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Thread: How to find AIC rating from existing panel

  1. #1
    Join Date
    Jan 2006
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    Angry How to find AIC rating from existing panel

    Every time I do field servey. I try to find the AIC rating for each panel especially the main serive switchboard. I can find other electrical information form nameplate but it alway say see label inside for AIC rating. So I have to remove the cover plate or other cover to find out. Do you guys have any good method to find it? Why the panel manufacturers (Simens, Square D, GE etc) put the label inside? It is pretty dangerous to remove the cover since all the panels are hot. Thanks.
    David Lin

  2. #2
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    I think the most important AIC is on the breakers. Panels may have an overall withstand or something, but the limiting factor is the AIC of the individual breakers. That is printed on most breakers in such small print, that I have to ask for someone to read it to me.

    Hope that helps.

    Jim T

  3. #3
    No, No...Do not use the OCPD markings as they could have been incorrectly installed. The only way to know the panel's AIC rating is to perform a short circuit analysis on the entire system. This should not only take into account the utility transformer's AIC but the cable impedances and any contributors like motors or secondary transformers.

  4. #4
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    Quote Originally Posted by Snorks
    No, No...Do not use the OCPD markings as they could have been incorrectly installed. The only way to know the panel's AIC rating is to perform a short circuit analysis on the entire system. This should not only take into account the utility transformer's AIC but the cable impedances and any contributors like motors or secondary transformers.
    Transformers don't have an AIC, but I think I understand what you're saying. Your suggestion will determine the available fault current not the ampere interrupting capacity. I agree that the available fault current must be less than the AIC, but your approach gives no information about the equipment that is installed, which is what I understand the OP asked.

    Jim T.

  5. #5
    No problem jtester. Looking at the OCPD markings or even the panel board's buss rating can lead to a very disasterous result. And transformers do have an AIC contribution which is noted as their bolted fault value. This is how a short circuit is started, with the utility transformer's AIC or bolted fault value. From there use IEEE Std 141 for an accurate short circuit analysis of the system. Cooper Bussmann has a booklet, I think the title is A Simple Approach to Short Circuit Calculations or something simular, that is free in a PDF format from their website. It stays pretty close to IEEE Std 141 but is simplier to understand.

  6. #6
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    Snorks

    What do you think AIC stands for? I believe it stands for "amperes interrupting capacity" If the device doesn't interrupt current it doesn't have an AIC rating. I agree transformers contribute to available fault currents, but since they don't interrupt current they don't have an AIC. I still stand by my post. I also agree that reading AIC values from a breaker doesn't indicate that they are properly rated, you must compare the AIC on the breaker to the available fault current

    Jim T

  7. #7
    Join Date
    Sep 2003
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    If the panel is part of a "fully rated" system, the panel AIC is the AIC of the lowest rated CB in the panel. So you have to look at them all and use the lowest rated CB, whether installed when new or added later. If a panel comes new as 18kA and someone adds a 14kA CB, then the (fully rated) AIC is now 14kA. If the panel is series rated (newer panels) then there should be a warning label to that effect.

    [EDIT] I agree, it's really boneheaded to put the AIC on a label INSIDE the panel. Thanks for reminding me. I wanted to add that to my master spec.

  8. #8
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    I have a lot of renovation projects. For those projects, the switchboard or MDP is existing. We need add some new panels to the existing switchboard or MDP. That is why we need to know the AIC rating of the existing switchboard or MDP to do short circuit current calcualtion to find out the AIC rating for the new panels.

    Basically, the AIC rating of existing panels shall be larger than available short circuit current. I don't know why Snorks say that it can lead to disasterous result based on breaker's marking or panel's AIC rating. Thanks.
    David Lin

  9. #9
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    Sep 2003
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    Quote Originally Posted by dahualin
    That is why we need to know the AIC rating of the existing switchboard or MDP to do short circuit current calcualtion to find out the AIC rating for the new panels.
    You don't need to know AIC to do a short circuit CALCULATION. You obtain the utility available fault current and go from there. But after you calculate the available short circuit current and X/R ratio, you use that info to check if existing CB's are rated OK and to spec new CB's. Is that what you meant?

  10. #10
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    Hi Sparkie,

    Do you mean to find the information from service transformer such as KVA and %impedance? Sometimes the nameplate on transformer is hard to read because of aging and other reasons. That is why I try to find the AIC rating of switchboard or MDP.
    David Lin

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