Originally Posted by btrayfors
ANL fuses are unlike any other CPDs, in that they blow at a much higher than nominal rating, whereas most other fuses and breakers blow at about 130% of their rating.
ANL blowpoints can exceed their ratings by 140% to as much as 266%. A 100A ANL won't blow until current reaches 175A or more for 500 seconds -- over 8 minutes!
The ampacity tables cannot take the differences between CPDs into account; you must do that yourself, or find a table in which the specific CPD is referenced.
I believe that MRBFs are like most other fuses found in the marine environment -- relatively fast blowing at 130% of their nominal rating. ANLs are very different.
Here are the curves for the MRBFs: Detailed Specifications for Terminal Fuses (MRBF - Marine Rated Battery Fuse) - Blue Sea Systems
The ABYC and Blue Sea make no exceptions or derating suggestions for short circuit protection for ANL fuses vs. other types. Now if you're using it for an inverter many manufacturers suggest using Class T because they blow slightly faster but that would not be a short circuit protection issue.
You only need to look at this Blue Sea .pdf document:
of which I have photographed and posted the important information.
And you'll notice that Blue Seas own fuse sizing chart actually up-sizes on occasion to the next size fuse, which the ABYC recommends against:
You notice that the company you've referenced makes no changes in the ANL column for short circuit protection and the fuses rated value is what is used.
The face value rating of an ANL is perfectly acceptable to use with the ABYC ampacity chart for short circuit protection and Blue Sea makes it easy to understand in the above doc though I would down size rather than up size if pushing up against the 150% rule at least.
As I mentioned I have had this very conversation with John A. at ABYC, and Eric J., before he left, and they both made the point that the E-11 ampacity chart already takes all this into account.
Derating an ANL is certainly fine but there is no need for all that extra math if using it for short circuit protection and you can simply use the face value rating of the fuse...