First-cycle duty of molded- or insulated-case breaker = MF × first-cycle symmetrical RMS current

Interrupting ratings 10,000A symmetrical or less:
MF = 1.0 is applicable whenever the first-cycle short circuit X/R ratio at the fault point (i.e., source side of breaker) is less than 1.7. If the first-cycle short circuit X/R ratio is greater than 1.7, the following formula can be used to find the MF.

Interrupting Ratings between 10,001A and 20,000A symmetrical:
MF = 1.0 is applicable whenever the first-cycle short circuit X/R ratio at the fault point (i.e., source side of breaker) is less than 3.2. If the first-cycle short circuit X/R ratio is greater than 3.2, the following formula can be used to find the MF.

Interrupting ratings exceeding 20,000A symmetrical:
MF = 1.0 is applicable whenever the first-cycle short circuit X/R ratio at the fault point (i.e. source side of breaker) is less than 4.9. If the first-cycle short circuit X/R ratio is greater than 4.9, the following formula can be used to find the MF.

The 3-phase rated short circuit current in RMS kA must exceed the first-cycle duty in asymmetrical RMS kA.

Note: On 3-phase systems where the voltage across a single pole under a fault condition may exceed 58% of the rated maximum voltage (e.g., corner-grounded delta under single line to ground fault), the single-pole interrupting capability shall be 87% of the 3-phase rated short circuit current.