Calculate capacitor inrush current from supply voltage and series resistance, size the resistor for a current limit, or estimate motor starting current.

Include source, wiring, capacitor ESR and any fixed limiter.

Optional settings
0 V assumes a fully discharged capacitor.

Blank omits timing and energy.

Related Calculators

Inrush Current Formula

There is no single universal inrush-current formula that works for all devices. Inrush depends on the load type (capacitor input PSU, motor, transformer, etc.) and the total source/series impedance. The calculator above uses the following common estimation models:

Capacitor / PSU (RC model): Iโ‚šโ‚‘โ‚โ‚– & โ‰ˆ (Vโ‚› - Vแตข) / (Rโ‚œโ‚’โ‚œโ‚โ‚—), tau = Rโ‚œโ‚’โ‚œโ‚โ‚—Cโ‚œโ‚’โ‚œโ‚โ‚—, t95% โ‰ˆ 3tau\ Motor (rule of thumb): Iแตขโ‚™แตฃแตคโ‚›โ‚• & โ‰ˆ M IFLA

Variables:

  • I_{peak} is the peak capacitor-charge inrush current (A)
  • V_s is the supply voltage (V), and V_i is the initial capacitor voltage (V)
  • R_{total} is the total series resistance limiting inrush (ฮฉ)
  • C_{total} is the total capacitance being charged (F), and ฯ„ is the RC time constant (s)
  • I_{FLA} is the motor full-load (rated) current (A), and M is an estimated starting multiplier (unitless)

For an ideal DC step through constant resistance, the initial capacitor current is ฮ”V/R_total and decays exponentially with ฯ„ = RC. The 95% time is measured across the voltage change from the initial voltage to the supply. This is not a general rectified-mains PSU or transformer model. For motors, pick a typical starting method or supply a manufacturer or measured starting/full-load current ratio on a consistent measurement basis; RMS inrush and instantaneous peak are different. See TIโ€™s inrush-current guidance and Flukeโ€™s motor inrush measurement explanation.

What is Inrush Current?

Inrush current is the maximum (or near-maximum) transient input current drawn by an electrical device immediately after it is energized. Inrush current is typically higher than steady-state current due to effects such as charging input capacitors, establishing magnetic flux in inductive components (motors/transformers), and core saturation. It is a critical design and protection parameter, because excessive inrush can trip circuit breakers or blow fuses if not properly managed.

How to Calculate Inrush Current?

The following steps outline how to calculate the Inrush Current.

  1. First, choose a model that matches your load (capacitor-input supply, motor, etc.).
  2. Next, collect the inputs required by that model (for example: V_s, C_{total}, and total series resistance for an RC charge model; or FLA/power, voltage, and starting method for a motor).
  3. For a capacitor-input load, estimate the peak using I_{peak} โ‰ˆ (V_s โˆ’ V_i)/R_{total} and the time scale using ฯ„ = R_{total}C_{total} (with t_{95%} โ‰ˆ 3ฯ„ as a rough โ€œmostly chargedโ€ time).
  4. For a motor, estimate inrush as I_{inrush} โ‰ˆ M ร— I_{FLA}, using a multiplier M appropriate to the starting method (or a custom multiplier if known).
  5. Finally, sanity-check the estimate against equipment datasheets, protection curves, and (ideally) measured waveforms.

Example Problem:

Use the following variables as an example problem to test your knowledge.

supply voltage (V_s) = 120 V, capacitance (C) = 470 ยตF, series resistance (R_{total}) = 5 ฮฉ, initial capacitor voltage (V_i) = 0 V

peak inrush current I_{peak} โ‰ˆ (V_s โˆ’ V_i)/R_{total} = 24 A, and ฯ„ = R_{total}C โ‰ˆ 2.35 ms (so t_{95%} โ‰ˆ 7.0 ms)