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Author Topic: The physics of wire carrying DC current?  (Read 4370 times)
RobertKoernerExAE7G
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« Reply #15 on: August 02, 2006, 04:53:07 PM »

�stranded wire would have to have a slightly larger actual diameter due to the gaps between the wires�

You�d have to get random samples of wire, then measure the diameters.

Your hypothesis is that the average diameters are not equal.

The null hypothesis is that they are equal.

If  I remember correctly, this is a one tailed test, vrs a two tailed test.

Next, you�d set one diameter equal to the other.  Then You�d have to see, at a 95% confidence interval, if diameter cross equals diameter solid.  

If they are equal, you reject your null hypothesis.

Stats people will have to verify my memory.  I have empirical evidence that my memory is not accurate.

Course, you�ll need to know what your measurement error is.

Short of testing your theory, you can call a small wire manufacturing company.  One that I am familiar with is LAT, or L.A.T. International in NJ?

Have fun.
Bob
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W3JJH
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« Reply #16 on: August 02, 2006, 06:46:44 PM »

Stranded wire will not carry more direct current than a solid wire of equal cross-section.  Period.

AC is a different case.  Skin effect comes into play with ac but not with dc.

Stranded wire will not carry more alternating current than a solid wire of equal cross-section either--unless the strands are individually insulated as is the case with Litz wire.  If the strands are shorted together along the length of the wire, then the conduction skin forms around the entire bundle.  If they are individually insulated, then each strand will have its own skin, and all the skins will be in parallel.  I deal with this problem on a day-to-day basis designing switching power supplies.

There's a reasonable explanation of the skin effect at http://en.wikipedia.org/wiki/Skin_effect and a description of Litz wire at http://en.wikipedia.org/wiki/Litz_wire.
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W8JI
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« Reply #17 on: August 04, 2006, 04:21:12 AM »

If the two wires are exactly the same physical diameter and material the solid wire will handle slightly more current.

The reason is simple. A stranded wire ALWAYS has some air inside, and air does not conduct heat or electricity very well.

The standed wire, for exactly the same materials and outside diameter, will have more resistance. It will make more heat per unit length at a given current.

The stranded wire will not conduct heat away from the wire core as rapidly as the solid wire, so it will have more temperature rise for a given amount of heat all other things equal.

The stranded wire will not conduct heat along the LENGTH of the wire as efficiently as a solid wire, so it will "hot spot" more than a solid wire.

The stranded wire will oxidize more rapidly at high temperatures, and will have a lower fusing current for a given wire diameter and material.

The difference largely depends on the ratio of air inside the wire to "copper".

73 Tom
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AA4PB
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« Reply #18 on: August 04, 2006, 05:23:17 AM »

Now the question: Is the wire guage of stranded wire determined by the sum of the cross sectional areas of all its strands or the outside diameter? I thought the area determined the guage and if so, 16 guage stranded would have the same DC current carrying capacity and resistance per foot (given the same material and ignoring temperature rise) as 16 guage solid wire.
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W9OY
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« Reply #19 on: August 04, 2006, 07:20:46 AM »

So lemme see

stranded has more resistance

stranded has less resistance

stranded has the same resistance

Cracks me up!!

73  W9OY

http://www.electronics2000.co.uk/data/itemssz/wheatstn.htm
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W8JI
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« Reply #20 on: August 06, 2006, 04:23:44 AM »

by W9OY on August 4, 2006

stranded has more resistance>

Indeed it does for a given length, outside diameter, and given material! There always has to be a gap area inside the wire. It's physically impossible to build a stranded wire without space inside.

stranded has less resistance>>

We know that isn't correct, unless the OD is larger.

stranded has the same resistance>>

Ditto.

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W9OY
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« Reply #21 on: August 06, 2006, 08:04:00 PM »

Tom

If you read the original question, he was involved in an argument, which is also precisely what happened in this forum

the 3 positions of this forum are:

1 stranded has less res.  (the general consensus)

2 stranded has more res.  (your position)

3 stranded has equal res.  (the air force's position)

He asked for a methodology to determine the answer.  An accepted and sensitive method to actually do this experiment is to use the Wheatstone bridge.  It's easy to build and reproducible.  In his case all he would need is a regulated power supply, 2 equal length pieces of the same gauge wire, with the same metallurgical composition, one solid, one stranded, a bridge that wouldn't pass too much current, so every thing doesn't change temp much, and a meter.  Put in the solid piece of wire and balance the bridge.  Put in the stranded and one of three things will happen

1 stranded has less res

2 stranded has more res

3 stranded is equal

The result is not quantitative but qualitative and answers the question.  

