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60A fuse

    • 6 posts
    November 16, 2019 12:08 PM PST

    A 20 HP motor feeds a VFD with a 60A fuse from a 480v. Is 8 AWG enough for 60A fuse?

    • 39 posts
    November 17, 2019 12:05 PM PST

    (27A × 1.25) + 14A = 48A

    8 AWG rated 50A at 75°C [110.14(C)(1) and Table 310.15(B)(16)]

    largest branch circuit ground fault and short circuit protective device [430.52(C)(1) Ex]: 20 hp motor = 27A × 2.50 = 68, next size up = 70A

    the feeder protection: not more than 70A + 14A, = 84A.

    Next size down = 80A [240.6(A)]

    Therefore, for this specific arrangement, you would use 8 AWG conductor and an 80A breaker.

    • 27 posts
    November 18, 2019 10:42 AM PST

    Overcurrent protection for a motor is not based on the ampacity of the feeder, but the FLA of the motor, so your question asking if a 60A fuse in okay for a #8 feeder is not the correct way to consider the problem.

    I'm Canadian, so my expertise is in the Canadian Electrical Code, which does closely mirror the NEC.  I'm not sure why Ronnie added 14A to the cable minimum ampacity requirement.  Maybe it is buried in the NEC somewhere that I'm not aware of.

    The minimum cable ampacity is based on 125% of the motor FLA, which comes to 33.75A, so your #8 cable is adequate. 

    If a fuse is to be used for the overcurrent protection, we can choose between time delay (TD) fuses or non-TD fuses.  TD fuses are to be max 175% of FLA, and non TD fuses can be 300% of FLA max (we are allowed to go up the next higher available fuse size).   So you could use a TD fuse of 50A or a 80A non-TD fuse.  The 250% value Ronnie used should be limited to sizing circuit breakers only.  For motors, I always use Class J TD fuses simply for speed at which they operate for short circuits (and because I'm old-fashioned!), but I hear that the newer motor protection circuit breakers are pretty good too.


    This post was edited by Derek Nelson at November 18, 2019 10:43 AM PST
    • 67 posts
    December 7, 2019 9:30 AM PST

    Reference 2017 NEC:  If the installation is not under the NEC then cancel the rest of this.

    Suggest changing the question to: "Is #8 CU sufficient to feed the VFD?"

    Short answer: It depends on the input rating for the VFD.

     

    VFDs are covered under 430, Part X. 

    Conductors feeding the VFD:

    430.122 Conductors — Minimum Size and Ampacity.

    (A) Branch/Feeder Circuit Conductors. Circuit conductors supplying power conversion equipment included as part of an adjustable-speed drive system shall have an ampacity not less than 125 percent of the rated input current to the power conversion equipment.

    You didn't mention a bypass device, so branch/feeder conductors are covered under 430.122.A, Branch/Feeder Circuit Conductors.  The conductors feeding the VFD are required to be 125% of the of the rated input current of the VFD.

    The conductors are not sized by the motor FLA or T430.250.  Look at the VFD nameplate, or specifications, for rated input current.  For example if the drive is rated for a 20hp motor, and the VFD rated input current is 30A.  The VFD feeder conductors must be rated for at least 125% x 30A = 38A.  For this example, #8 CU (310.15.B.16, 50A) would be fine.

    Note:  If this were a 30HP drive with a rated input current of 45A, conductor rating is 45 x 1.25 = 56A.  #8 is not enough.  This example requires a #6 CU.  The smaller 20HP motor does not matter.

     

    Circuit Protection:

    430.130 Branch-Circuit Short-Circuit and Ground-Fault Protection for Single Motor Circuits Containing Power Conversion Equipment.

    (A) Circuits Containing Power Conversion Equipment.

    Circuits containing power conversion equipment shall be protected by a branch-circuit short-circuit and ground-fault protective device in accordance with the following:

    (1) The rating and type of protection shall be determined by 430.52(C)(1), (C)(3), (C)(5), or (C)(6), using the full-load current rating of the motor load as determined by 430.6.

    The branch/feeder circuit fuse/CB is not picked to directly protect the conductors.  430.130 sends us to the motor horsepower, 430.6, T430.250, and 430.52.

    For a 20Hp motor, T430.250 gives 27A FLC.  430.52.C.1, for time delay fuses, allows 175% x 27 = 47A.  Next size up, 50A is okay.

    For non-Delay fuses, 300% x 27 = 81A.  Next size up, 90A is okay.

     

    Recap:

    • Branch/feeder conductor to the VFD is sized by the VFD rated input current - not the motor FLA or T430.250 motor FLC.
    • Over current protection is sized per T430.250, and 430.52 - not the conductor size.
    • The NEC requires you read the manufacturer's manual and follow the manufacturer's instructions.  This can limit the size and type of overcurrent protection.
    • The pertinent NEC sections for 430, Part X cross reference other sections.  Follow the references.
    • The above example only covered standard stuff.  The NEC has exceptions to cover other installations where the equipment is different.  Highly recommend reading the additional pertinent sections, and exceptions.

    carl