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Thread: OCPD for four 30HP VFD's

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    OCPD for four 30HP VFD's

    I'm working on the design for an AHU replacement project that has four 30HP, 460V, 3 phase fans with each motor having its own VFD (soft start). The mechanical engineer is telling me that the fans will run at a low speed normally (around 18 or 19HP) but if one of the 4 fans breaks then the other fans would automatically speed up to accommodate the load.

    I was planning on providing a breaker in the switchboard and run a feeder up to the roof where the AHU is located. Then I would provide a wireway and four separate 3P100A disconnect switches with 80 amp fuses for each 30HP motor.

    My question is how to properly size the breaker in the switchboard that will feed the four 30HP fans with VFD's. Do I take 125% of the largest motor plus the sum of the remaining motors to size the breaker?
    Here are the specs for the AHU. 4 supply fans, 460/3/60, 35.85FLA, 44.81MCA, 80.0A FUSE SIZE.
    Would I size the breaker in the switchboard this way? 35.85 x 125% = 44.81 + 35.85 + 35.85 + 35.85 = 152.35A. Provide a 3P160A breaker?

    Also, the new AHU will be located outside of the existing AHU which is located in a roof top mechanical room. The new AHU will be put in place before the existing AHU is removed. The mechanical engineer is placing the new VFD's inside the mechanical room. Since the new AHU will be located outside the mechanical room would I place the 4 disconnect switches outside next to the new AHU even though the VFD's will be located inside the mechanical room? I'm not clear on where the correct placement for the disconnect switches should be since they are supposed to be in line of sight. The VFD's will not be in line of sight since they will be located inside the mechanical room and the new AHU will be located outside of the mechanical room.

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    Quote Originally Posted by john2016 View Post
    I'm working on the design for an AHU replacement project that has four 30HP, 460V, 3 phase fans with each motor having its own VFD (soft start). The mechanical engineer is telling me that the fans will run at a low speed normally (around 18 or 19HP) but if one of the 4 fans breaks then the other fans would automatically speed up to accommodate the load.

    I was planning on providing a breaker in the switchboard and run a feeder up to the roof where the AHU is located. Then I would provide a wireway and four separate 3P100A disconnect switches with 80 amp fuses for each 30HP motor.

    My question is how to properly size the breaker in the switchboard that will feed the four 30HP fans with VFD's. Do I take 125% of the largest motor plus the sum of the remaining motors to size the breaker?
    Here are the specs for the AHU. 4 supply fans, 460/3/60, 35.85FLA, 44.81MCA, 80.0A FUSE SIZE.
    Would I size the breaker in the switchboard this way? 35.85 x 125% = 44.81 + 35.85 + 35.85 + 35.85 = 152.35A. Provide a 3P160A breaker?

    Also, the new AHU will be located outside of the existing AHU which is located in a roof top mechanical room. The new AHU will be put in place before the existing AHU is removed. The mechanical engineer is placing the new VFD's inside the mechanical room. Since the new AHU will be located outside the mechanical room would I place the 4 disconnect switches outside next to the new AHU even though the VFD's will be located inside the mechanical room? I'm not clear on where the correct placement for the disconnect switches should be since they are supposed to be in line of sight. The VFD's will not be in line of sight since they will be located inside the mechanical room and the new AHU will be located outside of the mechanical room.
    I would design it with an individual breaker for each motor. But what do I know........

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    Yes. I would prefer to do it that way but the existing switchboard only has a limited amount of breaker spaces. There are a total of 4 AHU's on the roof that are being replaced with the same motor requirements. Currently each AHU is being fed by its own breaker in the existing switchboard. Otherwise, I would agree with you on designing separate breakers for each motor (VFD).

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    A lot of that info about running speeds etc., while interesting, is superfluous to the stated task. It's really kind of simple; you size the CONDUCTORS based on the rules for Feeders (article 215), then you size the OCPD to protect the conductors. So you actually had that almost right as far as sizing the conductors; 125% of the largest continuous load + the total of the remaining FLAs (from the NEC table for the HP, not the equipment label, unless the equipment label value is higher).

