Something I haven't seen mentioned here (apologies if it has) is to be sure the fuse you put in is as close to the battery as possible/practical. (usually less than 18 inches is best)
Unless the amp has a built in fuse (typical of most newer amps) the fuse you install near the battery is simply to protect the battery cable circuit, which supplies you amp's power and not the amp itself, in the event it shorts to ground (IE. in an accident
or wheeling mishap.
). Otherwise the fuse can be sized by the amp's wattage output. You can also go bigger if you'd like.
The formula for watts is (A x V = W) inversely the formula for amps would be W / V = A
For example: A 12V system with a 120W amp will draw 10 Amps. But remember you'll typically see closer to 14V while your rig is running so run that fuse as big as practical.
Another thing to consider is how many amps you will be running on that same fuse (the one near the battery) because it has to acommodate for all amp draws.
For example: If one amp draws a max of 10 amps and another draws a max of 20 amps then you will need a min of a 30 amp fuse.
Even if you over size the fuse near the battery and if you don't have built in fuses on the amp you're installing, it would still be best to install the proper size fuse for each amp at the amp's batt wire near the amp itself. This would be the amp's circuit protection fuse.
Note: the term 'fuse' is used here but my preference is to use a circuit breaker so the long term affect of heat doesn't affect the battery connection for the amp/amps which will often degrade the voltage supply for any good high powered amp system.
Reason being that as a fuse get close to it's amp limits (without exceeding the amp rating) it will get hot. Which in turn can cause the ends of the fuse strip to become desoldered from the end caps of the fuse creating an "open". This happens more often and faster than you might think. The problem here is....the fuse goes "open" on the ends and not in the middle because this "open fuse" is caused from resistance brought on by the heat created from the high amperage draw and NOT from over loading the circuit (too many amps). Remember we didn't exceed the amp rating of the fuse in this example.
If this is all new to you, you may have to read it a couple of times to fully understand it. I hope I was clear in my explanation. Trust me, this understanding is definitely worth the trouble because not doing this isn't worth the trouble you'll have later with blown fuses and other crap that will just frustrate you to no end. Been there done that.
On another note, also it's a good idea to solder all your high amp conections (battery cable ends) so they wont get hot and get corroded over time which again will quickly deminish the cable connections and let your stereo system voltage suffer. I do this with all my installs and have yet to have any problems. Even after all my years of leaky windshields and what not.
Sorry for the long post, it's just easier than multiple post for all the same words. I think.
Hope this helps.
