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Thread: AIC Ratings

  1. #1
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    AIC Ratings

    As a relatively new electrical inspector, I am experiencing new areas for the first time, so I need a little input here. I have 2 distribution panels (of many) that I have an issue with. The interrupting capacity rating for the main circuit breaker in both cases is 10,000 amps while the interrupting capacity on the branch circuit breakers is 22k.

    Can someone please explain if this is abnormal or not.

    These are, by the way, lighting panels 208y/120 3ph 4 wire w/grnd.

    Thanks, JM

  2. #2
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    Those are among the standard ratings for such components. It is not "abnormal" for the rating of the breaker to be higher than the rating of the panel. This particular rating only has to do with the ability of the component to withstand a short circuit, without something really bad happening (i.e., explosion, flying molten copper, etc.). So what matters is whether the component is rated for the amount of fault current that can be imposed by the distribution system. I think that most applications of 120/240 volt power in dwelling units would have a fault current less than 10,000 amps. In such cases, the equipment you describe would be fine.
    Charles E. Beck, P.E., Seattle
    Comments based on 2014 NEC unless otherwise noted.

  3. #3
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    masterelect1:

    You'll find stranger issues later. There is no problem mixing different AIC breakers in a panel. That depends only on what the contractor found and purchased. What it tells you is that the panelboard, as a whole with breakers and accesories, would be rated at 10KA. The interrupting rating shall be that of the lesser interrupting rating of any component in the paneloboard.

    And here, you have to check that this panel whitstands the calculated shortcircuit current (3ph and 1ph) at this point. Let's say, if you have a fault of 9KA, No hay bronca; but if it is 18KA, don't think that the 22KA breaker protects this panel.
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  4. #4
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    I do not have the necessary field experience to know if this is _common_ or not, but what you describe does not in and of itself indicate a problem.

    The AIC rating is simply the maximum short circuit current that the breaker is capable of opening against.

    The maximum available short circuit is determined by the supply characteristics, eg. the transformer, the service drop, the feeders, etc.

    The _requirement_ is that the AIC rating of the circuit breaker (or fuse) system be sufficient for the _maximum available short circuit current_. This may be achieved by making sure that each and every component can individually deal with the available short circuit (a 'fully rated system'), or this may be achieved by having a combination of components which together can deal with the available short circuit.

    If the available short circuit current is lower than 10KA at the panels that you describe, then the are fine.

    In general, the higher the AIC rating, the more expensive the part, so having a 22KAIC breaker downstream of a 10KAIC breaker sounds strange, but without knowing _why_ this is the case, I have no reason to see a problem. This combination of AIC rating might be because of something as benign as the higher rated devices simply being discounted, up to nasty things such as counterfeit parts.

    Perhaps if you provide the brand of breaker and brand of panel, others here could point you to more information.

    -Jon

  5. #5
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    Quote Originally Posted by charlie b
    I think that most applications of 120/240 volt power in dwelling units would have a fault current less than 10,000 amps. In such cases, the equipment you describe would be fine.
    OP said 120/208Y Sounds like 2 commercial lighting PNL's. In that case I would think the mains by nature would have a higher AIC rating. Are they 10k at 480 and higher at 208?
    What is the AIC upstraem?

  6. #6
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    Could it be a "Series-Rated panel" to achieve a higher AIR? I would look inside the panel to see if a Series Rated label is within.

    The test for Molded case circuitbreakers as allowed by UL 489.
    Standard interrupting rating tests for a 22,000 ampere sym. RMS
    interrupting rated circuit breaker will allow for a maximum 4 ft. of #10 rated wire on the line side for each lead, and 10 in. rated wire on the load side for each lead of the circuit breaker.
    Test station source impedance is adjusted to achieve a calibrated
    22,000 RMS symmetrical amps at 20% or less power factor. This
    circuit can achieve a peak current of 48,026 amps. For the calibration
    test, a bus bar (shorting bar) is inserted between the test station
    terminals.
    After the circuit calibration is verified, the shorting bar is removed
    and the circuit breaker is inserted. In addition, lengths of rated
    conductor are permitted to be added as shown. This extra rated conductor has a high impedance and effectively restricts the current to 9,900 RMS symmetrical amps. The power factor increases
    to 88% due to small conductor high resistance versus its reactance.
    So infact :
    The AIC rating is simply the maximum short circuit current that the breaker is capable of opening against
    is not true, You have to know how a component is tested to properly apply it. Having 22,000 amps available at the line side of a 22,000 amp Molded case circuit breaker may result in damage to the breaker. The same applies for 10 KAIR MCCB.
    Just my $.02
    Last edited by davidr43229; 06-27-07 at 05:09 PM.
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  7. #7
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    Quote Originally Posted by davidr43229
    Having 22,000 amps available at the line side of a 22,000 amp Molded case circuit breaker may result in damage to the breaker. The same applies for 10 KAIR MCCB.
    Just my $.02
    Correct me if I'm wrong, but damage to the breaker is irrelevant for AIC ratings. SCCR takes into account damage to the breaker, right?

