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I was reading glasswolfs article on charging systems... I was wondering about determining current draw of an amplifier... Instead of calculating watts divided by voltage divided by efficiency... Could i not just use the the fuse rating as a rough measure? So if i have two amps with 50A fuse each. I should assume at peak my alt will get hit for 100A of current? Also, i see some high quality amps doing 1200rms... Using 90A fuses... But if i do the math above... The current draw would be in the 100A+ range? Can somebody set me straight?

So far... In building my system, my rule of thumb has been i should not have amps with fuses totaling more than what my alt can produce (90A hot in my case)... Is that right? Again, somebody please set me straight here?

I'm a newbie, hope you don't mind all the questions!

1200 wrms/13.8 volts= like 85-86 amps...

Ryan from Ohio wrote:1200 wrms/13.8 volts= like 85-86 amps...

But what about accounting for efficiency? And shouldn't the voltage be 12?

Thanks for the reply.

No, really you could use 14.5 since thats what it will be at since it's chargin. But with 85 amps you will need a bigger alt to handle everthing else the that's drawin current.

thats also if the amp is running at full power constantly remember music is dynamic.

you can use fuse ratings if you have a thorough understanding of fuse ratings, but remember that just because a fuse is rated for "100A" and may be a "fast blow" fuse, that doesn't mean it blows the instant it sees 100A of current.
Fuses are rated to blow based on the % of over-current per amount of time exposed... for example, the same 100A fuse at 10% overcurrent (110A) may blow in 15 minutes at 110A, whereas at 30% OC, or 130A, it may blow in 5 minutes, and at 50% over, or 150A, it may only last 10 seconds.
Knowing this, and knowing that music is dynamic in nature, and power output of the amp is based both on the dynamics of the source signal, and the volume level of the music/system, you need to realize that if an amp has 2 100A fuses, that doesn't mean the amp can only draw 200A of current.
It can draw substantially more than that, just not indefinitely.. usually for shorter periods, though those periods could last for minutes without doing anything to the fuses.
Quite often in fact, amp makers use fuses rated below the RMS peak output of the amp for just this reason. The amp can still deliver it's full power, and draw a lot of current, but the fuses are there to protect against dead shorts, which would blow the fuses instantly. Otherwise, they count on you never trying to run the amps at full output for long periods, thus fuses are underrated for the amp's output specs.

This is why I don't delve into using fuse ratings, and why I steer away from suggesting this course of action. Most people don't really have a good grasp of how fuses really work.. although http://www.bcae1.com has a terrific primer on fuse ratings.

My method is a bit more precise when it comes to figuring the most current your system will demand.
remember though, most times you don't run systems at full volume for long periods, so you can get by with a charging system rated for a mere fraction of that peak output and never notice a difference, because you're only using a small portion of the power the system is rated to produce anyway.
example: a 2Kw rated system, during daily use, probably never draws more tahn about 200 watts on average.

Thanks glasswolf!

If im running about 1200Wrms does my 90A alt have anything to worry about if im not being ridiculous with the bass? Could the momentary spikes above 90A hurt my alt?

the battery will stabilize the voltage rails for momentary peaks over 90A. You should be fine.

1200 watts from a class D amp will draw up to 110A or so at most, but in reality, probably a small fraction of that during normal use. figure maybe 40 or 50A at most for most applications.

I always thought mfg's using 14.4V is bogus. Granted I've always owned Honda's, but even measuring voltage at the battery with the engine revved, I've never had anything greater than 13.8V. At idle, with my amps in the trunk and measuring at the amp's (+) terminal I get a reading of about 12.6V.

Distance from battery is about 14' using 4 ga.