User Tag List

Results 1 to 7 of 7

Thread: Oversized VFD / 430.122(A) violation

  1. #1
    Join Date
    Sep 2011
    Location
    Boise, Id
    Posts
    11
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)

    Oversized VFD / 430.122(A) violation

    I'm working on a project where the owner wants to use a spare VFD (rated 150HP 480V 180A) to replace an FVNR starter that controls a well pump (100HP 460V 127FLA). The feeder conductors to the motor starter are #3/0 copper and the upstream overcurrent protection is a 200A fused disconnect. Using the 75 deg. ampacity table and no adjustment factors it appears the #3/0 conductors are rated for 200A, which is short of the 225A (125% of 180A) required by NEC 430.122(A). So the question is.... Do I have to replace the #3/0 feeder conductors with #4/0 even though they're protected by 200A fuses?

  2. #2
    Join Date
    Jun 2003
    Location
    NE (9.06 miles @5.9 Degrees from Winged Horses)
    Posts
    11,105
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    The 180 amps you quoted is the FLA for a 150 HP, Table 430.250.

    You need to multiply the Drive rated input current * 125%. I would venture a guess that it will be higher than the 180.

    The answer is yes, you will need to increase the size of the wire.
    Tom
    TBLO

  3. #3
    Join Date
    Sep 2011
    Location
    Boise, Id
    Posts
    11
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by ptonsparky View Post
    The 180 amps you quoted is the FLA for a 150 HP, Table 430.250.

    You need to multiply the Drive rated input current * 125%. I would venture a guess that it will be higher than the 180.

    The answer is yes, you will need to increase the size of the wire.
    According to the literature I have, most of Toshiba's H3 series drives have output ratings that are very close or exactly the same as the FLA values in Table 430.250. The 150HP drive the owner wants to use is rated 180A.

    Thanks for your response.

  4. #4
    Join Date
    Feb 2003
    Location
    Illinois
    Posts
    24,133
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by Steamdonkey2011 View Post
    According to the literature I have, most of Toshiba's H3 series drives have output ratings that are very close or exactly the same as the FLA values in Table 430.250. The 150HP drive the owner wants to use is rated 180A.

    Thanks for your response.
    The conductors that supply power to the VFD must be have an ampacity of at least 125% of the VFD input amps. Is the 180 amps that the drive is rated at input or output amps?
    Don, Illinois
    (All code citations are 2017 unless otherwise noted)

  5. #5
    Join Date
    Sep 2011
    Location
    Boise, Id
    Posts
    11
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by don_resqcapt19 View Post
    The conductors that supply power to the VFD must be have an ampacity of at least 125% of the VFD input amps. Is the 180 amps that the drive is rated at input or output amps?
    Unfortunately the only info I have at the moment is the rated output of the drive. As you pointed out the NEC refers to the input current, hopefully that's stated somewhere on the drive.

    Thanks.

  6. #6
    Join Date
    Dec 2007
    Location
    NE Nebraska
    Posts
    41,100
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Unfortunately the NEC does not consider the fact that you possibly can put a smaller motor on the output of the drive than it is rated for. As far as I'm concerrned you could probably even go with 175 amp fuses and 2/0 conductors for 100 hp motor but NEC doesn't seem to think so.

    The motor is 100 hp whether connected directly to the line or via the drive. The only increase in power with the drive would be any inefficiencies in the drive - which should not be very much. Poor power factor is not an issue with a drive but is with direct line connection, so that makes up for any other inefficiencies right away.

  7. #7
    Join Date
    Dec 2006
    Location
    Eastern Oregon
    Posts
    3,763
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Yeah, you've really got to be careful with this code article. We've used drives a little oversized in the past and if you're not paying attention sometimes it bumps the wire size up a notch. Not something you want to find out at inspection time.

    Fortunately, I haven't had this come up where I've had to explain to the customer why we have to replace the wire that's been perfectly fine for years just because they want to use a little bit bigger spare VFD....

Tags for this Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •