Originally Posted by 808MGuy
Batteries are actually capable of putting out very high amounts of current. The thing is they can only output that high current for a very short amount of time. Cold cranking amps (CCA) is a rating you see on most batteries and that rating is the amount of current that the battery can put out at 0 degrees F for 30 seconds while maintaining a cell voltage of 1.2V (6 cells per battery would mean your battery never dips below 7.2V). The OEM BMW battery is rated for about 650 CCA so you can see that capability is there to put out way more current than any audio system would need for a very short amount of time. Now battery capacity is typically expressed with a unit of time. For example, a battery like an Odyssey PC1200 has a rated capacity of 70Ahr. Odyssey defines this as their 20 hour rating. So if required to run for 20 hours, the battery could put out 3.5A and maintain a minimum cell voltage of 1.2V.
Now since audio is transient that means the current draw is changing very rapidly. It is highly unlikely that you would ever draw the max rating of any amplifier all at once because of the transient nature of music. The only time it would be remotely possible is if you were playing pink noise or sine waves for long periods of time at very high volumes. With music material, the transient peaks are too short to cause an large demand for an extended period of time. The fuse probably would have enough time to blow even if there was an overcurrent condition due to a transient peak however, you probably wouldn't want to allow the amp to see that on a regular basis because you will eventually burn out the output devices. Typically the fuse will blow on a fault condition like a short or an amp failure which causes an overcurrent condition at the amplifier's power input.
I have also ran smaller than usual fuses on some high powered systems. In my last, the recommended size for my amps were 125A each amp of which I had 2. The main circuit breaker at my battery was 150A. It never did trip because the instantaneous demand was always a lot lower than the max fuse rating of the amp.
Still, even though it would take a fault condition to burn that fusible link in the OEM distribution block, it would be nice to know what it's rated for. I'm guessing its somewhere between 50 and 70A since the connector is made for a 6 or 8 AWG conductor.
Think about this, it takes twice the current rating of a fuse to pass through it for two seconds before it will blow. And that's a normal "fast blow" type fuse. A slow blow fuse needs to see double the current for even longer. That's why you fuse at half the reserve capacity. As you said, music is transient so there is no need to ever run fuses over 60A or so. I've never run a fuse larger than 60A and I've never blown one. Ever. And you know i've run some serious power. Plus it keeps your car safer since less current will have to flow to blow the main fuse.