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  #16 (permalink)  
Old 02-26-2013, 12:42 AM
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The fine wire cable, like welding cable is so much more flexible and easier to work with, but you still need the gauge. The longer the cable, the greater the voltage drop. A short 4 gauge wire will work, but put that battery in the trunk and run 15 feet of that 4 gauge and you will have a bad voltage drop - you make up for that by using the larger gauge. I used 1/0. If that was heavy wire like most battery cable, it would be hard to work with because it would be so stiff.

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Old 02-27-2013, 01:23 PM
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Quote:
Originally Posted by alittle1 View Post
Finally, someone got it right. It's not the gauge of the cable, it's the number of strands in the cable. A fine wire cable will carry a current load better than a coarse wire cable in the same overall gauge.
I trust that this will satisfy all.
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Old 02-27-2013, 01:58 PM
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Fine wire or coarse, for a long battery cable you still need a large gauge. Fine wire in 2 gauge will not carry more load that 1/0 coarse. Your own qualifier "...in the same overall gauge" may be true (I have seen it debated more than once), but in this instance you have to have a large gauge to minimize the voltage drop.

I have read that when dealing with high voltages, that the current travels over the surface of the conductor, meaning that the fine wire havine more surface area, carries more current. However, with low voltages such as a 12v auto electrical system, the current is carried through the conductor. There is nearly an identical amout of copper in both cables.
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Old 02-27-2013, 04:31 PM
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alt to batt

sounds excellent with good insulation and away from heat
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Old 02-27-2013, 05:33 PM
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Quote:
Originally Posted by alittle1 View Post
Finally, someone got it right. It's not the gauge of the cable, it's the number of strands in the cable. A fine wire cable will carry a current load better than a coarse wire cable.
huh? 10 gauge wire is 10 gauge wire. current carrying capacity is calculated from wire gauge, insulation and location. fine strand wire is flexible compared to solid or coarse strand wire.

thnn solid wire has the same current carrying capacity as thnn stranded wire.
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Old 02-27-2013, 06:19 PM
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Welding cable is for welding, that's why it's called welding cable. It's made of fine strand wire so it will give the flexibility needed for welding. Look at the ratings for insulation on a wire distributors web site, (Try Waytek.com). Welding cable insulation is normally only rated for water resistance. Automotive, aircraft, and marine wire is rated for oil, grease, gas, and acid resistance. The gauge of the wire is what matters. The number of strands does not matter to D.C. voltage, D.C. current flows through a conductor and "skin effect" where the current flows on the outside of a conductor only happens in the upper radio and microwave frequencies.
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Old 02-27-2013, 06:31 PM
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Quote:
Originally Posted by ogre View Post
huh? 10 gauge wire is 10 gauge wire. current carrying capacity is calculated from wire gauge, insulation and location. fine strand wire is flexible compared to solid or coarse strand wire.

thnn solid wire has the same current carrying capacity as thnn stranded wire.
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Originally Posted by sparkchaser View Post
... The gauge of the wire is what matters. The number of strands does not matter to D.C. voltage, D.C. current flows through a conductor and "skin effect" where the current flows on the outside of a conductor only happens in the upper radio and microwave frequencies.
Thank you both!
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Old 02-27-2013, 08:06 PM
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Old 02-28-2013, 07:10 PM
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Been using welding cable for years with no problems...
There's a good reason they use it to weld with...
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Old 03-14-2013, 09:31 PM
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professional electrician here. unless you are running 20ft of wire or more 4ga wire will suffice. you can buy 4ga THHN from home depot or lowe's but its expensive and doesn't bend as easily as welding cable but the insulation is rated for outdoor use and burial, very u/v resistant. good wire overall. the number of strands and it's relationship to current carrying capacity is null. 4gauge wire(bare) is rated for 100 amps whether it's THHN, SOOW, or type W. If you can find EPDM that would be the best for automotive use if you are that particular about it. the types of wire insulation will also determine the ampacity of the wire. 4ga SOOW is not rated for 100 amps, its rated at 65. the insulation has to be able to safely transfer the heat created by the load. SOOW is low cost compared to type W(100 amp rating). insulation specs also address environmental issues that can de-rate the wire. however, for the application in question, 4ga ANYTHING will work. unless its bare copper(duh). my personal preference in my camaro is 2ga EPDM because i have access to it for free. it's fine strand, dissipates heat well, takes the harsh underhood environment, and is more than big enough. the specific resistance of copper doesnt change with strand consistency (fine/coarse). the disadvantage of fine strand is when it starts to corrode it ampacity goes to hell in a handbasket quickly because of the air gaps between the strands. but if you seal the wire ends with a good heat shrink with adheisive like 3M you should never have a problem. fine strand wire handles heat dissipation better than, say, THHN(coarse). unless you are running 8 million watts of electrical crap you wont need to worry about heat from amp draw. clean connections, good grounds (size them as you would + wire), and good wire and you're all set. and yes you can use welding cable.
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Old 03-15-2013, 03:13 AM
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Well stated Gonzo.
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Old 03-15-2013, 04:02 AM
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Why not use real SAE spec. J1128 automotive wire? It's what is in your wifes new car and you can get it at Napa, Waytek, or dozens of other suppliers.
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Old 03-15-2013, 01:28 PM
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Interesting topic for sure. I have used welding cable for remote location of a large dump truck type battery in my pickup truck box(enclosed in a custom containing box with wood lining)for use in very cold winter to start without fail. What I did have happen is corrosion where the battery connectors connect to the cables(seemingly no physical corrosive elements got into the connections). The solution was a baking soda and water cleaning, shorten the cable to remove the affected cable and re connect cable ends, but adding petroliun gelly to the connections before the heat shrink wrap and on the battery posts has eleminated the corrosion..Just my thoughts. Den
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Old 03-15-2013, 04:52 PM
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The idea of using a large gauge wire for a trunk mounted battery is to reduce the voltage drop. Will a 4gauge wire make the connection without burning up - probably, but the voltage drop is almost 1 1/2 volts over 15 feet, versus a drop of only .6 volts with 1/0 gauge. The sizing is to handle peak load (starting).
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Old 03-15-2013, 08:12 PM
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math samples

The battery brand has nothing to do with the wire gauge. The wire size is determined by the starting needs of the engine (ampere load) at a given ambient temperature, and the battery that is properly rated for the job expressed as (CCA) cold cranking amps.
We will assume that in a climate of (0*), a big cube high compression engine the CCA could be 400 amps or more.
The formula is Vdrop=2xLxRkxI/1000 or more clearly expressed as - voltage drop of the conductor = 2xdistance from the battery to the load x 1000 foot resistance of the conductor x load in amps divided by 1000.
Still with me? Good.
Ok - using 400 amps as a load.
And using 10 feet as a conductor distance.
And for #4 copper use - .308 ohms per 1000 feet. And for 1/0 copper use - .122 ohms per 1000 feet.
NOTE -- resistance numbers are from the NEC tables Chapter 9, table 8 conductor properties.

2x10x.308x400/1000= 2.46 volts dropped using #4 copper
and
2x10x.122x400/1000=.976 volts dropped using 1/0 copper.

I hope that somewhat demonstrates the ranges of conductors used for motor starting.

vicrod
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