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Thread: How do I apply AIC or Interrupting Rating?

  1. #1

    Default How do I apply AIC or Interrupting Rating?

    I am involved in a tenet improvement project on a commercial property. As part of the scope of work I rewired the building, installing a new meter/main disconnect box and panel.

    I went into the building department to speak to the electrical inspector about the sequence of inspections and I was told that I need to obtain a letter from SCE detailing the AIC of their protective device(s) at the vault that supplies the building in question. I have never been asked this before. Why would he ask for this kind of information? What is he really looking for? Is he looking to have me install a main breaker with a certain Interrupting Rating? If SCE has a device that can handle 42,000 amperes, why would it matter if the main can only handle 10,000 amperes? What should the rating be on the my main breaker? What does the NEC say?

    Thank you

  2. #2
    In the Trades Bob NH's Avatar
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    You need to find out from the power company what is the maximum fault current that they can deliver to you. That depends on the characteristics of the transformer and the conductors between the source and your disconnect.

    Your main breaker must have a current interrupt rating at least as great as they can deliver if your system has a fault.

    Your disconnect must be able to safely and quickly disconnect without welding the contacts together.

    The process usually involves an computer analysis that includes the source characteristics and the rest of the circuit.

  3. #3

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    Quote Originally Posted by Bob NH
    You need to find out from the power company what is the maximum fault current that they can deliver to you. That depends on the characteristics of the transformer and the conductors between the source and your disconnect.

    Your main breaker must have a current interrupt rating at least as great as they can deliver if your system has a fault.

    Your disconnect must be able to safely and quickly disconnect without welding the contacts together.

    The process usually involves an computer analysis that includes the source characteristics and the rest of the circuit.
    Thank you so much for the straight forward response. Do you know the code section in NEC by any chance or a topic heading?

  4. #4
    Electrical Contractor/Instructor jwelectric's Avatar
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    Jun 2007
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    Your best code answer was given here

  5. #5

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    If the rating is 10,000 AIC, standard circuit breakers will work. If it is higher, say 23,000 or 42,000, or more, then the less expensive alternative is to use fuses. Make sure that the local inspector allows it. Why not get an old contractor to design your service equipment? [/I]

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