Question for Electrical Geniuses (Genii?)

DatsunBucky · Post by **DatsunBucky** » Sat Jul 17, 2004 6:39 pm

Thanks for the help on the spring question. Now I have another question. This one is a bit more techincal in nature than the last, but considering the cross-section of people we have on this forum, perhaps someone will know the answer.

This is about wire gauge. I heard a long time back that when current flows through a wire, it actually travels on the surface of the conductors as opposed to the interior of the conductor. Is this true? If this is true, then the more conductors in a wire, the lower the resistance. Right?

If it's not true, then if a given wire had 1 conductor, it would flow just as much current as a wire of the same gauge but with 100 conductors. Is this right?

The reason I'm asking is I'm getting around to planning to relocate the battery (to where the passenger seat was), and I have some 7-conductor #6 AWG cable that was used in telephone central offices for carrying current to ground. The stuff we used for heavy duty battery supply could carry up 1000 (yup, that's one thousand) amps at 48VDC, and probably had no more than 25-30 conductors in a one-inch diameter bundle.

Cable length from the battery to the starter will be maybe nine feet, with a stop at a battery distribution strip at about six feet from the battery. That strip is where I plan on bringing in the feed from the alternator, too. So, do I need to spend $25 on finer-conductored cable, or is what I have sufficient? This will be for autocross only, so no need for taking into account lights, radio etc.

Thanks in advance for any help.

Ted928 · Post by **Ted928** » Sat Jul 17, 2004 8:42 pm

I am no genius but the only advantage for multi-filament wiring for a Direct Current application is that it is more flexible.

The "skin effect" is a very high frequency AC thing.

See http://www.audioholics.com/techtips/aud ... Cables.htm
for a description of the "skin effect"

TR · Post by TR » Sat Jul 17, 2004 10:13 pm

If those are the lengths of the runs, you cannot go wrong with Taylor's pre-terminiated wires. Good insulation, stainless braided for protection, good connectors. Price isn't bad either, tons of lengths available from summit...

You can grab one of their battery cut switches at the same time...

I've always heard that DC "pushes" a lot of current through the core of a wire, but have also heard that most of a signal goes over the surface of a wire. Never stopped to think about it, though...I'll leave that to the sparky's...TR

toolsnob · Post by **toolsnob** » Sat Jul 17, 2004 11:13 pm

Electonics 101...

I failed but I do know this much. Current travels on the outside of the wire. The larger the gauge the more surface area for the current to travel. Copper conducts better than aluminum. Another cheap cable to get is go to a welding shop and buy some of there 00 gauge that is used for are welder leads.

Alexi

TR · Post by TR » Sat Jul 17, 2004 11:57 pm

Now you guys have my damn curiosity going...

I have heard the statement before, but it cannot be that big of an effect. Otherwise big conductors would be designed to optimize the surface area to amount of material (basically perimeter to area ratio of a cross section). This would save big amounts of money in raw materials.

Granted, stranded wire increases the surface area, but it is very common for equivalent load carrying wire of the same gauge to be offered in either form, solid or stranded...

I'll have to look at my "electrical engineering for dummies" on Monday at work...TR

Minh · Post by **Minh** » Sun Jul 18, 2004 2:15 am

I'm very rusty on this, but I'll try refreshing my memory years with the custom stereo installations in the past where debates of what wire would be appriate for what application is starting perk a little.

It is true the the current mostly travels through the surface or in other words through the path of least "resistance." Imagine having to run thorough a crowded hallway; it much easier to run along the side than right down the middle.

The thicker the strand/wire the larger the surface area, yes. Also the thicker the wire cooler the flow. Imagine the varying widths in in the fuses in the fuse box. Higher "amperage" fuses have thicker fuses. To much heat then the wire overheats and cathces fire or melts (i.e. like frying the wiring harness for example).

Then you also have to consider the load - "ohms". To thick a strand/wire and you actually weaken the current by giving too little resistance. Imagine the amount of water that flows in a creek and have it instead flow through a canal the size of the Mississippi - not a whole lotta water going through per second.

That is why fat (not too fat) speaker cables are good for bass speakers - tsunami size flow.

And the smaller (not too small) wires are good for tweeters - accurate high frequency signal response.

Yada, yada...

Hope this leads you to some conclusion.

Ted928 · Post by **Ted928** » Sun Jul 18, 2004 9:29 am

The science on this "debate" is well established. Of course people are free to believe what they want to believe.

For example;
I am having a new water well drilled and the driller used bent brass rod to "identify" the well location. Of course Science does not provide any validity to this old wives-tale but he explained that this method doesn't cost anymore than merely throwing a rock over your shoulder. I certainly can not disagree with that logic, especially since the spot he picked was where I wanted it anyway. I'll keep my fingers crossed and hope for water - not that science has validated the powers of finger crossing either.

DatsunBucky · Post by **DatsunBucky** » Mon Jul 19, 2004 9:54 am

Thanks for the replies, guys. "Before", when I relocated the battery to the trunk, I used welding cable. I remember where I bought it, but not how much I paid for it.

The main run of the cable could be single conductor for all the flexing it will take - being clamped about every 6" or so. I wasn't thinking about the flexing at the battery itself. Come to think of it, the run from the battery strip to the starter will flex a lot.

I guess I'll bite the bullet and buy cable instead of using what I have on hand. In the grand scheme of things, another $25 or so isn't that big a deal. I'm looking at #4 AWG cable, which is, of course, larger than the #6 I had on hand.

I was already thinking about putting a battery cut-off switch in, but forgot all about it when planning on the size of the cable. Hmmm, maybe #2 cable.

Minh · Post by **Minh** » Mon Jul 19, 2004 10:21 am

From what I've beeh lead to believe you can not have too much ground...

Well you actually can, only because it would look awfully wierd have ground wires everywhere...