I have an existing feeder in an MCC with an IPD TC100L gG fuse (100A) with a feeder cable with a derated CCC of 132A.
At first i thought it was okay as the fuse is rated at 100A but when I saw the fuse TCC I saw it starts at 140A @ 10,000sec. Does this mean this fuse will actually only start melting at current >140A? What happens between 100A and 140A? My cable is 132A so not sure if its protected at all.

This seems to be same the same for all fuse sizes, the TCC starts at a much higher current that the rated value, for a 160A the TCC starts at 210A!

How do you normally size a fuse based on your cable derated current capacity?

Many Thanks!

 

Welcome to real-world fuses, which require both time & current to heat their internal fuse-wire or -ribbon to the melting point of the fuse wire alloy. The nominal rating often assumes "will blow after many hours at the rated current". Similar behaviour is seen with modern PTC "resettable fuses" and various types of circuit breakers. You also need to understand, in your situation, how the "derated CCC" of your cable was determined, and why it matters. Any wire will get warm at it's "rated max current", but will it start a fire with a 50% overcurrent? Will it survive repetitive large overcurrents of short duration for decades? These questions can only be answered by you thinking carefully about what matters & what does not. Best wishes... BTW, fuses are often selected by starting with one of lower current than you expect to be appropriate & collecting data over days or weeks as to what are called "nuisance trips". If the fuse blows too often, go to a larger fuse until you see only a very, very few nuisance trips. Crude technique...