Calculating current draw

Problem:

I have a 60 amp alternator on my car. I am currently running 1 Alpine MRVT-420 2 CH amp to my fronts. (90w x2), there is 40 amps of fusing on the amp. I am planning on adding subs and I am wondering how many watts I can push (or better yet, amps I can draw) without replacing my alternator. I also have a 1 farad capacitor. I am considering a DIamond Audio D3600.1 (600 watts at 1 ohm, 70 amps of fusing) and a Fosgate P6001 (700 watts at 1 ohm, not sure how much fusing)

Also, how can I calculate the current draw on my 2ch amp? I was figuring watts divided by volts. is that correct? Can I test this with a DMM?

Thanks

You can figure the draw of the two channel with the amp specs and the ohm load.  Your fronts are no doubt at 4 ohms, so the Ohms Law equation you would use is I=sq.rt.(P/R).  If the power is 180 watts into 4 ohms, then amperage draw at a maximum would be 6.7.  You could also measure voltage output of the amp with a DMM, but you have to set up a careful test with a reliable tone at 0 db.  You would measure voltage in AC volts, then use P=E^2/R to find watts.  Then do the above I=sq.rt.(P/R) to get a truer amperage draw.  But it will probably be close enough just going by specs.

Seems to me that a 60 amp alt doesn't leave much room for aftermarket amplifiers.  You might want to start looking into a reasonably priced upgrade.

I always bring up http://www.bcae1.com/ohmslaw.htm when the subject of Ohm's Law comes up.

stevdart, the current you found is the output current of the amp, not the current draw of the amp.

When adding up current draw in an electrical system, always take the worst case scenario, which in this case is what the fuse is rated for (well, a little under it)

 To measure current draw you can use an inductive meter.. either electronic (known as a 'clamp on' type,) or mechanical (not as accurate as electronic, but gives you a rough idea).

 If you don't have an inductive meter you can buy a shunt and wire it inline.. use the DMM to measure the current. A common shunt ratio is 1 amp equals 1mV, so if you were drawing 50 amps, you'd be reading 50mV on the meter.

 Regardless of current draw, a voltmeter is a nice way to monitor your charging system.

 Hope this helps..

 Jim