In order to select the right amperage of the fuse, you first need to know the full-load steady-state current of the circuit at an ambient temperature of 20°C (68°F). Once the current value is determined, then a fuse rating should be selected as to be 135% of this value (taken to the next standard value).
For example, if the normal steady-state current is calculated to be 10 amps, then a 15A fuse rating should be selected [10 amps x 135% = 13.5 amps, the next larger standard size is 15A].
It is important to note that if the fuse is intended to be used in an environment with possibly very high or low ambient temperatures, then the nominal fuse current would need to be sized significantly higher or lower.
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