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Thread: voltage drop in power cable

  1. #1
    Variable Bitrate warnockm's Avatar
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    voltage drop in power cable

    as a background, my car computer/car audio system works pretty well. I have an amp for the front speakers and an amp for my subwoofer. The front amp is configured to run 80watts into 4 channels. I'm running a 40amp fuse and 10 gauge wire. i don't think i'm using near 80 watts. However at stop lights, after a few seconds, the sound will drop out and come right back in. if i rev the engine slightly it doesn't cut out. it doesn't it every time w/ the lights or AC on, but only rarely w/o any of those loads. I assumed I needed a new alternator, but someone mentioned it could be the wiring. I took a few measurements and i want to know if this is enough to cause my cutouts:

    when the engine is off:
    Vbatt = 12.60v
    Vamp = 12.58v

    when the engine is on:
    Vbatt = 13.97v
    Vamp = 13.90v

    and when i turn the music up it drops to 13.6v. It could be high because the battery is fully charged and at the light it drains, but i haven't measured it when the sound cuts out. Does this shed any light on my voltage problem?
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  2. #2
    Variable Bitrate roadhog's Avatar
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    Knee jerk response -

    Alternators can lag a little. The current sensing may not be sensitive enough to keep the voltage constant at idle without a large load ie lights and a/c blower.
    Try wiring in your voltmeter somewhere where you can check it as you're driving.

  3. #3
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    Simple answer, Use bigger wire gauge like 8 or 4.
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  4. #4
    Variable Bitrate warnockm's Avatar
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    Quote Originally Posted by pancit175
    Simple answer, Use bigger wire gauge like 8 or 4.
    I know that's a simple answer. can you explain your reasoning? is a .1v drop on the wire a problem, is it going to be fixed by increasing the gauge, and will this fix my sound drop problem?
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  5. #5
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    80w x 4 = 320w.

    * 1.2 to account for inefficency = 384w
    / 12vDC ~ 30amps RMA, 60 peak

    A bit much for 10 ga.

  6. #6
    Variable Bitrate warnockm's Avatar
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    Quote Originally Posted by greenman100
    80w x 4 = 320w.

    * 1.2 to account for inefficency = 384w
    / 12vDC ~ 30amps RMA, 60 peak

    A bit much for 10 ga.
    hey, thanks for the reply. i was going by this webpage: http://www.powerstream.com/Wire_Size.htm which has the 10ga limit for chassis wiring at 55 amps. i have a 30 amp fuse inline and never blew it. I'm more concerned about the voltage drop than the current limit. that's why i'm wondering if the .1v voltage drop on the wire is enough to be concerned about. To me it's not, but that's why i ask you guys
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  7. #7
    Variable Bitrate HHdesign's Avatar
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    I dont think it should be much of an issue. My reasoning, My Mobo Needs +12VDC, and +5VCD, 3.3V etc...

    Right now I'm running 12.56 VDC, and that seems to be about where it always is, my 5VDC line is 5.1VDC, so on and soforth.

    Upgrading to a thicker wire will help. I have a 800 Watt Amp, Now I could run 4 Gauge and Proably get away with it, but I ran 0 Gauge and it works great.
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  8. #8
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    Me and a friend of mine did some calculations with a few assumptions as follows:

    • when you said 80 W amp you meant 80 W per channel
    • you were using 4 speakers on this amp (80 W * 4 = 320 W)
    • you had 20 feet of 10 gague wire between your power source and your amp
    • the resistance due to length of the conductor was based upon the measurements given on powerstream.com's site (which gives 10 gague 0.9989 ohms/1000 ft)


    Given this current draw while your amp was using all 320 W (a rare situation) and the resistance per foot used, we calculated roughly 23 A of current drawn from the amp. If 23 A were in fact drawn, the most you'd lose from the wire was 0.46V. For an amplifier starting at almost 14 volts with negligable current draw while the engine is running, this is insignificant. What you're experiencing is the lag of the alternator that is trying to catch up.

    I do not think that thicker wire would help in this case, although if you want you can go for it. However what would help you the most is if you ran one of those huge 0.5 or 1 Farad capacitors parallel to your amplifier. The capacitor would store energy so that if the amp needed a sudden burst of current, the capacitor would supply it instantly instead of relying on the laggy alternator thus solving your problem of random cutouts.

  9. #9
    Variable Bitrate roadhog's Avatar
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    Quote Originally Posted by warnockm
    However at stop lights, after a few seconds, the sound will drop out and come right back in. if i rev the engine slightly it doesn't cut out.
    I don't think a capacitor will hold a charge for long enough to help here, although under normal conditions your amp will definately sound better.
    It still sounds to me like the alternator is dropping out at idle revs. I suggest you check the voltage and current flow from the alternator while idling. It may just be a slightly loose belt

  10. #10
    Variable Bitrate warnockm's Avatar
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    Quote Originally Posted by fugue
    Me and a friend of mine did some calculations with a few assumptions as follows:

    • when you said 80 W amp you meant 80 W per channel
    • you were using 4 speakers on this amp (80 W * 4 = 320 W)
    • you had 20 feet of 10 gague wire between your power source and your amp
    • the resistance due to length of the conductor was based upon the measurements given on powerstream.com's site (which gives 10 gague 0.9989 ohms/1000 ft)


    Given this current draw while your amp was using all 320 W (a rare situation) and the resistance per foot used, we calculated roughly 23 A of current drawn from the amp. If 23 A were in fact drawn, the most you'd lose from the wire was 0.46V. For an amplifier starting at almost 14 volts with negligable current draw while the engine is running, this is insignificant. What you're experiencing is the lag of the alternator that is trying to catch up.

    I do not think that thicker wire would help in this case, although if you want you can go for it. However what would help you the most is if you ran one of those huge 0.5 or 1 Farad capacitors parallel to your amplifier. The capacitor would store energy so that if the amp needed a sudden burst of current, the capacitor would supply it instantly instead of relying on the laggy alternator thus solving your problem of random cutouts.
    wow, thanks. you are correct in saying the amp is 80 watts per channel, my mistake. I'm actually running less than 20 feet of power cable, but still very close. I did some calculations like this and i agree with you that i'm in line w/ my power ratings and that thicker wire would not help, but i wanted to ask the group before exhausting this idea.

    My alternator is a 75 amp alternator, which is pretty small, but since it only cuts out for a second while at a stop light, i'm hoping for a very small change to make everything work.

    Again, thanks for the calculations
    Progress - VIA EPIA SP8000 | 120 Opus Power Supply & Case | 1GB Ram | 120GB 2.5" Hard Drive | Bluetooth 2.0 | GPRS/3G | Wifi | Road Runner/LSX 2.0 (waiting for a day skin for 3.0) | iGuidance 4.0 | Lilliput 7"

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