How to Calculate the Short Circuit Rating

Short circuit current calculations are most valuable in power distribution systems.
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Formally, calculating short short currents is a complex task because of the many variables involved. For this reason, many engineers use computer programs to calculate the currents. However, you can use a simplified method to approximate the short circuit currents for high voltage 3-phase power distribution systems. Power distribution systems are driven by 3-phase transformers and you can use the data on the nameplates of the transformers to calculate short circuit currents.

    Locate the nameplates on a transformer associated with a power distribution system. Find the kilovolt-amperes rating, or "KVA," the secondary voltage, or "Vsecondary," and percentage impedance, or "Zpercent." As an example, assume the KVA is 1200 KVA, Vsecondary is 480 volts and Zpercent is 7.25 percent.

    Calculate the transformer secondary load current using the formula: SLC = KVA/(Vsecondary/1000) x 1.73. Continuing with our example:

    SLC= 1200/(480/1000) x 1.73 = 1200/0.48 x 1.73 = 2500 x 1.73 = 4325 amps

    Calculate the transformer secondary short circuit current using the formula: SSC = (SLC x 100)/Zpercent. Continuing with our example:

    SSC = (4325 amps x 100)/7.25 = 59,655 amps.