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Thread: Reducing AIC at a machine.

  1. #21
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    Quote Originally Posted by Jraef View Post
    Point taken. The OP is in Indiana, so they are still on the 2008 code and the latest changes wouldn't apply, unless the machine is considered "industrial" (Article 409 covers "Industrial" control panels and was introduced in the 2005 code).

    PS: Actually, the relevant wording hasn't really changed since the 2005 code. 409 requires it for "industrial controls", then defines that. But 430.8 requires it as well, for "motor controllers" in general.

    MSHA uses 1968
    iirc PA recently went to 2008 from 2002



  2. #22
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    Quote Originally Posted by Jraef View Post
    Hmmm...
    Having the correct SCCR at the panel and having it meet or exceed the AFC is now a CODE REQUIREMENT.
    Your methodology, while probably good for a guesstimate (I'm not doing the math), is NOT going to suffice in lieu of a proper fault study and the required labeling.
    Proper SCCR has been a Code requirement for decades. But because everyone seems to have routinely ignored 110.10, the CMP's have been adding additional SCCR requirements to other code articles as well.
    Just because you can, doesn't mean you should.

  3. #23
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    I am going to nitpick on a couple points.... Sorry

    Quote Originally Posted by drktmplr12 View Post
    you need a calculation that has been signed and sealed by an engineer regularly engaged in the practice of calculating fault currents.

    you want to ask if they have an existing short circuit study or set of plans listing the amount of fault current available at the equipment.
    Unless someone is asking who has authority or you need to listen to, I dont see why this would necessarily need to be done by an engineer. Basic fault current calculations are very easy and can be done by a competent electrician. Sure, get into a bunch of motor contribution, or a more complex system then maybe you will need an engineer to be involved.

    Quote Originally Posted by wbdvt View Post
    I would add that the available fault current has to be obtained from the utility and be careful that you are not provided with the standard utility response of an infinite bus fault current based on transformer size.
    I dont quite follow you here. First where is it stated that it "has to be obtained from the utility"? Perhaps it is a local requirement, but the NEC does not contain such language. If you do obtain the value form the utility, I think 94% of the time you will get their generic response. IMO and experience, if you dont want the generic figure, you have to calculate it yourself from the actual equipment data and still good luck getting any information on the primary impedance. Maybe you mileage has varied in that department. Finally, for AIC purposes, there is nothing wrong with using a higher than actual fault current value (other than possibly spendng money on unneeded AIC, but sometimes worth the simplicity).
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  4. #24
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    Quote Originally Posted by electrofelon View Post
    Unless someone is asking who has authority or you need to listen to, I dont see why this would necessarily need to be done by an engineer. Basic fault current calculations are very easy and can be done by a competent electrician. Sure, get into a bunch of motor contribution, or a more complex system then maybe you will need an engineer to be involved.
    someone regularly involved in calculating available fault current, in any case. the path of least resistance would be an existing study, assuming someone knows where to look inside said study

  5. #25
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    In the previous posts, there has been discussion on the NEC for code compliance regarding SCCR. What is overlooked quite frequently is the OSHA requirement below:

    1910.303(b)(5)
    Circuit impedance and other characteristics. The overcurrent protective devices, the total impedance, the component short-circuit current ratings, and other characteristics of the circuit to be protected shall be selected and coordinated to permit the circuit protective devices used to clear a fault to do so without the occurrence of extensive damage to the electrical components of the circuit. This fault shall be assumed to be either between two or more of the circuit conductors, or between any circuit conductor and the grounding conductor or enclosing metal raceway.


    This seems to indicate that all the equipment in the circuit has to be suitable for the fault current. So having properly rated equipment installed is also an OSHA requirement.

  6. #26
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    Quote Originally Posted by electrofelon View Post
    I am going to nitpick on a couple points.... Sorry



    Unless someone is asking who has authority or you need to listen to, I dont see why this would necessarily need to be done by an engineer. Basic fault current calculations are very easy and can be done by a competent electrician. Sure, get into a bunch of motor contribution, or a more complex system then maybe you will need an engineer to be involved.



    I dont quite follow you here. First where is it stated that it "has to be obtained from the utility"? Perhaps it is a local requirement, but the NEC does not contain such language. If you do obtain the value form the utility, I think 94% of the time you will get their generic response. IMO and experience, if you dont want the generic figure, you have to calculate it yourself from the actual equipment data and still good luck getting any information on the primary impedance. Maybe you mileage has varied in that department. Finally, for AIC purposes, there is nothing wrong with using a higher than actual fault current value (other than possibly spendng money on unneeded AIC, but sometimes worth the simplicity).


    If you don't know actual primary impedance yet still calculate service fault current based on infinite primary - and your values at load end devices is under their rating - you are fine with equipment selected, it will only get lower if you did figure out actual primary values. You don't even need to fully know how to calculate this, just need to know how to read transformer impedance off the nameplate, need to know how to determine conductor size, type, length, and a few other reasonably simple details and enter that information into spreadsheets that are readily available out there and can easily come up with values that are "worst case scenario".

  7. #27
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    This is an interesting thread especially for us non-engineers who don't deal with this stuff very often. There seems to be some opinions that this is a very complex question and some other opinions that it really isn't. So getting back to the OP where it said this:

    We installed several 30 & 60 amp type TEY 14K AIC 3 phase breakers. Ran the conduit and wire to where each new machine. Each machine has 3 fuses that our feeds connect to. Just received a call from the company that the machine manufacture wants the AIC on some of the machines to be 10K AIC
    We have a circuit breaker and some conductors in a branch circuit feeding a piece of equipment. Given the information above it is difficult to do the calculation and find the answer to the OP's question?
    Rob

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    All responses based on the 2014 NEC unless otherwise noted

  8. #28
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    Quote Originally Posted by infinity View Post
    This is an interesting thread especially for us non-engineers who don't deal with this stuff very often. There seems to be some opinions that this is a very complex question and some other opinions that it really isn't. So getting back to the OP where it said this:



    We have a circuit breaker and some conductors in a branch circuit feeding a piece of equipment. Given the information above it is difficult to do the calculation and find the answer to the OP's question?


    For a branch circuit, especially one of significant length, the impedance of the circuit conductors should limit the AFC (Available Fault Current), possibly enough to solve the whole problem.

  9. #29
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    there was a thread recently that had links for a checklist of requirements for a code review/permit app process
    it required utility furnished primary values



  10. #30
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    Quote Originally Posted by infinity View Post
    This is an interesting thread especially for us non-engineers who don't deal with this stuff very often. There seems to be some opinions that this is a very complex question and some other opinions that it really isn't. So getting back to the OP where it said this:



    We have a circuit breaker and some conductors in a branch circuit feeding a piece of equipment. Given the information above it is difficult to do the calculation and find the answer to the OP's question?
    Rob,
    This is the problem with the OP's statement as it stands:
    ...machine manufacture wants the AIC on some of the machines to be 10K AIC...
    That statement as typed is not using valid terms and makes no sense. MACHINES cannot have an "AIC", only the BREAKERS can. On top of that, an "AIC rating" on a breaker is a "not to exceed" value and the ones he ALREADY installed are 14kAIC, so wanting them to have a lower interrupting capacity value is ridiculous; the breakers he provided ARE at least 10kAIC, in fact they are 40% MORE than 10kAIC. So their "want" is indicative of someone in this situation not knowing what they are talking about. I'd hazard a guess that the machine manufacturer is not saying it right, and the OP didn't quite know enough to push back yet. Hence my attempt to edumacate him and have him go back to get them to say what they REALLY need.
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