73  W9OY
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W8JI
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« Reply #22 on: August 07, 2006, 06:18:15 AM »

RE: The physics of wire carrying DC current?  Reply  
by W9OY on August 6, 2006  

If you read the original question, he was involved in an argument, which is also precisely what happened in this forum>>

I expect that. He didn't state the case in enough detail.

the 3 positions of this forum are:
1 stranded has less res. (the general consensus)>>

That's very clearly **wrong** for the same outer diameter. The starnder wire contains air, and the air can't conduct. So a stranded wire of a given size.

2 stranded has more res. (your position)>>

It's very easy to understand why I'm correct.

3 stranded has equal res. (the air force's position)>>

I doubt the Air Force has an opinion on this subject.

He asked for a methodology to determine the answer. An accepted and sensitive method to actually do this experiment is to use the Wheatstone bridge.>>

Actually you need to go back and read his question. He never asked about resistance at all. As a matter of fact, he never once mentioned electrical resistance in his question.

Everyone is obviously not even giving an answer when they stop at resistance, so why focus on that?

The fellow asking the question had a better handle on the problem than almost all of the replies!

<<It's easy to build and reproducible. In his case all he would need is a regulated power supply, 2 equal length pieces of the same gauge wire, with the same metallurgical composition, one solid, one stranded, a bridge that wouldn't pass too much current, so every thing doesn't change temp much, and a meter. Put in the solid piece of wire and balance the bridge. Put in the stranded and one of three things will happen>>

A completely useless test for his question.

He asked which wire has more current carrying capacity. You suggested a test that excludes the only real limitation in current carrying capacity, heat!

The **maximum current carrying capacity of a conductor**, all other things equal, ties only to temperature rise in the conductor. Nothing else.

All external conditions equal, temperature rise is a function of resistance per unit length and abilitly to get heat out of the conductor. Nothing else.

The stranded wire has several things working against it.

1.) The air in the wire and poor contact between strands means stands INSIDE the core have mush less ability to get rid of heat than the core of a solid wire.

2.) The gaps around each wire stand mean there is less conductor cross section available for a given wire maximum outside dimension limit, so for a given size a stranded wire has more resistance per unit length. This is true even for RF signals.

3.) When the heating isn't even, the resistance in the hotter areas increases faster than colder areas. This makes the wire have higher resistance and make more heat.

4.) Stranded wire oxidizes faster. It has more area exposed to air. That means when a temperature is reached that compromises the conductor surface, stranded "goes away" faster.

All of this is why lightning grounds should use solid wire unless the conductor needs to flex, in  which case a larger stranded wire can do the same job of a smaller solid wire.

Common sense should prevail and eliminate the need for such proof, but if he wanted a test he would have to obtain several samples of identical alloy material with identical outside dimensions and run enough current through them to see when they actually fail.

That is the only meaningful test.

I'm not amazed a number of people argue in any forum. That's mostly because they never really read the question, so they argue about answers to questions that were never asked.

73 Tom
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N4CR
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« Reply #23 on: August 07, 2006, 01:29:49 PM »

W8JI said: "Common sense should prevail and eliminate the need for such proof, but if he wanted a test he would have to obtain several samples of identical alloy material with identical outside dimensions and run enough current through them to see when they actually fail.

That is the only meaningful test."

Thanks, and that is near to the test that we finally came up with.

Our objective measurement will be generated heat. We don't actually need the wire to fail.

We will take equal lengths of the two wires, put them in series, condense the lengths into identical zig-zags, run enough current through them to bring up the temperature and measure it with an infrared temperature gun. Since the wires are in series, we know the current is equal. Getting the zig-zags nearly identical should be the only critical element of the test, given trusted wire samples.

No resistance measurements are required to perform this test. Of course, resistance enters into the physics of the result, but it's not the test.

The test can't be conclusive unless we run metallurgical tests on the copper. It is out of our reach to do this on a budget. In lieu of metallurgical tests, we will test many different manufacturers wires. Alternately, we might be able to get lab grade samples from one of the wire companies to run the test.

We're still working out the details of how we will proceed.
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73 de N4CR, Phil

Never believe an atom. They make up everything.
W8JI
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« Reply #24 on: August 07, 2006, 02:39:58 PM »

That sounds like the one way to test it.

A few words of caution.

You are measuring temperature rise and not ultimate failure points.

You have to be specify if wire diameters are the same, or gauge is the same.

If you have a given volume of space to fill a solid wire will have the largest cross section and lowest resistance and I^2 R losses, all other things equal. BUT (big but)stranded wire is larger in diameter for a given gauge to offset the conductor cross section reduction caused by air.

