Fuse Sizing Calculator — rating, inrush and derating

Fuse rating for a load including inrush, with I²t and fast/slow-blow guidance.

Example: A 2 A load at 25 °C needs a 2.5 A slow-blow fuse.

Inputs

Continuous current(A)

Inrush current (optional)(A)

Motors and capacitive supplies can draw 10× or more at start-up.

Inrush duration (optional)(s)

10 ms for a capacitive supply; hundreds of ms for a motor.

Fuse type

Ambient temperature(°C)

Result

Fuse rating

2.5 A

Minimum required: 2.5 A
From continuous current: 2.5 A
Temperature derating: 100%
Headroom over load: 1.25×

Tolerates start-up surges while still clearing a sustained fault. A 2.5 A slow-blow fuse carries 1.3× your continuous current.
No inrush entered. If the load contains a motor, a transformer or significant input capacitance, its start-up surge is usually what determines the fuse — enter it above.

Standard ratings and their effective capacity at 25 °C

Rating	Effective at 25 °C	Max continuous load	Suitable
1 A	1 A	800 mA	no
1.25 A	1.25 A	1 A	no
1.6 A	1.6 A	1.28 A	no
2 A	2 A	1.6 A	no
2.5 A	2.5 A	2 A	yes
3.15 A	3.15 A	2.52 A	yes
4 A	4 A	3.2 A	yes
5 A	5 A	4 A	yes
6.3 A	6.3 A	5.04 A	yes
8 A	8 A	6.4 A	yes

Formula

rating ≥ I_continuous × 1.25 / derating    and must survive I²t of inrush

Worked example

A 2 A load at 25 °C needs a 2.5 A slow-blow fuse.

minimum = I_continuous × 1.25 / derating
2 A × 1.25 / 1
2.5 A
Fuses carry only about 75% of rating indefinitely without ageing, hence the 125%.
rating = next standard value ≥ required
2.5 A → 2.5 A
2.5 A

Frequently asked questions

How do I choose a fuse rating?

Start at 125% of continuous current, divide by the temperature derating factor, then round up to the next standard value. If the load has an inrush surge, check that too — it usually decides the answer.

Why 125% and not the exact current?

Because a fuse only carries about 75% of its nameplate rating indefinitely without ageing. Running one at its rating makes the element slowly degrade until it opens for no apparent reason, often months later.

When do I need a slow-blow fuse?

Whenever the load surges at start-up — motors, transformers, and any supply with significant input capacitance. A fast-acting fuse sized for continuous current will not survive repeated inrush. Slow-blow tolerates the surge while still clearing a sustained fault.

What is I²t and why does it matter?

The energy a fuse must absorb without opening, in ampere-squared seconds. It is the honest way to compare an inrush pulse against a fuse's capability, because a brief high current and a long moderate one can carry the same energy. Fuse datasheets publish an I²t rating for exactly this comparison.

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