how does the fuse rating on amps work?

I read somewhere that a good rule of thumb to determine if the amp your looking at will give what company says it will, is by looking at the fuses (20,30,40 etc)? Any input as to if this is true?

Absolutely. Fuse rating times voltage will tell you how much power the power supply can safely process. If you have a 20A fuse, and your battery voltage is 14.4V, that's only 288 watts in. After losses, how can you possibly expect that amplifier to produce 500 watts into a load? It can't!! There are a couple threads already in the forum right now... But yes. Amps times volts equals power in. If you are seeing a claim of twice the power out, walk away.

Thanks bro I appreciate it. I'm a rookie in car audio. So if I see a amp with two 20A fuses claiming to push 1000 or so Watts it's safe to say they're full of crap?

Absolutely - if they are implying RMS or "meaningful" output! [ REMember, traditional "music" or "peak" is/was merely 2x RMS from a root-2 times root-2 of the RMS voltage & current because the peak value of a sinewave is root-2 its RMS - i,e Vrms = 0.7071 x Vpk ~0.7*Vpk, & Vpk = 1.414 x Vrms ~ 1.4*Vrms ] Power = Volts times Amps. And you cannot "create" energy (only store & convert). And power comes from energy - its the usage of energy over a certain time. Hence input can NEVER exceed output - ie, max efficiency can only ever be 100%. (Longterm & a closed system, internal caps etc may discharge by need recharging; 300% efficient air-conditioners use the atmosphere as a heatsink.) Insist on RMS and "steady-state" figures. IE - a "peak" for one cycle or 1 micro-second is meaningless. And what it can do for a short time (before blowing up if continued) is also meaningless. What maximum will it put out on average all day, or for an hour, at what voltage eg; RMS output @ 14.4 V or 13.8V etc (forever at 25C unless stated otherwise). It is such a simple "sanity check" and theory for ANYONE that should be in charge of money. The watch the sales people squirm with their "internal heat recovery" or capacitor storage. Sorry guys, it is still a maximum of Volts times fuses-input over time!    FYI: Simple calcs: 2x20A = 40A total. Hence 12V x 40A = 480W max input (DC = RMS). Hence max output if 100% efficient is 480W RMS. (Which traditionally means 960W Music Power or peak power, but these days 480W RMS can be quoted as 1920W or even 10,000W peak! Useless!) Best "practical" calcs" Assume fuses operate all day or for one hour at 110% = 40 x 1.1 = 44A. Assume V = 15.6V (during rapid recharging of an Optima battery). 15.6 x 44 = 686W RMS. Reality is far less. EG - fuses must handle its MAX current - ie, at it minimum voltage. If amp operated from 11V to 16V, then 11V x 40A = 440W (hence 27.5A at 16V etc). (Hi-power amplifiers are "constant power" loads, ie, use SMPS.) Further more, fuses should normally operate at no more than 70% of rating, though can be at 90% etc. That's for headroom/margins for short surges, higher fuse temperatures etc. Add to the above the amps efficiency of (maybe) 80% - ie, deduct 20% from power input for its output.

oldspark wrote: Hence input can NEVER exceed output - ie, max efficiency can only ever be 100%.

You mean output can NEVER exceed input, right?

No - I'm reverse engineering and using the amp's SMPS ability to return its inductive energy to the battery (through phase-angle manipulation) and subs/air elasticity. Yes - quite correct - I meant output power can never exceed input power. (Long term etc.) (I shouldn't read foreign books whilst Forumming!)

Man seriously thanks guys, this really opened my eyes as far as amps go. It makes sense to because my RF Prime R-500 has 2 20A fuses which equal 576 and the amp has a RMS of 500 watts @ 2 ohms. My list of amps that i was considering buying is getting a complete overhaul. Also, this question my be "beating a dead horse" as well but I figure why not ask people who know more than I do. When trying to match a amp to a sub you should always attempt to match according to the subs ohm rating right? Like say for instance I just bought a RF T1 800 watts @ 4 ohms. I should attempt to match a amp that puts out 800 watts(or close to it) @ 4 ohms not 2 or 1 ohms right?

Scratch that my RF R 500 has two 30A fuses for a total of 864..just got done with finals my brain is hurting lol

Good link. And a good pdf in that link albeit glass 205 fuses. It shows (eg) how a 6A fuse will take over 100 seconds to blow with an 8A load (if indeed it blows at all). That is similar to ATC and modern fuses. Even a 100A ANL "non-delay" fuse takes 20 seconds to clear (blow) at 200A. So much for those that expect standard fuses to "offer exacting protection".... And hence why component protection is usually internal to any design.