Ambient Temperature Derating Curve for Fuses

What is Derating a Fuse?

In electronics, derating a fuse is a technique used to prolong the life of the fuse by adjusting the current capacity of a fuse for changes in ambient temperature.

How to Temperature Derate a Slow Blow, Medium or Fast Acting Fuse

Ambient Temperature Derating Curve for a Slow Blow Fuse, Medium Fuse or Fast Acting Fuse

The current capacity of fuses is 110% of rating when installed in a standard UL test circuit and tested in open air at 25℃ ambient. This allows for derating to 100% of rating in an enclosure at 40℃ ambient. At higher ambient temperatures, continuous current carrying capacity will be decreased as shown in the chart above. This closely follows the derating tables for all electrical equipment, and reduces equipment burnout due to high ambient conditions.

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Warning: When using this information to perform electrical work, call a licensed electrician and consult the NEC® for safety. All licensed electricians have passed examinations covering the National Electric Code®, know state and local building codes, and may carry insurance to cover damages.