Free Fuse Size calculator with clear step-by-step results.
A fuse must carry the continuous load indefinitely while still clearing a fault, so the rating is set at 125% of the derated operating current and rounded up to a standard size. The inrush energy figure - peak current squared times duration - is what determines whether a fast-acting fuse will nuisance-trip on startup.
Minimum rating
Minimum = operating current / (1 - ambient derating) x 1.25
Inrush energy
I2t = (operating current x inrush multiple)^2 x duration in seconds
Overcurrent protection must be coordinated with conductor ratings, equipment listings and the applicable electrical code. Selection should be verified by a qualified engineer or electrician.
Whenever inrush exceeds roughly three times the running current for more than about ten milliseconds - typical of motors, transformers and switch-mode supplies charging input capacitors.
No. The fuse protects the wiring and the device, and oversizing it removes that protection. A repeatedly blowing fuse means the wrong characteristic or an actual fault.