  8. #8
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    David,

    Thanks for the correction and the expansion. I guess that quite a bit gets hidden in the UL standards.

    Could you help me to understand this better:

    1) Is it fair to say that the AIR rating of a breaker is _related_ to the current which it can actually interrupt, but that the test procedure means that the _actual_ current which the breaker is required to interrupt during a test is lower than the _rated_ number?

    2) Is it correct to say that a breaker with a 22KAIC _rating_ may be applied at a point in the system where the available fault current is up to 22KA? In other words, the 22KAIC breaker may be applied if the available fault current is up to 22KA, even though (as you describe the testing) the breaker design was only tested up to 9KA of actual current flow?

    -Jon

  9. #9
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    Jon,

    David will do a good job of presenting a fuse manufacturers views of series rating of circuit breakers. I suggest that you also get information from a circuit breaker manufacturer like Square D.

    From Square D's catalog 0600DB0301



    Myth:


    Circuit breaker ampere interrupting capacity (AIC) does not equal its ampere interrupting rating (AIR).



    Fact:


    The UL 489 test standard calls for four feet of rated copper wire on the line side and an optional 10 inches of rated wire on the load side for a multipole circuit breaker. The circuit breaker is tested in an enclosure. However, when testing its circuit breakers, Square D often uses a shorting bar instead of 10 inches of wire on the load side.


    Some industry members persist in believing and perpetuating the argument that four feet of wire affects the validity of a device’s interrupting rating because of the effects of wire impedance in the test circuit. So, why specify four feet of wire? UL 489 established four feet as the standard length for testing all

    circuit breakers—regardless of frame size.



    Why are the wires included in short circuit tests at all? UL states that wires are used in field applications, and, therefore, safety requires demonstration of performance in the following areas:


    Protection from wire damage due to heat


    Protection from wire pull-out



    Protection of the structural integrity of the circuit breaker (or the molded case) from the effects of wire whip.



    UL stipulates the use of rated wire in its test sequences to



    prove that the circuit breaker can protect it. Our industry therefore recognizes that AIC and AIR are the same and that they can be used interchangeably.




    edited: tried to correct appearance

    Last edited by jim dungar; 06-27-07 at 09:25 PM.
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  10. #10
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    Myth:
    Circuit breaker ampere interrupting capacity (AIC) does not equal its ampere interrupting rating (AIR).
    Tis not a myth, as you so state, but a fact Because the Ampere interrutping rating (AIR) as determined by UL is not it's Ampere Interruting Capacity AIC).
    Please find enclosed a shortcircuit calculator:
    http://www.cooperbussmann.com/software/index.aspx
    I would respectfully submit that within that 4' of #10 wire on the line side of that MCCB it proves mathmatically that 22,000 calibrated amperes turns into 9,900s amp at the actual line side of the MCCB due to reactance and impedence of that #10 wire, which the breaker is tested but yet gets a 22,000 AIR sticker, however that breaker was NEVER tested with 22,000amps at the line side (truely).
    I would again caution, you have to know how a component is tested (UL489) to properly apply it.
    "Tis a myth" to think that if you have 10,000 available at the line side of a breaker, simply installing a 10,000 AIR breaker and think you are fine. I'm sure that Jim will have more to say on this.

    Correct me if I'm wrong, but damage to the breaker is irrelevant for AIC ratings. SCCR takes into account damage to the breaker, right
    CB nor fuses have a SCCR. They may have current limiting propertities that can be used within UL508A SB to modify, to attain a higher combinational rating.
    Just my $.02
    I found my easter eggs & my car keys, life is good!

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