Question: If the starter can draw more than the fuse rating then why doesn't it blow?


Contrary to popular belief ANL, Class T or marine rated battery fuses (MRBF's) do not blow or trip at face value unless the duration is long enough.


As you can see below a 200A ANL can support 500% of it's rating for about .7 seconds, longer than average inrush by more than double. It can support over 300% for 1 second and 200% for as long as 5 seconds. You can push 150% through for up to 500 seconds.


Even if you figured an inrush that, by freak chance, lasted for 1 second, which should not happen, a 200 amp ANL can supply 600A which is almost double what the peak inrush is for the 2QM20 in the video above so in that scenario you'd still be fine.


Just as a point of reference a 1987 Universal M-25 draws roughly 225A +/- depending upon temp and other factors. This is the in-rush load though. All engines are slightly different. Some draw more and some draw less but the in-rush is still a very short duration on all of them. None of this changes the fact that you are still protecting the battery cable with the fuse and not the battery or other systems. The fuse is still sized to protect the battery cable and this in-rush data is simply a reference point..


200 AMP ANL Seconds vs. Amps


.7 Seconds =1000A
1 Second = 600A
5 Seconds = 400A
500 Seconds =300A


NOTE: Photo taken of original image by Blue Sea Systems