Fuse Size Calculator

Size a fuse from load current or power. Includes continuous-load margin, inrush allowance, and Show Work.

How to Use

  1. Choose your input method: Current or Power.
  2. Enter the system voltage (for Power mode) and the load value.
  3. Set whether the load is continuous and add any inrush margin.
  4. View the recommended fuse rating and open “Show Work” for the steps.
Protection View
Quick readout + visual: recommended fuse sizing for your circuit.
V
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I
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P
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Fuse
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Guidance: —
Source Fuse — Load — Visuals scale with sizing margin (overlay) and recommended fuse value (label).
Inputs & Settings
Enter current or power. Fuse guidance updates instantly.
Current mode sizes from load amps. Power mode uses V to compute I.
Use AC RMS voltage for mains loads.
Typical: 5V, 12V, 14.4V, 24V, 120V, 230V
In Current mode use A/mA. In Power mode use W/kW.
Continuous loads commonly use ~125% sizing margin.
Time-delay may tolerate brief inrush without nuisance blowing.
1.25×
Set 1.00× for none. Motors/compressors can be 2–5×.
Used for simple derating guidance (exact curves vary by fuse).
Show Work (step-by-step)
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Work is shown in base units (V, A, W). Fuse sizing varies by standard and manufacturer curves—this tool provides practical guidance.

Fuse Sizing Reference

  • Current mode: Base current is your measured/estimated load current.
  • Power mode: Current is computed as I = P ÷ V (use RMS for AC).
  • Continuous load: Common sizing guidance is ~I × 1.25 (125%).
  • Inrush: Add margin for motors/inductive loads; consider time-delay fuses when appropriate.
  • Safety: A fuse protects the wiring/circuit—verify wire gauge and connector ratings for your chosen fuse.

FAQ

Is a bigger fuse “safer”?

No. Oversizing can let wiring overheat before the fuse opens. The fuse should protect the circuit and wire, not the device.

Why does my fuse blow at startup?

Many loads have inrush current (motors, capacitors, incandescent bulbs). Add inrush margin or use a time-delay fuse if appropriate.

Do I fuse based on device rating or measured current?

Best practice is to use measured/realistic operating current, then apply continuous/inrush guidance and choose standard fuse sizes.

Can I size from watts?

Yes—use Power mode. The tool converts watts and volts into amps using I = P ÷ V (AC uses RMS voltage).

Tool Info

Last updated:

Updates may include standard fuse series, better rounding tables, and edge-case handling.