Sizing conductors for multiple VFD system

[evanc44]

I'm looking for some help interpreting 430.122. On a branch circuit with multiple VFDs, do I size the branch and feeder conductors based on the sum of each individual VFD input current rating multiplied by 125%? For example, if I'm using qty. (3) 3HP AB drives each with an input current rating of 7.5A, must the conductor size be capable of handling (3)*(1.25*7.5A)? I understand the OPCD sizing will remain a function of the motor FLA, but conductor size gets large very quickly if I am understanding 430.122 correctly.

 

[Jim1959]

I'm looking for some help interpreting 430.122. On a branch circuit with multiple VFDs, do I size the branch and feeder conductors based on the sum of each individual VFD input current rating multiplied by 125%? For example, if I'm using qty. (3) 3HP AB drives each with an input current rating of 7.5A, must the conductor size be capable of handling (3)*(1.25*7.5A)? I understand the OPCD sizing will remain a function of the motor FLA, but conductor size gets large very quickly if I am understanding 430.122 correctly.

Check out 430.131 which refers you to 430.53

430.131 Several Motors or Loads on One Branch Circuit
Including Power Conversion Equipment. For installations
meeting all the requirements of 430.53 that include one or more
power converters, the branch-circuit short-circuit and groundfault
protective fuses or inverse time circuit breakers shall be
of a type and rating or setting permitted for use with the power
conversion equipment using the full-load current rating of the
connected motor load in accordance with 430.53. For the purposes
of 430.53 and 430.131, power conversion equipment shall
be considered to be a motor controller.

 

[Sneak85]

I'm looking for some help interpreting 430.122. On a branch circuit with multiple VFDs, do I size the branch and feeder conductors based on the sum of each individual VFD input current rating multiplied by 125%? For example, if I'm using qty. (3) 3HP AB drives each with an input current rating of 7.5A, must the conductor size be capable of handling (3)*(1.25*7.5A)? I understand the OPCD sizing will remain a function of the motor FLA, but conductor size gets large very quickly if I am understanding 430.122 correctly.

Just got burned on this by the local inspector. We had a 225A breaker, feeding a 4/0 cable to two 75HP VFDs each with factory main input disconnect and fuses. Each VFD's fed three 25 HP motors, each motor with factory provided over current protection in cabinet. Inspector held that we needed to take the 107A input current to the drive (which was larger than what was submitted on for the project). Did (107A x 2 x 125%) for a total of 267.5A. Ending up putting in a 300mcm cable which requires pins in the breaker to accept a wire size much larger than what we have.

Inspector noted that 430.120 states "The installation provision of Part 1 through Part IX are applicable unless modified or supplemented by Part X. In this case 430.120 and sections thereafter set precedence.

He would not sway to reason that it would be impossible for the breaker to allow the 300A on the cable he is having us provide now, as 430.122 states 125% for the conductors to the VFDs. We had 225A cable to each VFD (only needed 133.75A), he also would not accept our setup being a tap (since we were going from one cable to two drives). So his reasoning was all encompassing, any cable that leads up the VFD needs the 125%. He had no issue with the breaker being properly sized for the twelve 25HP motors we had sized it for.

I also asked if we had to carry that 125% back from panel to panel to panel to utility, he thought for a second and thought that would be a really good idea. So hopefully he doesn't come back and note that our distribution panel with main and wire size of 1000A would likely end up to be about 1600A since all loads on it are properly sized VFD motor loads.