Motor Short-Circuit Contribution (First Cycle)
The first-cycle current the utility number leaves out: for the first cycle after a fault, every spinning motor becomes a generator, feeding the fault at roughly its full-load current divided by its subtransient reactance (~4-6x FLA). contribution = motor_FLA / x_subtransient; total = utility_fault + contribution. 500 A of running motors at 16.7% reactance on a 22 kA bus adds 2,994 A (6x FLA) -> 24,994 A total, so a panel rated exactly 22 kA AIC is under-rated by the motors. A first-cycle effect that decays in a few cycles -- it drives the momentary and interrupting duty, not the steady-state fault. Grouped small motors are often lumped at 4x FLA per IEEE C37.13. A design aid; a full short-circuit study governs.
Formula and source
contribution = motor_fla / x_subtransient_pu; total = utility_fault + contribution; multiple = 1 / x_subtransient_pu.
First-cycle motor short-circuit contribution (IEEE C37.13 / IEEE 141), first-principles, by name; a full short-circuit study governs.
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