Calculating current draw
Printed From: the12volt.com
Forum Name: Car Audio
Forum Discription: Car Stereos, Amplifiers, Crossovers, Processors, Speakers, Subwoofers, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=42774
Printed Date: July 15, 2025 at 2:53 PM
Topic: Calculating current draw
Posted By: Jeffpaulus
Subject: Calculating current draw
Date Posted: November 11, 2004 at 6:41 PM
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 ------------- Deck - Eclipse CD8443 - $475
Speakers - Boston BAZ5 - $1000
Amp - Alpine MRV-T420 - $400
Car - 1988 PLymouth Colt - $450, it's been rolled!
Replies:
Posted By: stevdart
Date Posted: November 11, 2004 at 11:16 PM
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 https://www.bcae1.com/ohmslaw.htm when the subject of Ohm's Law comes up. ------------- Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.
Posted By: dragonrage
Date Posted: November 13, 2004 at 10:16 PM
I'd say you should have at least an 85A alternator for any decent amp. 105A would be better, those ridiculous like 140A ones are generally overkill, but if you're upgrading, overkill doesn't hurt, especially considering alternators don't work at full power at idle. and stevdart, you have to include amplifier efficiency. Class A is only 25-35% efficient, class AB maybe 50%, class D and class T around 85%. But if the amp has a 70A fuse then just make sure you can supply 70A to it. And that's not going to happen with a 60A alternator. So keep the gain low or replace that alternator.
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2009 Pontiac G8 in planning stage
HU: ?
Speakers: ?
Amps: ?
Posted By: stevdart
Date Posted: November 13, 2004 at 10:27 PM
dragonrage, the efficiency of the amp wouldn't play a part in my answer because the amperage was figured on the output rating of the amp, not the fusing. Ohm's Law figured on the power of 180 watts @ 4 ohms, which is what was posted . " how can I calculate the current draw on my 2ch amp? " was the question I was answering.
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Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.
Posted By: dragonrage
Date Posted: November 13, 2004 at 11:15 PM
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) ------------- 2009 Pontiac G8 in planning stage
HU: ?
Speakers: ?
Amps: ?
Posted By: stevdart
Date Posted: November 13, 2004 at 11:20 PM
Okay, I gotcha. I have a mental block in that area.
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Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.
Posted By: Mad Scientists
Date Posted: November 14, 2004 at 11:32 AM
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
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