Calculator Inputs
Fill the fields below, then press Calculate. The result will appear above this form.
Formula Used
1) Load current from power
- DC: I = P / (V · η)
- AC single-phase: I = P / (V · PF · η)
- AC three-phase: I = P / (√3 · V · PF · η)
2) Minimum fuse rating
Imin = (Iload · Kcont · Kmargin · Kinrush) ÷ Ktemp
- Kcont = 1.25 for continuous loads, otherwise 1.0.
- Kmargin = 1 + (margin% / 100).
- Kinrush is your inrush multiplier, adjusted by fuse type.
- Ktemp is an ambient derating factor (approximation).
3) Recommended rating is the next common size not below Imin, unless you choose exact output.
How to Use This Calculator
- Select DC, AC single-phase, or AC three-phase to match your supply.
- Choose whether you will enter current directly or calculate from power.
- Enter voltage, and then either load current or real power.
- For AC, set power factor and efficiency. Use 1.0 if unknown.
- Set inrush multiplier based on the equipment starting current.
- Enter ambient temperature and a safety margin for practical headroom.
- Enable continuous load if operation is sustained for hours.
- Press Calculate, then export CSV or print to PDF if needed.
Example Data Table
| System | Voltage (V) | Power (W) | PF | η | Inrush (×) | Ambient (°C) | Continuous | Min Rating (A) | Suggested Rating (A) |
|---|---|---|---|---|---|---|---|---|---|
| AC single-phase | 230 | 1500 | 0.90 | 0.90 | 2.5 | 40 | Yes | ≈ 11.6 | 12.5 |
| AC three-phase | 400 | 5500 | 0.85 | 0.92 | 6.0 | 50 | No | ≈ 18.9 | 20 |
| DC | 48 | 800 | — | 0.95 | 1.2 | 25 | No | ≈ 21.2 | 25 |
Examples are illustrative. Always validate with the exact fuse series and installation conditions.
Fuse Sizing Guide
1. Why fuse sizing matters
A fuse is a calibrated weak link that opens when current and time exceed its curve. Correct sizing protects wiring and equipment while avoiding nuisance trips. Undersizing can open during normal startup, while oversizing can let conductors overheat before protection operates.
2. Start from load current
The best starting point is the steady-state RMS current. If you only know power, this calculator converts power to current using voltage, power factor, and efficiency. For three-phase systems, the √3 term reflects line-to-line voltage relationships in balanced loads.
3. Continuous loads and the 125% idea
Many installation practices treat loads running for extended periods as “continuous” and apply a 1.25 multiplier. This provides thermal headroom for warm enclosures, aging, and manufacturing tolerance. If your duty cycle is well below 100%, heating can be lower, but conservative design still helps reliability.
4. Inrush and motor starting
Loads with magnetic or rotating components often draw several times rated current at startup. Small motors may see 3×–7×, transformers 5×–12×, and capacitor-input supplies can spike sharply. Time-delay fuses tolerate short surges better, while fast-acting fuses respond quickly to faults.
5. Temperature derating and real installations
Fuse elements run hotter in warm air, so the same nameplate rating can carry less current. The calculator applies a simple ambient derating factor to reflect common datasheet trends. Always confirm the exact series derating curve, especially inside sealed panels or near heat sources.
6. Selecting a standard rating
Real products come in preferred steps such as 10 A, 12.5 A, 15 A, 16 A, 20 A, and higher. The tool recommends the next common size that is not below the computed minimum. This keeps protection predictable and simplifies spares.
7. Don’t forget voltage and interrupt rating
Current rating is only part of the selection. Ensure the fuse voltage rating meets or exceeds the circuit voltage, and verify breaking capacity (interrupt rating) is above the available short-circuit current. High-energy circuits may require special high-rupturing-capacity fuses and holders.
8. Verification and good practice
Use this result as a disciplined estimate, then verify with manufacturer time-current curves and coordination requirements. Confirm the fuse holder rating, conductor ampacity, and upstream protection selectivity. When commissioning, measure inrush and steady current to refine inputs and reduce downtime.
FAQs
1) Should a fuse be rated exactly at the load current?
Usually no. Real loads vary, and fuses heat with ambient conditions. A margin plus any continuous-load multiplier helps prevent nuisance openings while still providing protection.
2) What inrush multiplier should I use for a motor?
For many small induction motors, 3×–7× is a practical starting range. Use measured starting current or a datasheet value when possible, then confirm the time-delay behavior.
3) Why is power factor included for AC?
Power factor links real power to apparent power. If you size from watts, PF affects the current drawn at a given voltage. Lower PF means higher current for the same watts.
4) How does efficiency change the result?
If the device is not 100% efficient, input current must be higher to deliver the same output power. Including efficiency prevents underestimating current and choosing too small a fuse.
5) Is a slow-blow fuse always better for inrush?
Not always. Time-delay types handle short surges, but they may allow higher fault energy before opening. Choose based on inrush profile, protection goals, and coordination with upstream devices.
6) Can I ignore temperature derating in a cool room?
If ambient is near the rating reference and airflow is good, derating is smaller. However, panels often run hotter than the room. Use conservative inputs unless you have measured enclosure temperature.
7) Does this replace code compliance checks?
No. It estimates a fuse rating using common engineering assumptions. Always verify conductor ampacity, installation rules, and local electrical codes, and confirm final selection with product datasheets.