stranded vs. single conductor resistance

Thread Starter

TBayBoy

Joined May 25, 2011
148
I've been looking at charts for resistance of copper wire, in general, I've been seeing stranded wire has less resistance than single conductor wire. This seems to go against my understanding.

Is it solely due to the stranding increasing the circular mil. area, or some skin effect, or some other think I'm not getting.

thanks.
 

Thread Starter

TBayBoy

Joined May 25, 2011
148
So with more conductor surface, there is less skin effect? So with resistance of 22 awg single strand is more than 20 awg not specifically because the wire is thicker and more free electrons, but more surface area.
 

KJ6EAD

Joined Apr 30, 2011
1,581
I seem to remember some other effects playing a role even in DC such as eddy currents and interstrand capacitance which may affect resistance.
 

mcgyvr

Joined Oct 15, 2009
5,394
Skin effect is basically only accounted for with for AC currents (higher frequency the closer to the surface)
 
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Thread Starter

TBayBoy

Joined May 25, 2011
148
OK, so we seem to be back to the original question, why does a single conductor wire have more resistance than the same sized multi strand wire?.
 

Ron H

Joined Apr 14, 2005
7,063
I've been looking at charts for resistance of copper wire, in general, I've been seeing stranded wire has less resistance than single conductor wire. This seems to go against my understanding.

Is it solely due to the stranding increasing the circular mil. area, or some skin effect, or some other think I'm not getting.

thanks.
The cross-sectional area, and hence the resistance, of any particular AWG should be identical between stranded and solid wire.
Show us a link that says otherwise, and we'll look into it.
 

Thread Starter

TBayBoy

Joined May 25, 2011
148

Ron H

Joined Apr 14, 2005
7,063
At this site for solid core, 20awg is noted at 10.15 ohms

http://www.cirris.com/testing/resistance/wire.html

and here for stranded, 20awg is noted at 10.36 ohms

http://www.stealth316.com/2-wire-resistance.htm

for 1000 ft.

I know this is a very small difference so is it all just maths rounding, skin effect, combination of the two.

This isn't a problem that is gonna throw the earth out of orbit, just a curiosity I can't put down.
Your second link has this to say:
The resistance values for the stranded copper wire sizes shown in the tables below (sizes that are typically found in an automobile) are from http://www.mogami.com/e/cad/wire-gauge.html, a 2% lay factor is assumed and diameters are approximate. The lay factor relates the diameter of the helical wire path and the length of braided strands (360º of rotation) and is but one factor used to determine the ratio of a wire braid to the equivalent tubular, or solid, conductor (please see http://www.fiskalloy.com/Percon/cf-lay.pdf for some additional information). AWG is the American Wire Gauge (formerly Brown and Sharp) size. Also shown is the corresponding millimeter wire gauge size, which is the cross-sectional area of circular wire. Circular mils (CM) is the diameter of a solid wire in 1/1000th inch diameter circle (a 1 inch diameter wire would be 1 million circular mils). For a solid wire of the same gauge or CM, the resistance will be slightly less than that for a stranded wire. For example, a 12 AWG solid copper wire has a resistance of about 5.21 ohm/km compared to 5.32 ohm/km for a 12 AWG stranded wire.
Here is my interpretation of the lay factor:
Each strand of twisted wire is in the shape of a helix. If you took a length of stranded wire, then unwound all the strands and straightened them out, each would be longer than its length when twisted, and therefore the resistance will be greater than that of an equal length of solid wire of the same gauge.
 
I know this is an old thread but I figure I would give an answer that I think will clear up things up for a lot of people because it is one of the first links if you google "solid vs stranded wire resistance"

Strand wire and solid wire of the same gauge are NOT the same diameter. The diameter of the same gauge strand wire is larger so that it has the same equivalent cross section area of metal as a solid wire. (this depends on the number of strands.) I think this is what confuses most people.

I am unsure why you see less resistance even with the same cross section area, which its how I ended up here.

source:
http://en.wikipedia.org/wiki/American_wire_gauge
read the first few sentences under "Tables of AWG wire sizes"

http://en.wikipedia.org/wiki/Wire#Stranded_wire
read the second paragraph under (forms or wire -> stranded wire)


 

MrChips

Joined Oct 2, 2009
30,618
You have to read the wikipedia article carefully (second link):

A stranded wire will have higher resistance than a solid wire of the same diameter because the cross-section of the stranded wire is not all copper; there are unavoidable gaps between the strands (this is the circle packing problem for circles within a circle). A stranded wire with the same cross-section of conductor as a solid wire is said to have the same equivalent gauge and is always a larger diameter.
Stranded wire of same diameter will have more resistance.
Stranded wire with same cross-sectional area will have larger diameter and is given the same gauge.


At high frequencies, the skin effect comes into play. The wire with greater surface area will perform better.
For better performance at high frequencies, litz wire, which has the individual strands insulated and twisted in special patterns, may be used.
 

baconc

Joined Sep 1, 2015
1
Reading the above information, I have one important question.

On copper braid screened cables, does the lay length of the braid affect the DC Resistance of the screen?

I ask this question as I am informed by a far eastern manufacturer that shorter braid lays make the resistance deteriorate due to the distance the signal has to travel along the cable to reach it's end point.

My opinion was always the shorter the braid lay, the better the resistance as you have more copper within the product.
 
try this on for size. Take a look here: http://www.seas.gwu.edu/~ecelabs/appnotes/PDF/techdat/swc.pdf

Lok at 18 AWG wire. There are MULTIPLE ways of making 18 AWG wire. It consists of a number of strands of a lesser guage wire. None can have the identical circular area of 18 AWG solid wire, You can see it in the table.

I used low stranding to be flexible, but positionable when I wired panels.

Skin effect: Ac voltages only. In high power RF amplifiers, you will see wires as silver plated copper tubing. Silver has lower resistivity than copper.
 
I've been looking at charts for resistance of copper wire, in general, I've been seeing stranded wire has less resistance than single conductor wire. This seems to go against my understanding.

Is it solely due to the stranding increasing the circular mil. area, or some skin effect, or some other think I'm not getting.

thanks.
Surely, from my college years ago, this just comes down to basic parallel and serial resistance. Although, the individual thinner wires in multistrand wire have a higher resistance than the single thick wire in a single strand wire, you have resisters in parallel. Double the number of resisters in parallel and you halve the resistance. In the multistrand wire the overall resistance = Resistance of 1 strand of wire divided by the number of strands. If the resistance of 1 strand is 100 ohms and you have 20 strands then the overall resistance = 100 / 20 = 5 ohms.
 
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