Motor Short-Circuit Contribution (First Cycle)
The fault current spinning motors add in the first cycle.
Example
You enter
- Summed motor full-load current (A) 500
- Subtransient reactance (per unit, 0.167 = 16.7%) 0.167
- Utility / transformer fault current (A) 22000
You get
- Motor contribution (first cycle) 2994
- Total first-cycle fault 24994 A
Details, formula, and sources
Every 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.
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.
The motor-contribution relation (FLA / subtransient reactance) is a published IEEE first-cycle method; the summed motor FLA, reactance, and utility fault come from the system data.
Estimate. AHJ and licensed professional govern.
Field names used by the API: motor_fla_a, x_subtransient_pu, utility_fault_a, contribution_a, total_a
- Contribution a running motor feeds the fault at FLA / subtransient reactance (~4 to 6x FLA)IEEE 141
- First cycle decays within a few cycles; drives the momentary and interrupting duty, not the steady-state faultIEEE C37.13
- Lumped small motors grouped small motors are often taken at a lumped 4x FLAIEEE C37.13