Can someone tell me what the formula is for determining the number of amps drawn by an audio system...I assume it has something to do with watts x efficiency...

Thanks,

FB

There isn't really a way because it's difficult to know excatly how many watts your amps are putting out or the exact effieciency. If you really wanna know, replace your fuse with smaller and smaller values until she pops. Then you know. Remeber speaker impedence always changes so just because your running a 4 ohm sub does not mean it's gonna be a 4ohm load on the amp.

The formula would be
I=P/(eff x V)

I=Current (Amps)
P=Power Output (Watts)
V=Batt Voltage(approx 14.4V)
eff=Efficiency (eg .75)

If you determined the current by swapping out the fuse you could use the same formula to determine you output power. This would be somewhat accurate.

P=I x V x eff

Thanks...I am trying to determine the size of altenator that I need...sub amp is 1500 watts total and the mid/high amp is about 400 watts...both Rockford Power series.

So you have 1500bd and a 350s? What kinda car?

the only true way is to check it with an ampmeter
if not i just add up the fuse sizes in your amp it should get you close
jay fuses arn't a dead accurate device so using the pop theory may make it look like it draws less current
fuse size is relative to current draw and time

I didn't suggest an ammeter because not to many people have access to one that can hanle over 10 amps. Rockford amps don't have fuses in them and that method would give you a value nearly double of what you are actually drawing. I've got an old 25x4 clarion amp that has a 25A fuse in it. You would be lucky for that amp to draw 10 amps. The fuse pop method is still the best method for a fairly close measurement. It would probably be plus minus 5A. It's a heluva lot cheaper than buying a $350 clamp meter.

the reason i said it wasn't very accurate was that a fuse can pass something like 300% of its rating for like 10-30 seconds i don't have they exact number but its probably close

You're definetly right. But if he put's in a fuse rides it a day or two pounding the bejesus out of her and then tries a smaller one and repeats, I think it's pretty much the only way to do it. I would love to get my hands on a fluke clamp meter that does dc amps. That would be a great tool in car audio. Put in a sine wave and then you could figure out current draw and amplifier power. I think I might try and expense one through work [img]graemlins/thumb.gif[/img]

Amps are a 1500bd running the subs and an 850x running 4-channels, 2 of those into an 8-ohm load...car is a Mustang with a 130 amp alt on it.

You should be fine with that alt if it is working well. Try upgrading the big 3 wires, that should help. Just don't run the system full out with all of your accessories on. If you really want to do that, and don't care about the money I'd make it simple and get a 200 amp alt. That will do just fine.