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When I measure a stranded copper wire or see a resistance spec from mfg it is about 25% higher than what is listed in the tables I see online for solid copper wire. About 1 AWG difference. ex: 10 AWG stranded resistance is closer to that of 11 AWG solid; 18 AWG stranded is closer to that of 19 AWG solid.

Would expect a slight difference, maybe a percent or two, perhaps due to the helix of the strands. But 25%. Where is that huge difference coming from?

I realize there is the packing air gap between the round strands. But AWG is based on the cross sectional area of the conductors (the copper). Is it not? So the gap should not play a roll in DC resistance. Should it?

For instance. Looking at the resistance spec of a 10 AWG copper wire with 413 strands of 36 AWG, the mfg spec is 0.00424 ohms per meter. In the online tables I see 0.003277 ohms per meter for 10 AWG solid copper wire. 413 strands of 36 AWG by my calculation is about 2 strands shy of 10 AWG solid wire cross sectional area. About maybe 0.5%. But the resistance spec is about 25% more than what is found in the online tables for 10 AWG solid wire; falling more in line with 11 AWG solid wire instead.

What is the source of this large discrepancy in DC resistance between same AWG solid and stranded copper wire? I see this large difference both in published mfg specs and what I measure.

This is all copper. No CCA.

NOYB
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  • _"Would expect a slight difference, maybe a percent or two"_ - how do you figure it would only be 'a percent or two'? – Bruce Abbott Oct 01 '21 at 05:10
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    https://en.wikipedia.org/wiki/Circle_packing_in_a_circle – Bruce Abbott Oct 01 '21 at 05:16
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    Draw a series of close packed circles on 2 D.compare total area of an enclosing boundary for a significant number strands. You'll find the air space larger than you now expect. Hexagon packing is the most efficient at 0.9069 packing density. – Russell McMahon Oct 01 '21 at 05:17
  • "*But AWG is based on the cross sectional area of the conductors (the copper). Is it not?*" Not according to the labels on my wirecutters which only have a single hole for 12AWG, and not two a hole for 12AWG stranded and 12AWG solid. – DKNguyen Oct 01 '21 at 05:22
  • @Bruce Abbott The helix makes each strand slightly longer. – NOYB Oct 01 '21 at 06:49
  • @Russell McMahon How does circle packing change the resistance of a given cross sectional area of conductive material (copper in this case)? Sure it makes the overall diameter larger for a given cross section of conductive material. But it is the conductive material and it's DC resistance that is of interest here. – NOYB Oct 01 '21 at 06:51
  • @DKNguyn How precise are your wire cutters? Most have a range for each position. Do yours have a position for each single AWG size? – NOYB Oct 01 '21 at 06:53
  • @NOYB My impression has been as yours is. Fluke say no. I'm not convinced, but: I realised the difference in the two cases BUT I should have said so. :-( || Fluke [here](https://www.flukenetworks.com/blog/cabling-chronicles/considerations-choosing-stranded-vs-solid-cable) note that its the bundle size that is specified. || Test: Take an equal length of each cable. Strip out copper and weigh. Immerse copper in water and see displacement. IF they are equal in both cases, as they should be "it's interesting". If they are different it's also interesting, but differently so. || – Russell McMahon Oct 01 '21 at 11:50
  • Disagrees with Fluke :-) - as you expect and as I have so far always believed - [engineering toolbox](https://www.engineeringtoolbox.com/wire-gauges-d_419.html) - say: "The AWG table below is for a single, solid, round conductor. Because of the small gaps between the strands in a stranded wire, a stranded wire with the same current-carrying capacity and electrical resistance as a solid wire, always have a slightly larger overall diameter." – Russell McMahon Oct 01 '21 at 11:59
  • "the mfg spec is 0.00424 ohms per meter" at what temperature? – Ben Voigt Oct 01 '21 at 19:40
  • @Ben Voigt Mfg resistance spec is: 0.00424 Ohms per meter at 20C. – NOYB Oct 01 '21 at 21:42
  • @DKNguyen: *"the labels on my wirecutters which only have a single hole for 12AWG,"*. Compare with Ideal Industries 45-121 where each position has two labels, one for solid and one for stranded. It adds 2 to AWG for stranded. – RedGrittyBrick Oct 01 '21 at 22:58
  • @RedGrittyBrick Interesting. – DKNguyen Oct 02 '21 at 00:55
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    @RedGrittyBrick The overall diameter will be greater for stranded given equal cross sectional area of conductive material as solid. The AWG number is determined by the cross sectional area of the conductive material. Solid and stranded of same AWG are to have the same cross sectional area of conductive material (copper in this case). – NOYB Oct 02 '21 at 04:01
  • _"For instance. Looking at the resistance spec of a 10 AWG copper wire with 413 strands of 36 AWG, the mfg spec is 0.00424 ohms per meter."_ - please provide a link to this spec. – Bruce Abbott Oct 02 '21 at 05:56
  • https://www.parts-express.com/pedocs/specs/audtek-electronics-skrl-10-speaker-wire-specifications.pdf – NOYB Oct 02 '21 at 08:48