So you have to decide if you want to compare the copper crosssection of the conductor (AWG or circular mils) or the actual physical size occupied.

Stranded #10 has an OD of .116 inches and is 10380 mils with 1.038 ohms per 1000 feet at 20 dgrees C.

Solid #10 is .101 inches OD and is 10380 circular mils with essentially the same resistance.

Solid wire is manufactured smaller for the same AWG, because it doesn't have air between the strands.

That's partially why I said for a given diameter solid carries more current than stranded, and not a given gauge carries more current.

If you were going for wire failure stranded wire has a handicap caused by slower heat transfer and uneven heating in strands. Since you are only measuring slow temperature rise far from failure points the air gaps aren't so important.

73 Tom
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N4CR
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« Reply #25 on: August 07, 2006, 09:39:50 PM »

W8JI said: "So you have to decide if you want to compare the copper crosssection of the conductor (AWG or circular mils) or the actual physical size occupied. "

The original premise was that the gauge of the wire was the same, so I believe we are talking about cross sectional areas being the same and not diameters.

I have a big lab grade DC supply that will go from 0 to 36 volts and 0 to 25 amps. It's a huge thing. We can use that to quickly perform real destructive tests with wires in series and open air. That could be an interesting test. I believe those tests would be best performed outdoors. That should be enough current to destroy 18 gauge or smaller wire for most types of insulation. The following chart says 24 amps on 18 gauge wire with teflon insulation will take the wire to 200C.

http://www.bnoack.com/index.html?http&&&www.bnoack.com/data/wire-resistance.html

I'll try to get some lab samples from some wire companies.
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73 de N4CR, Phil

Never believe an atom. They make up everything.
W8JI
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« Reply #26 on: August 09, 2006, 09:30:21 AM »

I see that now.

If cross sectional area is the same, and if the heat isn't enought to cause wire failure, the standed and solid will be the same.

If wire diameter is the same, the solid wire will be better.

If wire gauge (cross sectional area)is the same and the wire is tested for wire failure from excessive current, solid wire will be better. It gets rid of heat better.

It sounds like a good experiment if it can be controlled!!

73 Tom
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K7PEH
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« Reply #27 on: August 09, 2006, 08:25:49 PM »

So, yesterday when I picked up the mail, I discovered that my latest "Physics Today" journal had arrived.  Just before dinner, I was thumbing through the pages glancing here and there and ran across an ad for the perfect instrument for these 'physics of wire' type questions.

It is a Keithley meter (www.keithley.com) that measures resistance down to 10 nanoOhms, voltage down to 120 picoVolts, and charge down to 10 femptoCoulombs.  That should cover the measurements fairly well.  I am not sure how much they cost but it doesn't look like too much, probably less than an Icom IC-7800 but more than the new Yaesu FT-2000.

As far as running current through wire until it melts, I do have a power supply for that task.  It is a 5-volt, 150 amp peak power supply that I picked up at Boeing Surplus here in the Seattle area.  That's right, 150 amps.  That means you can short the terminals with a copper bus bar and the power supply just hums along as if this is a normal everyday thing.  I think this supply was used for electro-plating applications.
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K8MHZ
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« Reply #28 on: August 11, 2006, 09:54:52 PM »

The question of ampacity of stranded vs. solid wire is not a matter of the physical characteristics of the conductor, but that of the insulator.

A conductor's ampacity is based upon the amount of current it can carry continuously for 3 hours without exceeding the thermal rating of the insulation.

You will find the ampacity of bare wire run overhead is enormously more than insulated wire run through conduit, even if the wire is of the same gauge size.

Please refer to the NEC section 310.16 if my memory serves me correctly.  (Sorry if that is the wrong section but I don't have an NEC right next to me)  There should be a table there with the ampacities of various conductors.

JJH has it correct. Skin effect while present in 60 Hz does not seem to be much of an issue, at least not like it is at RF levels.  In fact, AC electricians really don't even consider it to be a factor.  We also apply Ohm's law to AC as if it were DC in some instances by using 'effective values' and 'DC equivalents'

If you want to see something amazing, bust open a 400 amp fuse.  Check out how small the conductor inside is.  It is literally made of foil.  The insulator is a sand like substance that can get hotter than molten metal before it breaks down.  The conductors can operate at very high temps. A very small amount of metal can carry a very large amount of current if it's design temperature is high.
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K8MHZ
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« Reply #29 on: August 11, 2006, 10:00:34 PM »

Oh, and BTW,

I belong to IBEW Local 275 and am a Journeyman, both licensed and Union.
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