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testing amp wattage


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haemphyst 
Platinum - Posts: 5,054
Platinum spaceThis member has been recognized as an authority in Electrical Theory. Click here for more info.spaceThis member has been recognized as an authority in Mobile Audio and Video. Click here for more info.spacespace
Joined: January 19, 2003
Location: Michigan, Bouvet Island
Posted: June 25, 2007 at 4:38 PM / IP Logged  
bassmechanik wrote:
Ok bare with me here as I'm new to all the tech stuff. Kenwood 9152 900rms / 60a fuse / 14.4 is the voltage rating, what do you come up with and what calc do you use?
14.4v*60a=864 watts in.
900 watts out/864 watts in = 104.2 percent efficient.
12.0v*60a=720 watts in.
900 watts out/720 watts in = 125 percent efficient.
Both of those results show more power OUT than power IN. Impossible, and welcome to the world of Kenwood. Like I said, either the power rating or the efficiency "truth" was stretched... There's my math, you decide.
It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."
aznboi3644 
Gold - Posts: 2,600
Gold spaceThis member consistently provides reliable informationspace
Joined: May 01, 2006
Location: United States
Posted: June 25, 2007 at 7:41 PM / IP Logged  
haemphyst wrote:
No, you're using the wrong numbers. If the amp is 900 watts RMS, and it has a claimed 62.5%, then the INPUT side would be 1440 watts. 1440 watts, divided by 14.4 volts SHOULD be meaning 100A worth of fusing on the power cable.
If you are not seeing 100A of fusing or MORE, then either the efficiency has been stretched, or the real output power has been, or a possible combination of both. That's all there is to it.
haem...does this factor affect the fuse ratings
http://www.bcae1.com/fuses.htm
"Fuse Opening Time:
    A fuse does not blow when the current reaches its rated current. It is designed to pass its rated current without opening. A fuse will take varying times to blow under different conditions. A fuse will pass significantly more than its rated current for a very short time. It may take 10 minutes or more to blow a fuse at 25% over its rated current. The table below is an example of the specifications for a slow blow fuse. You can see that a 20 amp fuse may pass 40 amps of current for as long as 5 minutes before blowing although it probably wouldn't take a full 5 minutes to blow. The times for other fuses will be slightly different."
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