Fuse Size Calculator
Estimate minimum and practical standard fuse current ratings from normal load, design margin, startup inrush, fuse characteristic, and continuous operation. Use the result as an engineering starting point before checking the selected fuse datasheet.
Engineering tool
Fuse Size Calculator
Estimate minimum and practical standard fuse ratings with margin, inrush, and fuse-characteristic context.
Result console
- Minimum fuse rating
- 6.25A
- Recommended standard rating
- 6.3A
- Inrush current estimate
- 10A
- Continuous current margin
- 26%
Fuse selection status
Good
The entered margin meets the common 25% screening target. The selected slow-blow characteristic is better suited to short startup surges, subject to its time-current curve. Because the load is continuous, verify manufacturer derating at the expected ambient temperature.
Formula reference
Fuse Sizing Formulas
The calculated minimum is rounded upward to the next practical standard fuse current rating.
Inrush current = Load current × Inrush multiplierMinimum fuse rating = Load current × (1 + Safety margin)Recommended fuse rating = next standard value above minimumContinuous current margin = (Recommended / Load - 1) × 100%Variable definitions
- Load current
- normal operating current
- Safety margin
- additional continuous-current allowance
- Inrush multiplier
- startup current divided by normal current
Worked Example
For a 5 A continuous load with 25% margin and a 2× startup surge:
Minimum rating = 5 × 1.25 = 6.25 A. The next practical standard value is 6.3 A. Estimated inrush current is 10 A.
A slow-blow fuse may tolerate the short surge, but its time-current curve and ambient derating must be checked.
Engineering Notes
- Fuse rating should exceed normal operating current.
- Slow-blow fuses tolerate short inrush current better than fast-blow fuses.
- Continuous loads should use derating margin.
- Fuse voltage rating must meet or exceed circuit voltage.
- Final fuse selection should check datasheet time-current curves.
Support reference
FAQ
How do I calculate fuse size?
Multiply normal load current by one plus the desired safety margin, then select the next practical standard fuse rating and verify the datasheet time-current curve.
Should fuse rating be higher than load current?
Yes. The fuse must carry normal operating current without nuisance opening while still protecting conductors and equipment under fault conditions.
What is the difference between fast-blow and slow-blow fuse?
Fast-blow fuses open quickly under excess current. Slow-blow fuses tolerate short startup or charging surges before opening under sustained overload.
How much fuse margin should I use?
Twenty-five percent is a common starting point for continuous loads, but ambient temperature, enclosure conditions, inrush, and manufacturer derating rules may require more.
Does fuse voltage rating matter?
Yes. The fuse voltage rating must meet or exceed the maximum circuit voltage so it can safely interrupt a fault without sustained arcing.
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