Transformer Inrush Current Calculator

Estimate energization inrush, source sag, and limiter effect. Use rated current + inrush multiplier (or VA + V).

How to Use

  1. Enter transformer size (VA) and primary voltage, or enter rated primary current directly.
  2. Set an inrush multiplier (common ranges vary widely; use datasheet if you have it).
  3. Optional: add source impedance and/or an inrush limiter resistance to estimate sag and reduced peak.
  4. Open “Show Work” to see the math in base units.
Inrush Lab View
Peak estimate based on multiplier and series impedance (simple model).
Irated
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Iinrush
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V sag
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Z total
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Stress: —
Source V, Z Series R / NTC Transformer Inrush @ energize Visualization: spike bar scales with estimated inrush multiple (simple model).
Inputs & Settings
Enter transformer size and assumptions. Results update instantly.
If you only know watts, VA is often close for many loads (depends on PF).
Common: 120V, 230V. Use primary/input voltage.
If provided, overrides VA/V for rated current.
Datasheet is best. Toroids can be high. Use a conservative estimate.
Includes wiring + breaker + upstream. Use estimate if unknown.
Optional: NTC cold R or a fixed series resistor.
Show Work (step-by-step)
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This is a simplified estimation tool. Real inrush depends on switch-on phase angle, residual core flux, transformer design, and upstream impedance.

Reference

Basic relationships used here:

  • Rated primary current: I_r = VA / V (approx.)
  • Inrush estimate: I_inrush ≈ I_r × multiplier
  • Series impedance estimate: Z_total = Z_source + R_limiter (if enabled)
  • Voltage sag estimate: V_sag ≈ I_inrush × Z_total
Use datasheet or measured values whenever possible.

FAQ

Why is transformer inrush so high?

Core magnetization can momentarily drive the transformer into saturation depending on the instant of energization and any residual flux, producing a large current spike.

What’s a reasonable inrush multiplier?

It varies widely by transformer type and conditions. If you don’t know, use the datasheet or choose a conservative multiplier and compare with breaker/fuse behavior.

How does an NTC limiter help?

An NTC has higher resistance when cold to reduce the initial surge, then warms up and drops resistance during normal operation.

Is this an exact breaker trip predictor?

No—breaker trip curves depend on time, waveform, temperature, and type. Use this as a first-pass estimate.

Tool Info

Last updated:

Updates may include UI improvements, more limiter models, and edge-case handling.