Ampere usage calculations
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=16805
Printed Date: August 02, 2025 at 6:12 PM
Topic: Ampere usage calculations
Posted By: hobbes6981
Subject: Ampere usage calculations
Date Posted: July 28, 2003 at 8:23 PM
I seem to be getting some mixed messages from two different sources. According to Crutchfield, to get the approximate amperage draw from your system, you first add up all the rms values per channel of all amplifiers, add together, double this sum.....then divide by 13.8 to get the figure. My system will be 450 rms, using up 65 amps. (450watts x 2 = 900 ...900/13.8 = 65amps). I noticed a post a few days ago stating the following. here is the link www.crutchfield.com/installguide/S-SqgcLNgM79q/cable_gauge.html
From the specs you give above, 80 watts bridged (40 watts per sub) and then 40X4 your total wattage is 80+160 = 240 (I don’t know if this is the RMS or MAX wattage) now the current draw on 240 watts RMS is 18.9 amps. So with a 55 amp alternator you should have sufficient power for both your audio system and your cars electrical system.
I was just wondering how the final rating was determined...and if my calculations are accurate. Thanks
Z
Replies:
Posted By: Big Purds
Date Posted: July 29, 2003 at 1:51 AM
the simplest way to figure it out is to look at the fuse values in the amps you plan on using (if they have fuses...)
you know that the amp cant draw anymore than its fuse rates, so by using that standard, you will be placing nothing in danger...so if you have an amp with 2 25A fuses you should allow at least 50A for your stereo on top of the cars own needs...
does that make sense to you?
Posted By: bberman1
Date Posted: July 29, 2003 at 10:33 AM
I used good old Ohms law: Total RMS watts divided by the voltage. So if you have 240 watts RMS you would divide by 12.7 volts =18.89 amps. The 12volt has a ohms law calculator that you can plug the numbers in and it will calculate the current draw for you (I = P / E) https://www.the12volt.com/ohm/page2.asp#current Now in your case if you say your system is 450 watts rms total. You are adding an extra step in there by multiplying by 2 (unless you have a 2 channel amp that puts out 450 X (2) channels for a total of 900 watts RMS). You should calculate the 450 watts rms 450 /12.7=35.4 amps
Posted By: hobbes6981
Date Posted: July 29, 2003 at 10:40 AM
yup. that makes sense. I used those specs to determine my in-line fuse (currently 60 amp), but will upgrade to an 80 amp fuse when I put the sytem in this weekend. My power draw is currently around 45 amps, so increasing to 65 amps isn't a huge power need increase.
Thanks Big Purds
Z
Posted By: hobbes6981
Date Posted: July 29, 2003 at 10:55 AM
well bberman1, I guess you posted just before I did, so I didn't see yours before I replied. That Crutchfield formula was specifically for finding proper wire gauge, so I guess it doesn't pertain specifically to current draw. Well....35 amps sounds better than 65.haha...so I guess my power draw right now (300 rms) is only 23 amps. I'm not sure what my alternator is rated at for amperage (1995 Plymouth Neon) but only 35 amps shouldn't be a problem.
Thanks bberman
Posted By: bberman1
Date Posted: July 29, 2003 at 11:06 AM
IF you call the dealer or an auto parts store they will be able to provide you with the output of your alternator
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