Hey All

so this is my system,

1000CCA Motomaster battery (12V)
O gauge to 2 yellow tops in back (12V) (to and from)
O gauge to a 2200-2500RMS American bass amp
the amp is powering 6 10" sony Xplod's (330RMS/1200PEAK)

just wondering what size fuse i should go with?? 250 - 300?
idk. is there a formula?

 

quick answer ~ 200 amps
The correct answer is somewhat unknown, the question is how much impedance is the amp seeing since the lower the impedance the more power is being produced. The amp is pretty powerful if loaded, certainly powerful enough to give those Sony's a heart attack but we need to know the speakers impedance and how they are wired.
I am guessing 150 should be good to keep your Sony's in tact. The amp should max out (2500 W) at 200 amps or so (the owners manual should tell you the correct answer). You have a butt ton of miss-matched batteries so you have the power to arc weld your problems into slag, be careful

I (current sent to speakers) is determined by the series/ parallel impedance of the Sony's and directly dictates the amount of I (current sent to the amp from the car)

Voltage from car x I (current to amp)= Power Consumed by the system

 

the amp is seeing .67 Ohm
wired normally (all + to + and all - to -)
only two diff types of batteries lol
they are 4 ohm single voice coil.
and i have no manual for the amp

 

Look here it will explain everything. Basic Car Audio Electronics On the right side scroll down to #14 fuses. Should be what you need.

 

I would rather start off with a fuse that might be a little too small and have it blow when I am cranking it up. The worst that would happen is I spend a few bucks on a fuse. Much safer than the other possible results.

I agree with JohnVroom. 200 amps would be a good place to start.