Capacitor Bank Switching Inrush Calculator
Compute single bank and severe back-to-back capacitor switching inrush peak currents and transient frequencies per IEEE Standard C37.99.
Capacitor Bank Ratings
Source Impedance / Paralleled Bank
Transient Inrush Results
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Frequently Asked Questions
Why is back-to-back capacitor switching so much more severe than single bank switching?
When a single capacitor bank is energized, inrush current is limited by the large inductance of the upstream utility substation transformer. In back-to-back switching, an uncharged bank is paralleled across an already-energized bank separated only by a few feet of busbar; the only impedance limiting the discharge is minuscule busbar inductance (microhenries), causing tens of thousands of amps of inrush at frequencies up to 50 kHz.
How do capacitor-duty contactors protect equipment from inrush?
Capacitor-duty contactors feature auxiliary early-make contacts fitted with high-resistance nichrome damping wire loops. During closing, the damping resistors pre-charge the capacitor for 2 to 5 milliseconds to quench inrush before the main silver contacts close, preventing contact welding.
What is the role of an inrush current limiting reactor?
An air-core inrush reactor installed in series with each capacitor bank increases loop inductance L_eq. Because inrush current varies inversely with √L, adding even 20 to 50 µH of inductance reduces peak inrush current and di/dt by 70% to 85%.