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Thread: Circuit breaker AIC ratings

  1. #1
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    Circuit breaker AIC ratings

    I recently took my masters exam. One question they asked, What does AIC mean? I know that it has to do with the inrush current at the service equipment,and that the equipment be rated for the right AIC for the utility company. I want to know what the definition is. Can anyone give me the answer!

  2. #2
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    Re: Circuit breaker AIC ratings

    Amperes Interuption Current. This different than the thermal rating of the breaker. This AIC rating is "in short" a short circuit interuption capacity.

    Roger
    Moderator

  3. #3
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    Re: Circuit breaker AIC ratings

    AIC is now being referred to as AIR -ampere interrupting rating. Not to be mistaken with short circuit rating.
    AIR is the value used to determine if the ocpd is capable of interrupting a fault current, without causing damage to itself or the equipment.

    If an overcurrent protective device is inserted into the circuit and it's rating falls short of the available short circuit current available, damage or personal injury downstream can occur.
    Refer to 110.9.
    Pierre

    [ January 04, 2004, 05:37 PM: Message edited by: pierre ]

  4. #4
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    Re: Circuit breaker AIC ratings

    Pierre, are you really wanting to make a blanket statement at this time that AIC is AIR industry wide?

    Also, I agree ASCC is different than AIC or AIR but that is another conversation and AIC or AIR is only a part of this.

    AIR is the value used to determine if the ocpd is capable of interrupting a fault current, without causing damage to itself or the equipment.
    and what would you call a fault of this magnatude if not the generic term of "short circuit" or "bolted"?


    Roger
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  5. #5
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    Re: Circuit breaker AIC ratings

    I was told it was available instantaneous current.

  6. #6
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    Re: Circuit breaker AIC ratings

    To: mgnh, I would recommend that you contact the Cooper Bussman Company to get their book on "SPD Selecting Protective Devices, Based on The 2002 NEC. Their phone number is 636-394-2877. They will send one to you free of charge. If you were close by I have several in my possession.
    This book will explain all that you would ever want to know about AIC, and Selective Fuse Co-ordination.
    Now to answer your question, "What does AIC mean?"
    If you took a bar and shorted out the secondary of the transformer that feeds your facility, that would be determined to be the Available Fault Current. Suppose that this current would be 50,000 amperes. Any fuse down stream of that fault would have to have an interrupting rating of at least 50,000 amperes.
    Today, if you would look on the barrel of the fuse, you would find a rating saying 200ka or 300ka. Meaning 200,000 or 300,000 amperes.
    Now, as you go further downstream from the 50,000 amp fault, the fault current will decrease because of the resistance of the wiring in the circuit. For example, generally at a lighting panel, the circuit breakers are in the range of 5000 AIC or 10,000 AIC.
    But bottom-line, get the book that I suggested, and it will answer all of your questions.

  7. #7
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    Re: Circuit breaker AIC ratings

    Friebel, the Bussmann SPD you are talking about is available on line.

    By the way, the AIC that is marked on circuit breakers is a rating. In the smaller sized circuit breakers, if they received an actual bus shot at their rating, they would likely fail to open and destroy themselves. Circuit breakers are tested with a representative piece of conductor (4' if I remember correctly) that will significantly cut down on the amount of fault current a small circuit breaker actually sees. There was a proposal to require the actual interrupting capacity be placed on circuit breakers during the 2002 Code cycle but the circuit breaker people managed to stop it from happening.
    Charlie Eldridge, Indianapolis, Utility Power Guy
    Responses based on the 2011 NEC, unless stated otherwise.

  8. #8
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    Re: Circuit breaker AIC ratings

    Thank-You for your reply Charlie, I did realize that you could download the manuaul that I mentioned. The manual that I have has 190 pages, both sides, that is why I suggested to contact Bussman for a manual. But again, thank-you for your input. This is what makes this forum great, with knowlegable people like you.

  9. #9
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    Re: Circuit breaker AIC ratings

    Mgnh,this may or not be helpfull.Ampacity inrush current AIC is a rating.There is a formula that exceeds the competence of the common electrician.It;s application however is present in all of the work we do.The farther one is away from the transformer the better off the aic formula is because of the resistance inherent in the wire.When the transformer gets real close the rating jumps up dramatically.And so do the costs.

  10. #10
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    Re: Circuit breaker AIC ratings

    The AIC rating is the Asymmetrical Interrupting Capability. When making the normal fault current caculations, we end up with the Symetrical Fault Current. Due to the inductive nature of electrical systems, the actual fault current can be much higher. The inductance of the concuctor,
    of motors and transformers adds a factor the the sysmetrical fault current. This factor is determined by the systems X/R ratio. The resultant
    fault can be in the range of 1.6 x the symetricaL
    fault for the first few cycles. It begins to decay after that.

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