1 Answers1

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From Wire Strand Chart (with a bit of creative cut and paste) enter image description here

You are correct 413 strands of #36 AWG copper will have a similar resistance to #10 solid copper. 1.0232Ω / 1000ft vs 1.0175Ω / 1000ft. A 0.56% difference.

If your copper is tinned to allow easy soldering and prevent corrosion, then the resistance will go up. 1.1Ω / 1000ft vs 1.0175Ω / 1000ft if we compare it to bare copper. A 8.1% difference.

Resistance is inversely proportional to area.

$$R = \frac {\rho \ell}{A} $$

Adjacent wires in American Wire Gage are 1.261 greater in area. 26.1% more copper means resistance goes down by an equivalent amount. 3 wires apart, double area, half resistance.

OP is correct in their 413 strands of #36 copper will be just over 2 strands short of #10 solid copper.

A 8.1% difference does not equate to 26.1% difference, but it can add to the difference.

The most likely cause has to be measurement error. Most likely resistance in probes. Try measuring resistance with probes shorted out.

Regardless of the temperature, it wouldn't account for a 26% difference.

AWG is based on inches, mils and circular mils. #36 is 25.000 CM. 413 x 25CM = 10325CM. #10 is 0.327Ω/100m and #36 is 136Ω/100m, which means that 413 strands of #36 would be 0.329Ω/100m.

OP has a wire equivalent to #10 Copper, but measuring a resistance equivalent to #11. OP is not comparing measurement with #10 solid, but theoretical calculation in some Table.

StainlessSteelRat
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  • Because the question was asked in SI units -- 1.0232Ω / 1000ft ~ 3.33 Ω/m. So the problem is not in solid vs stranded, it is in mfg spec vs online table. Perhaps the manufacturer is quoting the resistance as a different temperature than the table you are using? – Ben Voigt Oct 01 '21 at 19:39
  • How are you measuring this resistance? Are you using Kelvin connections. Is your meter accurate at those low resistances. – Gil Oct 01 '21 at 20:01
  • Mfg resistance spec is: 0.00424 Ohms per meter at 20C. Meter is zeroed out and accurate to 10 mili-ohms. Measuring 15 foot and 53 foot lengths of 18 gauge stranded copper. As stated earlier the same ~25% difference is observed/corroborated by both my own measurements and published mfg specs. – NOYB Oct 01 '21 at 21:44
  • And resistivity is 10.37 Ω · CM/ft @20°C or 1.72 x 10^-8 Ω · m. The temperature coefficient is not ebough to have a meaningful effect on measurement. – StainlessSteelRat Oct 01 '21 at 21:51
  • Give us the link to published mfg spec! – StainlessSteelRat Oct 01 '21 at 21:52
  • https://www.parts-express.com/pedocs/specs/audtek-electronics-skrl-10-speaker-wire-specifications.pdf – NOYB Oct 02 '21 at 04:07