Inrush Current Calculator
Estimate startup peak current, select first-order series resistance, or calculate capacitor charge timing for power inputs and switched loads. Use the results to screen stress on fuses, switches, connectors, rectifiers, and bulk capacitors.
Engineering tool
Inrush Current Calculator
Estimate startup peak current, required limiting resistance, or capacitor RC charge time.
Result console
- Peak inrush current
- 12A
Inrush status
Moderate
The estimated peak is moderate. Check fuse time-current behavior and startup stress in the complete power path.
Formula reference
Inrush Current and Charge-Time Formulas
The resistive model is a first-order estimate and uses total effective series resistance.
Peak current: Ipeak = Vsupply / RseriesRequired resistance: Rseries = Vsupply / IpeakTime constant: τ = R × C90% charge time: t90 ≈ 2.3τ99% charge time: t99 ≈ 4.6τVariable definitions
- Vsupply
- applied startup voltage
- Rseries
- total source and limiting resistance
- C
- input capacitance
- τ
- RC time constant
Worked Examples
Peak Current
24 V / 2 Ω = 12 A.
Required Resistance
24 V / 10 A = 2.4 Ω.
Charge Time
2 Ω × 1000 µF gives τ = 2 ms, t90 = 4.6 ms, and t99 = 9.2 ms.
Engineering Notes
- Inrush current occurs when capacitors or loads charge at startup.
- Series resistance, NTC thermistors, soft-start circuits, or active limiters can reduce inrush current.
- High inrush current can stress fuses, switches, connectors, rectifiers, and capacitors.
- Capacitor charge time depends on the RC time constant.
- Real inrush current also depends on source impedance and load behavior.
Support reference
FAQ
What is inrush current?
Inrush current is the brief high current drawn when equipment starts, capacitors charge, transformers energize, or loads transition from an unpowered state.
How do you calculate peak inrush current?
For a simple resistive first approximation, divide supply voltage by total series resistance: Ipeak = Vsupply / Rseries.
How do you reduce inrush current?
Use series resistance, an NTC thermistor, a controlled soft-start circuit, a pre-charge path, or an active current limiter suited to the load.
What is RC charge time?
The RC time constant is resistance multiplied by capacitance. A capacitor reaches about 90% after 2.3 time constants and about 99% after 4.6.
Why can inrush current blow a fuse?
If startup current exceeds the fuse time-current tolerance for long enough, the fuse element heats and opens even though steady load current is acceptable.
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