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what is the SCCR of this circuit ??

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    Originally posted by samvivi7 View Post
    i have two panels, my panel has a CB rated at 65kA with a transformer 15kVA which is feeding a second panel (third party vendor) rated at 5kA.
    So, do i have to have a 65kA CB with a 65kA transformer and the sub panel rated at 65kA ?
    You need to be very careful with your terminology. There are several things that need to be considered and they all sound similar, partly because they are all expressed in thousands of Amps, typically as kA.

    The Short Circuit Amps (SCA) or short circuit current available is the amount of current that can flow into a bolted fault condition. The starting SCA is often provided by the utility and then re-calculated at various points in the distribution system, such as after a length of conductor or a transformer. All equipment must be rated to handle the amount of SCA at its line side terminals. This value is may be calculated in the field. SCA is really what most people are asking for.

    Amps Interrupting Capacity (AIC) is a rating unique to protective devices that interrupt fault current, like breakers and fuses. This value is determined by the manufacturer based on testing. Devices should be selected so that their AIC exceeds the available SCA.

    Short Circuit Current Rating (SCCR) is the amount of fault current that a piece of equipment is rated to tolerate until a protective device operates clearing the fault. This value must be provided by the equipment manufacturer. There is no universal method for determining it in the field. Each type of equipment may get its SCCR value in a different manner. Panelboards, switchboards, disconnect switches, and motor control centers, may have a maximum tested/listed value with a label that limits its actual rating based on the field installed devices. An automatic transfer switch may have different tested SCCRs depending on the specific device protecting it. Control panels, like those built to UL508A, may have an SCCR calculated by the registered assembly shop, although many simply use the UL default value of 5kA.
    Last edited by jim dungar; 07-11-19, 03:30 PM. Reason: typos
    Just because you can, doesn't mean you should.


      I agree with Jim.