    So 30HP 460V from the NEC table is 40A, therefore you would size the feeder conductors at 40 x 3 + (40 x 1.25) = 170A minimum conductor ampacity. 2/0 THHN is rated 175A at 75C without adjusting for voltage drop, so I would use a 175A OCPD.

    As to the disconnect locations, you are not required to have the disconnect within site of the VFD, it must be within site of the MOTOR. So unless the VFD is right there next to the motor, you may need another LO/TO type disconnect down stream of the VFD anyway.If the VFDs are built into the AHUs, this may already be part of the assembly.

    Also take note that for the Branch conductors going from your disconnects to the VFDs, the rules on VFDs are special. Those conductors must be sized at 125% of the VFD NAMEPLATE INPUT CURRENT, not the motor current. So you will NEED that information in your planning because on some VFDs the RATED input current is actually significantly higher than the motor current. That's because these VFDs (mostly Asian) are designed around IEC motor sizes and adapted for use here, but because our motor FLAs are different, you end up having to use a larger VFD than the motor, resulting in that nameplate input current being a "gotcha" when it comes time to connect it.
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    If I’m not mistaken, you will also need to use disconnects that have aux contacts in them to shut the vfd down before the disconnect physically opens.

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    Quote Originally Posted by Stebs View Post
    If I’m not mistaken, you will also need to use disconnects that have aux contacts in them to shut the vfd down before the disconnect physically opens.
    That is what I do but from what I’ve learned here, it’s more important to have the drive OFF before the load is connected. Overlapping contacts.
    Tom
    TBLO

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    Quote Originally Posted by john2016 View Post
    I'm working on the design for an AHU replacement project that has four 30HP, 460V, 3 phase fans with each motor having its own VFD (soft start). The mechanical engineer is telling me that the fans will run at a low speed normally (around 18 or 19HP) but if one of the 4 fans breaks then the other fans would automatically speed up to accommodate the load.

    I was planning on providing a breaker in the switchboard and run a feeder up to the roof where the AHU is located. Then I would provide a wireway and four separate 3P100A disconnect switches with 80 amp fuses for each 30HP motor.

    My question is how to properly size the breaker in the switchboard that will feed the four 30HP fans with VFD's. Do I take 125% of the largest motor plus the sum of the remaining motors to size the breaker?
    Here are the specs for the AHU. 4 supply fans, 460/3/60, 35.85FLA, 44.81MCA, 80.0A FUSE SIZE.
    Would I size the breaker in the switchboard this way? 35.85 x 125% = 44.81 + 35.85 + 35.85 + 35.85 = 152.35A. Provide a 3P160A breaker?

    Also, the new AHU will be located outside of the existing AHU which is located in a roof top mechanical room. The new AHU will be put in place before the existing AHU is removed. The mechanical engineer is placing the new VFD's inside the mechanical room. Since the new AHU will be located outside the mechanical room would I place the 4 disconnect switches outside next to the new AHU even though the VFD's will be located inside the mechanical room? I'm not clear on where the correct placement for the disconnect switches should be since they are supposed to be in line of sight. The VFD's will not be in line of sight since they will be located inside the mechanical room and the new AHU will be located outside of the mechanical room.
    I a little confused at what you have. Is it one unit with four motors, four units with one motor each, four units with four motors each?

    4-30 HP motors is going to need more than 80 or 100 amps worth of supply, 4 motors totaling 30 hp may be able to have less then 80 amps of supply.
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    Quote Originally Posted by Jraef View Post
    ....
    As to the disconnect locations, you are not required to have the disconnect within site of the VFD, it must be within site of the MOTOR. So unless the VFD is right there next to the motor, you may need another LO/TO type disconnect down stream of the VFD anyway.If the VFDs are built into the AHUs, this may already be part of the assembly.
    ....
    (2017)
    430.102.A Disconnecting Means, Location, Controller
    430.102.B.1 and .2, and exceptions Disconnecting Means, Location, Motor

    Jraef -
    All the installations I have seen consider the VFD as the motor controller. And the disconnect is required to be "located in sight of the controller location" - no exceptions (for this case)
    Without data you’re just another person with an opinion – Edwards Deming

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