives a maximum of 250% of VFD max amps for inverse time OCPD.

Correct?

Also i will verify on the SC protection of the VFD should a SC present in the motor. I know the VFD we selected protects against overload.

Please let me kow your thoughts and any reference you may have.

Thanks

There is a direct statement to the 125% issue with specific reference to VFDs (called Adjustable Frequency Drives if you have a searchable pdf version), but it is not in 430.52, I think it is 430.122 (?). I don't have a copy of my code here. The reference has changed between 2002, 2005 and I think 2008.

you are right. There is a reference to the branch circuit WIRE sizing. It is part X, section 430.122 conductors- minimum size and ampacity... (page 929 in electronic version of NEC 2005)

But there is nothing on VFD's and OCPD (that i can tell) hence my question was on OCPD in the original post.

What i did was take the next higher CB of 125% of VFD rated current. After that the CB ends up being 180 % of rated current in one case and 140% of rated current in another. Then i took rule 430.62 for feeder OCPD sizing (where you take largest PD and add the FLA of the other motors and then take the next lowest OCPD)

That OCPD is about 120% of the sum of FLA of all the VFD's that circuit is feeding.

Please let me know your thoughts.