Need help understanding amplifier amp draw

[gsniegow]

Last month I replaced my alternator with a PowerMaster 75A alternator.  Inside the box the tech had done some test and reported back that the alternator generates 68A at idle.

I have picked up a stereo and amplifier for her.  Mind you, I do LOVE just listening to the car itself.  However there are days I want to listen to some tunes or on cruise nights will want some music as well.  Anyway...

I picked up some lower end equipment.  A Pioneer GM 5500T amplifier and a Pioneer headunit.  Only speakers in her will be two 6 X 9's in the back. 

I am also replacing the radiator and fan with a new aluminum radiator and electric fans as well.  So above the stereo draw I will also be pulling power for the electrical fans. 

My concern now is total amp draw.  Acording to the manual for the amp it reads "Average current drawn... 8A (4 ohm for two channels)  14A (4 ohm for one channel)".  There is a side note that reads, "The average current drawn is nearly the maximum current drawn by this unit when an audio signal is input.  Use this value when working out the total current drawn by multiple power amplifiers.".

This unit also has two 25A fuses in it.  Why?   No clue!

So my question then becomes, what is my average amp drawn?  What is the max drawn?

I have sent Pioneer a email asking for help.  I had a chat session with a rep from Crutchfield who told me the average drawn is 29A and the max drawn is 50A.

I found a formula on the web which reads...  Amp = watts / volts.  This is a rough calculation for estimation purposes. 

When I plug the number is I come up with...
     29.7   = 410 (max) / 13.8 and
     18.12 = 250 (125 X 2) / 13.8

So all these numbers and I am LOST!  Can anyone help educate me?  My fear before hooking all this up is that I will be draining my electrical system when I am at a stop light or idoling somewhere.

Hope SOME of this question made sense!  TY in advance!

[twodko]

The formula you mentioned is correct in calculating current draw (amps) per basic Ohm's law -

Amps (current) = power (watts) divided by volts ---- I = P over V

What those figures are telling you is the average expected amp draw from your system under max load conditions. You didn't mention the total power (watts) this amp generates. Was that the 410 number? Is that per channel. As far as the 2 25amp fuses, I can only suspect they are for each channel.

I would have gone with a bigger alt especially since you'll be running electric fans and a power amp(s). Even with a larger capacity alt. most outboard amped auto sound systems use huge capacitors to store amperage for heavy current draw. If you haven't install the alt yet, take it back for a more powerful one. 

[gsniegow]

Thank you and good catch!  My 410 is per channel, so my math is now off.  My max would now be 59.4 (guessing the amps fuses would blow before ever hitting this number, so a realistic high would then be 50???) and the continous was 125 X 2, which I had so the 18.2 number stands.

Unfortunately I did the alternator before even thinking about the draw from the amp.  Need to decide if I should even go with the amp now or use it for my daily driver.  Grrrr!     Maybe I can find a little baby amp to help in the Charger???

Really appreciate your feedback!  TY!