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Thread: Breaker Sizing for motors with Soft Starters and VFD's

  1. #1
    Join Date
    Feb 2010
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    Ottawa, IL
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    12

    Breaker Sizing for motors with Soft Starters and VFD's

    Hi all.
    I was wondering if there is a rule of thumb for sizing breakers that feed VFDs or Soft Starters that will in-turn operate motors.
    I typically spec soft starts to ramp up and down and know that I can limit the current to 150% FLA.
    With this in mind I've been specifying the breaker sizes at 150% of FLA or the next size up. I don't have to option to look up the Soft Starter or VFD recommendations as we have to simply state VFD or Soft Starter without being biased in the design. Please help if you can. Thanks

  2. #2
    Join Date
    Jul 2006
    Location
    San Francisco, CA, USA
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    3,805
    On VFDs you are required to have the feed conductors sized at a MINIMUM of 125% of the VFD's maximum rated amps, the motor is NOT factorerd into it. So if you have a VFD rated for 100HP but it is connected to a 50HP motor, you MUST size the conductors for the max amp rating of the VFD, not the motor. You can then use a smaller breaker if you so desire, but most people just size the breaker for the conductors.

    On soft starters, all of the motor circuit breaker rules that apply in 430.52 do not change just because there is a soft starter involved. Conversely you should be able to use whatever breaker is listed for the motor HP as per usual, there is usually no need to up-size the breaker because if the starting current is going to be long and high enough to cause the breaker to trip, more than likely you will have exceeded the rating of the soft starter first. If you were to up-size the soft starter to compensate, then make the breaker match the soft starter size (assuming you don't end up violating a 430.52 rule in the process).
    Last edited by Jraef; 09-08-11 at 06:58 PM.
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  3. #3
    Join Date
    Sep 2011
    Location
    Collinsville, IL USA
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    Quote Originally Posted by Jraef View Post
    On VFDs you are required to have the feed conductors sized at a MINIMUM of 125% of the VFD's maximum rated amps, the motor is NOT factorerd into it. So if you have a VFD rated for 100HP but it is connected to a 50HP motor, you MUST size the conductors for the max amp rating of the VFD, not the motor. You can then use a smaller breaker if you so desire, but most people just size the breaker for the conductors.
    Can you tell me where the code addresses OCPD and conductor sizing for circuits feeding VFD's?
    I assume the conductors on the load side of the VFD is sized per 430.250?

    Thanks,
    James

  4. #4
    Join Date
    Aug 2004
    Posts
    11,542
    Quote Originally Posted by East Side Jimmy View Post
    Can you tell me where the code addresses OCPD and conductor sizing for circuits feeding VFD's?
    I assume the conductors on the load side of the VFD is sized per 430.250?

    Thanks,
    James
    430.122
    Bob

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