This is the very first calculation for the device under maximum load. With a chosen "security factor" we get a starting value for our iteration. Most sources use a security factor of 33%, but in litearture any value from 33% to 100% can be found.

This value takes into account, that some devices consume a much larger current when switched on. If you use a toroidal transformer, the inrush current mayst be ten times larger. Or a switch mode power supply has a large inrush current, because the capacitor needs to be charged. You mayst select the type by using the table below :

LABEL	I2t*	SPEED	POSSIBLE APPLICATION
FF	0.18	Super-Quick-Acting, Superflink	Resistive Load
F	1.045	Quick-Blow, Flink	Sensitive Semiconductor Circuits
M	6.00	Medium-Time-Lag, Mittelträge	...
T	6.73	Slow-Blow, Träge	Toroid Transformer, SMPS
TT	9.00	Super-Time-Lag, Superträge	...

*) in A²sec. Example data from 1 A types (Littlefuse, Eska, Schurter)

Datasheets usually talk of tem- perature derating, but with fuses, this becomes a re-rating, as the curve is not horizontally below 25°C. As the temperature is usually higher than 25°C, the fuse must be re-rated to take this temperature into account. The graph on the left is from a littlefuse fuse. The one you select mayst have a different curve. Use the sketch below to calculate the necessary value.

Fuses are designed to work for a maximum voltage. This is defined by the geometry of the design and the resulting spark-gap. The fuse must be able to reliable extinguish any spark replacing the melted wire in the fuse.

Therefore the fuse used must have a least a voltage rating of the applied voltage, more is no problem.