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Thread: Calculating AIC ratings at service.

  1. #1
    Join Date
    Apr 2003
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    south Florida
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    Calculating AIC ratings at service.

    My inspector is requiring me to provide the AIC at the main breaker on a 100 amp, residential service. The utility company had given me the AIC at the secondary side of the transformer (13021). The service drop is approximately 40 feet of #2 alumminium. I would appreciate any information about how to calculate the ACI at the service. Thanks.

  2. #2
    Join Date
    Feb 2003
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    3,172

    Re: Calculating AIC ratings at service.

    p51: In other words you are guilty until proven innocent.

    Charlie, the utility man, can address this better than I can.

    Utility companies will not install a transformer that can create damage to an approved service, on a dwelling.

  3. #3
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    Seattle, WA
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    Re: Calculating AIC ratings at service.

    Here’s one simple way - not as precise as a computer program can do, and with no certifications behind it.

    Use Ohm’s Law in the form: R = E/I. If a 240V source has 13,012 amps available at the secondary, then the resistance of the transformer can be calculated as 0.0184 ohms. From Table 8, the DC resistance of 40 feet of #2 aluminum is (0.319 ohms/1000 ft) x (40 ft) = 0.0128 ohms. Adding 0.0184 to 0.0128 gives 0.0312 ohms.

    Now using the I = E/R version of Ohm’s Law: I = 240/0.0312 = 7,700 amps. A panelboard rated above 10K AIC would suffice. Note that if you went through the trouble to find an AC resistance (not just the DC resistance), the AIC result would be smaller. So using DC resistance is conservative.

    However, I agree with Bennie: This is not the kind of process that most electricians would (or even should) be called upon to undertake for a residential service. It is the kind of thing I would do for a complex distribution system, using a fancy computer program, and issuing the results under my seal and signature. {Please note that you may not use the above information as though it were issued under my seal - it is not!} But I can’t understand why an Inspector would want such a calculation, and why an Inspector would be willing to accept it without a PE seal.

    The “Other Charlie.”
    Charles E. Beck, P.E., Seattle
    Comments based on 2014 NEC unless otherwise noted.

  4. #4
    Join Date
    Feb 2003
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    Re: Calculating AIC ratings at service.

    Sorry Charlie, I forgot you are well informed also. Now I know where I can get accurate calculations :o

  5. #5
    Join Date
    Feb 2003
    Location
    Indianapolis
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    Re: Calculating AIC ratings at service.

    Did someone call me? LOL

    P51, starting at the known level of fault current available, you can calculate the downstream levels by using the Point-to-Point calculations on page 43 (page 4 in Section 7) of the Bussmann SPD. Generally speaking, the impedance of a #2 triplex is enough to never cause a problem with a 10 kAIC breaker.

    You will find the formulas and calculations are easy to use and the instructions are straight forward. :cool:
    Charlie Eldridge, Indianapolis, Utility Power Guy
    Responses based on the 2011 NEC, unless stated otherwise.

  6. #6
    Join Date
    Feb 2003
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    3,172

    Re: Calculating AIC ratings at service.

    p51: A think tank, with the knowledge of the two Charlie's, would cost a bunch.

    My information is cheap

  7. #7
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    Wisconsin
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    Re: Calculating AIC ratings at service.

    In my area, the fault current supplied by the utility is based on standard utility service drop lengths and conductors. Their fault level takes into account the worst case scenario of a future increase in size of the service supply transformer.

    No adjustments are made to the quoted value unless the service conductors are excessivley long or are provided/owned by the electrician/customer.

    Using the values provided by the utility, without adjustment, will always result in a safe system.

    Of course, a qualified person should be involved in calculating any fault currents.
    Just because you can, doesn't mean you should.

  8. #8
    Join Date
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    Seattle, WA
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    Re: Calculating AIC ratings at service.

    That's a very good point, Jim! Perhaps, P51, you should verify with the utility whether the AIC they gave you was at the secondary terminals of their transformer (the basis of my calc) or at the service entry point some assumed distance from their transformer.
    Charles E. Beck, P.E., Seattle
    Comments based on 2014 NEC unless otherwise noted.

  9. #9
    Join Date
    Feb 2003
    Location
    Indiana
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    Re: Calculating AIC ratings at service.

    The local Poco provides the fault current at their transformer secondary terminal(s).

    For a padmount 50KVA 120/240V 1Ø 3W it is 23640 Amps which is used for many of the residential services.

    If the transformer is close, in cable length, to the Service Equipemnt, often times the 10 KAIR rating of the panelboard is not code compliant.

    PS

    This size transformer is used for many small commerical services and the transformer is typically only about 12 to 15 feet of service entrance conductors from the Main OCPD.

    Hardly ever the 10KAIR equipment is code compliant.

    [ April 17, 2003, 07:35 PM: Message edited by: gwz2 ]

  10. #10
    Join Date
    Mar 2003
    Location
    Illinois
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    12

    Re: Calculating AIC ratings at service.

    I had a similar question, try thishttp://mikeholt.com/documents/calculations/formulas/Fault.xls

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