When you apply these requirements to a circuit breaker, you must calculate the maximum 3-phase fault current the breaker will be required to interrupt. Equipment intended to interrupt current at other than fault levels shall have an interrupting rating at nominal circuit voltage sufficient for the current that must be interrupted.” “Equipment intended to interrupt current at fault levels shall have an interrupting rating sufficient for the nominal circuit voltage and the current that is available at the line terminals of the equipment. Because these calculations are life-safety related, they're mandated by 110.9 of the NEC, which states: Therefore, it's important to protect personnel and equipment by calculating short-circuit currents during system upgrade and design. These currents can produce considerable thermal and mechanical stresses in electrical distribution equipment. All electrical systems are susceptible to short circuits and the abnormal current levels they create.
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