Contributions to wire resistance - Page 1
Forum section: electronics / beginners

author: c4757p
author_rank: Super Contributor
timestamp: November 21, 2012, 02:27:51 am
content: I've got a couple sets of alligator clip leads that I bought from Amazon a while back when I was a bit more... poor. Really, really crappy stuff. (Claims to be 22 gauge, actually is 28. Not even soldered to the clips.) Finally decided to rewire them today with some proper wire. Here's the question: They definitely seem to be copper, 28 gauge, stranded, about 40 cm. They vary widely in resistance, many up to an ohm. (Yep, that's right, these wires are so shitty they're almost perfectly goodresistors. They get noticeably hot carrying 3 amps.) I rewired them with the same length of stranded 26 gauge copper wire. Now they all measure under 60 milliohms. I highly doubt there is a 16-fold increase in resistance per meter from 26g to 28g. Why then does their wire have such a high resistance?(It's not the non-soldered connections, as I also tested the wire itself properly, after cutting it off.)

author: robrenz
author_rank: Super Contributor
timestamp: November 21, 2012, 04:02:17 am
content: I likethis chart

author: c4757p
author_rank: Super Contributor
timestamp: November 21, 2012, 04:26:47 am
content: I like that chart too, but that still leaves me wondering - an ohm in 40 cm is about 1300 ohms/1kft (yeah, I just said "kilofoot"), the unit used by the chart. I am fairly certain it is 28-gauge wire, as I can strip it with the 28ga section of my strippers, but the 30ga section breaks the wire. The figures that table quotes for 28ga are 59 to 67 ohms/1kft, not 1300. What would cause its resistance to be so ridiculously high?

author: SeanB
author_rank: Super Contributor
timestamp: November 21, 2012, 05:05:14 am
content: Poor quality wire most likely, with a high level of contaminants.

author: nanofrog
author_rank: Super Contributor
timestamp: November 21, 2012, 05:26:38 am
content: Quote from: SeanB on November 21, 2012, 05:05:14 amI'm thinking recycled copper.Suitable for plumbing, but it's not sufficiently pure enough for electrical use. Last I checked, processing recycled copper to the necessary purity for electrical use is hideously more expensive than refining raw ore to the same purity.

author: c4757p
author_rank: Super Contributor
timestamp: November 21, 2012, 05:56:13 am
content: Recycled copper - interesting. And it does seem to have a bit of a "dirty" look to it, though it's hard to really see when the strands are so small. I never knew electrical copper had to be so pure. No wonder it's expensive.This is the only time I have seen wire that awful used for electronics. Not even going in the scrap wire bin - the whole lot of it goes straight in the trash. (Or I could paint stripes on them and throw them in with the resistors...)

author: ejeffrey
author_rank: Super Contributor
timestamp: November 21, 2012, 07:44:50 am
content: There is no grade of copper that has that high of a resistivity, recycled or not. Low grade copper (up to about 1% impurities) has 10s of percent higher resistivity than pure copper. Common copper alloys like bronzes and brasses have resistivity 3-6 times higher than pure copper. Your measurements indicate 20 times higher than pure copper. That is in the range of stainless steel and resistance wire alloys like CuNi44.Either your wire isn't copper, it is much thinner than you think, or there bad contact resistance in your measurement. Low grade copper is somewhat brittle. It could be that most of the strands are broken, or even that all the strands are broken in different places and the current has to hop strand-to-strand.

author: johnwa
author_rank: Frequent Contributor
timestamp: November 21, 2012, 09:00:36 am
content: Sometimes you will find that the copper conductor has reacted with a component of the insulation and corroded, reducing its effective cross-sectional area significantly. Though I haven't seen this for quite some time - I think that all reputable wire manufacturers have figured out what chemicals cause this, and stopped putting them in their insulation.

author: T4P
author_rank: Super Contributor
timestamp: November 21, 2012, 09:20:36 am
content: I'm thinking intermittent broken strands tooLast year i bought a pair on ebay and only arrived to me 3 months later(Some odd reason i got refunded for everything i purchased)Back on the topic, mine was actually worse, like 1.78 ohms not counting the connectors

author: c4757p
author_rank: Super Contributor
timestamp: November 21, 2012, 03:13:41 pm
content: Yep. Intermittent broken strands. Thank you. I just pulled one of the wires' insulation off, and along the entire 40cm wire there was no single strand longer than 10cm! I bundled all of the strands properly into a much shorter wire and measured the resistance - down to 0.1 ohm. Notgood, but much, much better.

author: SeanB
author_rank: Super Contributor
timestamp: November 21, 2012, 04:02:10 pm
content: Recycling copper you have 2 options. Melt it down and cast into an ingot and roll it, or cast into anode slabs and refine again. The second method is preferred as you get pure copper, exactly like the mine produced, though the price is higher in production as you need to factor in the power cost. Still cheaper than digging it out, but less profit.

author: G7PSK
author_rank: Super Contributor
timestamp: November 21, 2012, 05:11:29 pm
content: Check the wire with a magnet, it could either be a copper iron alloy or a copper wash on iron or steel wire.