Inrush Current

Formula

Description

When a power supply or transformer is first energized, the input filter capacitors are fully discharged and appear as a short circuit, causing a large inrush current limited only by the source impedance and wiring resistance. The worst case occurs when the AC voltage is at its peak at the instant of switch-on. Inrush current can be 10-100 times the steady-state current and may trip circuit breakers, blow fuses, or damage connectors and relay contacts. NTC thermistors (inrush current limiters) are commonly used to reduce the inrush to safe levels.

Variables

• I_inrush — Peak inrush current (A)
• V_peak — Peak mains voltage at switch-on (V)
• Z_source — Total impedance including wiring, NTC, and source (Ω)

Practical Notes

For a 230 V mains supply, Vpeak ≈ 325 V. With only 0.5 Ω of wiring and source impedance, the inrush can reach 650 A. An NTC thermistor of 5 Ω cold resistance limits inrush to about 65 A. Active inrush limiters use a series MOSFET with controlled gate drive to gradually charge the input capacitors. IEC 62368 and other safety standards specify maximum allowable inrush currents.