Inrush Current Calculator

Estimate startup surge (peak) and steady current for common cases: capacitor charging (RC), motor/transformer, and resistive loads.

How to Use

  1. Choose a mode (Capacitor / Motor-Transformer / Resistive).
  2. Enter known values (voltage, resistance/impedance, capacitance, etc.).
  3. Review peak inrush and steady-state estimates.
  4. Open “Show Work” for formulas and assumptions.
Inrush Lab View
Visual feedback: surge level + time constant hints (RC mode).
IPEAK
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ISTEADY
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τ / Hint
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Risk
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Risk: —
Peak vs Steady Current Ipeak Isteady RC rise (approx) Note: RC mode uses ideal capacitor charge assumptions; motor/transformer mode is a quick estimate.
Inputs & Settings
Switch modes for the right inputs. Results update instantly (no URL changes).
Pick the closest real-world scenario. Use Custom Z if you know effective impedance at startup.
Common: 5V, 12V, 24V, 120V, 230V
Used only for risk indicator (does not change math).
Includes wiring, ESR, source resistance, and any limiter. Inrush ≈ V/R at t=0.
Used for time constant τ = R×C and current decay over time.
If unknown, leave blank and use stall/peak estimate only.
If you know locked-rotor/stall current, choose direct stall.
Typical: 3×–8× (varies a lot by motor type and load).
For purely resistive loads, peak = steady = V/R.
Peak ≈ V/Z. Use this if you have measured or modeled startup impedance.
Show Work (step-by-step)
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Work is shown in base units (V, A, Ω, F, s) for clarity. Motor/transformer mode is a simplified estimate.

Reference

  • Capacitor charging (ideal RC): I(0) = V/R, τ = R×C, current decays roughly exponentially.
  • Resistive load: I = V/R (peak equals steady).
  • Motor/transformer: Inrush is often several times steady current (depends on design, saturation, load, and supply impedance).
For safety-critical design (fuses, breakers, wiring), use measured data and manufacturer specs whenever possible.

FAQ

Why is capacitor inrush so high?

At startup the capacitor looks like a short (for an instant). Initial current is limited mostly by series resistance: I(0)=V/R.

What resistance should I use for RC inrush?

Include wiring, switch/relay contact resistance, source resistance, and the capacitor’s ESR. Adding an NTC or resistor reduces peak inrush.

Is motor inrush always 6×?

No—3× to 8× is a common rule-of-thumb, but real inrush depends on motor type, load, and supply. If you know locked-rotor/stall current, use it.

Does this tool replace datasheets?

No. It’s a fast estimator for planning and troubleshooting. Use manufacturer specs and measurements for final sizing.

Tool Info

Last updated:

Updates may include UI improvements, unit support, and edge-case handling.