We must be careful to define our terms here. It may indeed be sound practice to only operate a fuse at 75-80% of it's rated current to avoid long term nuisance tripping - but that's a different matter (similar to derating capacitors below their stated voltage). Here are a few examples, and I'll certainly read any Data Sheets you provide to the contrary: DigiKey Fuse Tutorial: "The current rating is the maximum current that can flow through the fuse under normal conditions. Fuses will typically be de-rated at 25% to avoid nuisance blowing at 25˚C". Schurter Fuse Selection Guide: "A fuse, which is designed according to IEC standard, can continuously operate at 100% of rated current of the fuse. A fuse, which is designed according to a UL standard, can continuously operate at 75% of rated current of the fuse." Littlefuse Normal Operating Current: "The current rating of a fuse is typically derated 25% for operation at 25ºC to avoid nuisance blowing." So it might be a matter of semantics, but I'll stand by my original statement - which basically said a 1 amp rated fuse will carry 1 amp forever at 25 degrees C. But certainly a host of factors can be used to justify derating under practical applications, including: temperature, peak inrush currents, vibration, type of fuse holder, etc.