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  • Wire resistance/voltage drop

    Ran into something kind of odd. I have a project where I need to run 12v to a light at about 25 amps. I picked some high flex 8 ga wire off ebay. The stuff has more resistance in a 10' section than an old piece of 12ga MTW in 15'.

    Any ideas what might cause this? Remembering Evan's post about the copper coated steel speaker wire he found I looked at the ends of the wire in a microscope and it appears to be all copper and it did cut pretty easily.

    Bad alloy? Dunno. Cant use it though. In 20 feet I get a 1.5v voltage drop. Should be around .6-.7 at that distance.

  • #2
    I would bet it's not all copper. Even a discrepancy in the purity won't cause the degree of resistance that you're seeing. Is it magnetic at all?

    I'm thinking that some manufacturer has found a way to keep the copper color, make it non-magnetic, but make it cheaper by adding some other metal to the copper. In other words, it's now an alloy, not copper with some degree of impurities.

    I'm not aware of any method to achieve high flexibility except by finer stranding of the wire. If there was less wire and some added plastic fiber in the bundle, I could understand that, but you would be noticing that. If the fibers were coated in something before being bundled, they maybe could claim the gauge number will less copper content, but then you would have a connection nightmare. Maybe that is the case- only some of the strands are carrying current. Can you solder it easily and normally?

    I'll be keeping an eye on this topic since this kind of thing does impact me.

    The latest round of magnetic speaker wire I found and reported to the cashier was met with a defense- 'does it say anywhere on the package that it's copper?' It says speaker wire, but does not say it's copper.

    Seems to me that in industry, we're going to have to keep a very watchful eye on the materials that we import, either raw or made into product. I think it will become increasingly inferior, and will start impacting the quality of the product that we still do make on this continent.
    Last edited by darryl; 12-11-2011, 06:57 PM.
    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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    • #3
      E=IxR

      http://www.allaboutcircuits.com/vol_1/chpt_2/2.html

      There are no free lunches. Increase resistance and amperage stays the same voltage will drop. It is Ohm's law in full demonstration.

      You need to watch the fine stranded wire. Welding machine leads are #2 copper not #1 in my experience. If you must have the flexibility look at car stereo shops for good fine stranded copper cable. If not plain old THHN from the hardware store will be my suggestion.

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      • #4
        A friend of mine ran into the same problem a few months ago. Bought some crap wire some where....... dollar store stuff I think it was. Thought he got a deal. The stuff had a wicked voltage drop in 30 ft. It looked like copper, felt like copper, cut like copper, soldered like crap.
        My guess it was some zink / white metal either some anodizing to the strands to give the copper color but it wasn't copper.

        JL....................

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        • #5
          300 watts? Thats a hell of a DC lamp. You will need some quality 4ga to suffer no voltage loss at all. Sorry, no free lunch. Maybe you could get some four strand SO cord and double up conductors.
          James Kilroy

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          • #6
            Lamp

            It sounds as though you do not have copper wire. 300 watts is not a big lamp. I remember a small search light when I was in the Army that used 100 amp at 28 volts.
            JRW

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            • #7
              Macona - count the strands, measure the diameter.. calculate and add up the sq. mils.

              Super flexible wire is often sparce (lots of air) and twisted loosely. The exact opposite is "compact copper" - it's crushed togther to reduce the overall wire size and in doing so removes the air gaps, but is very stiff.
              Last edited by lakeside53; 12-11-2011, 10:39 PM.

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              • #8
                Macona,

                Is it possible that the wire you have is 'fuse link' wire? I have found the link wire I have has a high resistance compared to standard stranded wire of the same gauge, but it is extreemly flexable. The fuse link wire has recommendations for using no more than 4 inches in a protective link due to the higher voltage drop. One thing you could try is heat the insulation of your wire with a soldering iron and see if it melts like normal wire. Fuse link wire has a much higher melting point to contain the hot wire ends when it blows ,
                Robin

                Happily working on my second million Gave up on the first

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                • #9
                  This is the wire that is sold for car stereo useage. High wire count of small diameter wires. The 12 ga stuff I have is very similar to thhn but a different coating and I am getting better performance per foot than the 8 ga stuff. It is not magnetic.

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                  • #10
                    12V aint much push. For 12 V lamp(s), even a small voltage drop results in major loss in incandescent output. Find a copper wire resistance chart and determine a wire size yielding less than 2% drop over the run. Also make sure your 12 V source is indeed 12V under load. Many outdoor lighting transformers (if that's what your're using) don't measure up at full load ratings. Anyway most NEC wire gage approved Ampacities are intended to limit resistive heating in wall cavities and conduit not minimise voltage drop.

                    You're gonna have to crunch some numbers. I did a quick calc and it looks like your original conclusion is correct. #8 AWG for 2% drop at 12V @ 25A over 20 ft assuming good connections should be plenty. I find welding lead is a good bargain for low voltage high current conductors but I don't know if #8 AWG is available. #6 is but maybe not #8.

                    http://www.powerstream.com/Wire_Size.htm
                    Last edited by Forrest Addy; 12-12-2011, 06:10 AM.

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                    • #11
                      From some tables I looked up, stranded wire whether consisting of very fine strands or fewer but thicker strands has about the same resistance as solid wire of the same gauge. The high strand count wire actually has a lower resistance rating in some gauges. In any event, the differences are only about 2%. The THHN rating is for the insulation, temperature resistance and resistance to moisture, and has nothing to do with the wire itself.

                      8 ga should have about .63 ohms per 1000 ft of resistance, or about .01 ohms in 15 ft. At 25 amps then, the voltage drop should be about 1/4 volt in a 15 ft length of #8.

                      In 20 ft, Macona should be seeing about .32 volts of drop at 25 amps, not 1.5 or so. If that wire was made for fusible link, it should be clearly identified as such, and I doubt that Macona would miss seeing that. I think it's fake wire, dangerously so in my opinion. It's a fire waiting to start if you're expecting it to carry high current according to the rating for that gauge of wire. I think the factory that produced it should be burnt down.
                      I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                      • #12
                        This is the wire that is sold for car stereo useage.
                        Wait a minute. The latest thing in speaker cables is "loss". You see, without a little loss the speaker will resonate and the connectors will cause ringing. They are now making cables alloyed with zinc because they sound much better.
                        Free software for calculating bolt circles and similar: Click Here

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                        • #13
                          Originally posted by Evan
                          Wait a minute. The latest thing in speaker cables is "loss". You see, without a little loss the speaker will resonate and the connectors will cause ringing. They are now making cables alloyed with zinc because they sound much better.
                          is that the cable or speakers ?
                          or do you have to use digital speakers ( 0 and 1 's )

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                          • #14
                            It depends on your ears, wallet and gullibility. Also, the Chinese are masters of product adulteration. The sellers of the product here are making a virtue of a defect. As is usual in the audio business, it's all BS.
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                            • #15
                              When working at low voltages and high currents it makes more sense to calculate the power loss instead of just the voltage drop. The lower the supply voltage the more difference resistance loss makes since power loss goes up by the square of the voltage drop.
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