Desert Spark
Member
- Location
- New Mexico
In a installation I worked on last year we fed a 500 hp 480v motor through a VFD with an input current rating of 529 amps on the nameplate. The conductors to the VFD and motor were three parallel sets of 350 Kcmil copper conductors protected by an 800 amp breaker.
Now, an additional 50 hp motor is required in the install and the electrical engineer is wanting to tap the feeders of the VFD and power the motor through an additional VFD that has an input rating of 180 amps.
I wanted to run a new circuit to the new VFD in a spare raceway from the MDP but the 180A input rating of the new VFD would require conductors too large for the spare.
For purposes of conductor sizing for a single VFD I know I would use 430.122. For multiple VFDs would I size conductors like you would multiple motors? (529A x 1.25) + 180A = 841.25A required for conductor size? The three 350 Kcmil parallel runs would have this ampacity.
For SC/GF protection 430.131 permits multiple VFDs on one circuit as long as the installation meets all requirements of 430.53 are met. Am I interpreting this correctly?
Now for 430.53, this install would not meet requirements of (A) or (B) for sure, it would not comply with (C) because I don't believe the VFDs are listed to be used together. (D) to me it seems could be interpreted to mean that I could tap off the parallel conductors and run to another OCPD then to the new VFD.
I'm having a hard time understanding the requirements of 430.53 applied to VFDs. An install complying with 430.53 (D) would also have to comply with (C) correct?
I do believe that this install would not be able to meet code because of the requirements of 430.53 (C). Even if it did I would not feel confident in my interpretation of this section to actually do the install this way.
I'm looking for a little help in seeing if my conclusion is correct or if there may in fact be a way to accomplish this install in the way the electrical engineer has envisioned.
Thanks
All code references from 2017 NEC
Now, an additional 50 hp motor is required in the install and the electrical engineer is wanting to tap the feeders of the VFD and power the motor through an additional VFD that has an input rating of 180 amps.
I wanted to run a new circuit to the new VFD in a spare raceway from the MDP but the 180A input rating of the new VFD would require conductors too large for the spare.
For purposes of conductor sizing for a single VFD I know I would use 430.122. For multiple VFDs would I size conductors like you would multiple motors? (529A x 1.25) + 180A = 841.25A required for conductor size? The three 350 Kcmil parallel runs would have this ampacity.
For SC/GF protection 430.131 permits multiple VFDs on one circuit as long as the installation meets all requirements of 430.53 are met. Am I interpreting this correctly?
Now for 430.53, this install would not meet requirements of (A) or (B) for sure, it would not comply with (C) because I don't believe the VFDs are listed to be used together. (D) to me it seems could be interpreted to mean that I could tap off the parallel conductors and run to another OCPD then to the new VFD.
I'm having a hard time understanding the requirements of 430.53 applied to VFDs. An install complying with 430.53 (D) would also have to comply with (C) correct?
I do believe that this install would not be able to meet code because of the requirements of 430.53 (C). Even if it did I would not feel confident in my interpretation of this section to actually do the install this way.
I'm looking for a little help in seeing if my conclusion is correct or if there may in fact be a way to accomplish this install in the way the electrical engineer has envisioned.
Thanks
All code references from 2017 NEC