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Thread: Amp Frame / Amp Trip

  1. #1
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    Amp Frame / Amp Trip

    Industrial one-line diagrams illustrate circuit breakers in terms of "Amp Frame / Amp Trip: AF/AT. Any one have a definition of what the terms mean?

    Thanks Folks

  2. #2
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    There are multiple meanings, but the most common is that the breaker can handle for example a maximum of 400A, but it has a built-in trip setting that allows you to adjust the long-time overcurrent protection down to say 320A, or within some other range. Some devices come with what they called plugs, that allowed the user to change the range by taking one out and replacing with another. Usually there is a limit to the lower setting before you have to change to a smaller frame.
    "Just because you're paranoid, doesn't mean they're not out to get you"

  3. #3
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    It similar to a fused disconnect. The frame of the CB will allow a certain maximum size, the trip setting is changeable by inserting different fixed sized or adjustable trip devices.
    Rob

    Chief Moderator

    All responses based on the 2011 NEC unless otherwise noted

  4. #4
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    Um, no, no and no.

    AF is the frame size rating of the breaker
    AT is the CT size on the breaker.

    Neither of these tell you anything about when the breaker will trip, nothing to do with trip settings, or rating plugs.

  5. #5
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    Quote Originally Posted by zog
    Um, no, no and no.

    AF is the frame size rating of the breaker
    AT is the CT size on the breaker.

    Neither of these tell you anything about when the breaker will trip, nothing to do with trip settings, or rating plugs.
    Ok so the print shows a 400 amp frame with a 250 amp trip how do I size the wire if the 250 amp number is meaningless?
    Rob

    Chief Moderator

    All responses based on the 2011 NEC unless otherwise noted

  6. #6
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    Quote Originally Posted by infinity
    Ok so the print shows a 400 amp frame with a 250 amp trip how do I size the wire if the 250 amp number is meaningless?
    The print dosent show the trip at all, thats my point. You size the wire to supply the necessary loads and set the trip settings (LSIG) on the breaker to properly protect the equipment it is there to protect.

  7. #7
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    Quote Originally Posted by zog
    The print dosent show the trip at all, thats my point. You size the wire to supply the necessary loads and set the trip settings (LSIG) on the breaker to properly protect the equipment it is there to protect.

    Sounds like semantics to me. The OP asked a simple question, the amp frame size pertains to the breaker frame size and the amp trip pertains to the trip setting. Why does it have to be more complicated than that?
    Rob

    Chief Moderator

    All responses based on the 2011 NEC unless otherwise noted

  8. #8
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    Quote Originally Posted by infinity
    Sounds like semantics to me. The OP asked a simple question,
    And I gave a simple correct answer, it is not semantics at all the CT size is not the same in any way shape or form as the trip setting.


    Quote Originally Posted by infinity
    the amp trip pertains to the trip setting.
    It most certianly does not.

  9. #9
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    Quote Originally Posted by zog
    Um, no, no and no.

    AF is the frame size rating of the breaker
    AT is the CT size on the breaker.
    Only if the breaker has CTs.

    A Square D LA36350 is a thermal magnetic trip breaker in a 400AF (this is the physical size of the breaker) with a 350A long time trip element.

    I have always used:

    AF = maximum long time current available in that physical package.
    AS = sensor. CT, or plug, size of the breaker (the LT setting is always a multiple of the AS)
    AT = Long time trip setting of the breaker.
    Just because you can, doesn't mean you should.

  10. #10
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    [QUOTE

    AF = maximum long time current available in that physical package.
    AS = sensor. CT, or plug, size of the breaker (the LT setting is always a multiple of the AS)
    AT = Long time trip setting of the breaker.[/QUOTE]

    And then hopefully you receive a coordination study to adjust the trip settings
    (LSIG).

    L=Long Time
    S=Short Time
    I=Instantaneous
    G=Ground Fault
    Brian John
    Leesburg, VA

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