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I have a friend that works with automotive mechanical sorts of things, and he told me that he had a pair of jumper cables that was ruined by too much current; it wasn't that the insulation melted: he said that the wire didn't carry hardly any charge after that.

I can't think of any reason why this would be. If a wire carried too much current it would obviously get hot, and that heat would eventually melt insulation, but even then, the copper (or whatever it is) should be able to carry charge just as well as before, shouldn't it? When it actively was hotter, the resistance would increase and this would impede the flow of charge, but once it cooled down, it should work like before, as I understand things. Does something occur when copper gets to a certain temperature that could permanently impair the electrical conductivity?

I'm not an electrical engineer or even an electrician. I just do some mechanical and automotive work and I have a little bit of experience with physics.

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  • \$\begingroup\$ "Ruined" is a little vague. How about a photo? Or at least what is the color and/or resistance of the "ruined" wire compared to how it was before or to new cable of the same kind? A digital multimeter costs less than a jumper cable these days... \$\endgroup\$ – Fizz Jan 17 '15 at 11:57
  • \$\begingroup\$ I have not yet seen the wire, but I do have a multimeter and if I can get him to find the cable, I will definitely test it now. I just wanted to check to make sure there wasn't something I was missing that explained it. I understand how jumper cables could break, but I didn't understand how copper could be inherently impaired at carrying charge just because of a temperature increase. Especially because it is molten when it is formed... \$\endgroup\$ – Conley Jan 17 '15 at 17:17
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Some newer jumper cables have fuse type elements included, (either a very large value fuse or just a thinner [sacrificial] wire that burns out before the main cable begins to super heat). This is done to protect those people who have trouble following simple instructions. Another possibility is that the wire to clamp connection point had a weak spot and that small area burned out during the current overload.

The small increase in resistance (with high temperature) of a thick copper wire is insignificant in this situation. Even molten copper would pass a lot of current.

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    \$\begingroup\$ Can you point me to some concrete jumper cable having this features listed in its sales-advert/"datasheet"? I've done a bit of googling but even self-advertised top-of-the-line cables, never mind the Amazon top sellers don't seem to have such features (made public anyway). I did find some with LEDs for indicating when they're connected wrong. \$\endgroup\$ – Fizz Jan 17 '15 at 12:31
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    \$\begingroup\$ I figured (if there wasn't some characteristic of hot wire that I wasn't aware of) that the latter explanation - or something of that nature - was correct. So, just to clarify, even if wire were to become molten and "reweld" itself, there's no way for the wire to be permanently impaired at carrying charge to a significant degree (other than if the wire were to somehow form a discontinuity)? My understanding of metals and "electron pools" and electrical conductivity leads me to this conclusion, but intuitive and intelligible explanations are usually wrong or have significant limitations. \$\endgroup\$ – Conley Jan 17 '15 at 17:15
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IN over 20 years as a mechanic in both the automotive and construction equipment, I have replaced many damaged components from overheating starter-motors, over cranking to staters, relays reversing switches on winches. Have seen copper jumper cables which turn a black color inside without melting the insulation yet when you bend the cable which seemed brittle and turn a bright dull orange. On a couple of occasions. Bakelite got so hot from heat it cracked and warped relay and solenoid housings without fire. One of the worst cases was a international diesel truck with two batteries connected in parallel with so many conections in between the battery cables, wiring, other multiple connections more wire ignition switch intermediate relay and the final solenoid the stater. If every connection on the system had the supposed ⅟10 /one tenth per connection in 8 connection you would have .8 volt less than whats supposed to be a acceptable level.

Any one knows what is the case when as DC starter motor starts overheating and how that may effect current draw.

Back in the mid '80s I had access to a 40K dollar SUN ocilloscpe computer that had the capibity to figuure the compression electrically on each cylinder and other odd problems.

https://www.quora.com/How-does-current-increases-when-resistance-increases

Jeff Verive Jeff Verive, BSEE/MSCSE Electrical Engineering & Computer Science, Illinois Institute of Technology Chicago - Illinois T... Answered May 2, 2017 Originally Answered: Does a splice of a wire have an effect on voltage?

Certainly, though the effect depends on the “quality” of the splice. Voltage drop in a conductor is equal to the resistance of the conductor multiplied by the current running through it. If I cut a wire in two pieces and then casually touch one cut end to the other I will have a higher resistance (and therefore a greater voltage drop) than if I squeeze the two ends together because squeezing increases the contact area, and the resistance is inversely proportional to the contact area (just as the resistance of a conductor is inversely proportional to its cross-sectional area). Indeed, if the effective contact area of the splice is greater than the cross-sectional area of the wire the resulting splice will have a lower resistance (hence a lower voltage drop) than the same length of an unspliced wire.

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    \$\begingroup\$ Welcome :-) Everything after your (currently shown) 1st paragraph does not seem related to the original question. Here on Stack Exchange sites, answers must address the original question (see the tour) e.g. the original question wasn't related to measuring engine compression electrically. Then you have copied and pasted part of an answer from a different Quora question than you linked (it was copied from this one). Again, there's nothing in the original question here about a splice. What's happening? \$\endgroup\$ – SamGibson Feb 25 at 2:16
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Perhaps there are no electricians willing to field this question. Wire doesn't carry a 'charge' it carries current. Excessive current does not cause the wire to melt (not in practice), it causes it to crystallize. Typically what happens in a home is - a connection at the duplex outlet (sometimes in a wire nut) fails and overheats, the insulation becomes brittle, and the wire itself gets an oxidized (green or black) coating. If you attempt to repair it without removing that portion of the wire you'll find that it is brittle. Current is carried along the surface of a conductor. With a dielectric coating (the oxidation) the wire cannot carry as much current, but it tries. What happens next I don't know. Finally the house burns down. (I had a stroke and sometimes don't express myself well.) I never used an inadequate jumper cable. With a battery that is long past needing to be replaced (I haven't done that in the most recent 50 years) your alternator has been badly stressed, your starter has likely also been badly stressed. Why compound that with a cheap jumper cable. Besides, lovely ladies give me kisses when I jumper their cars correctly and their engine starts immediately, as if nothing was wrong. If they've got an ancient battery I tell them to go directly to Walmart or wherever they like. If their alternator belt is badly worn, etc. Hope I haven't insulted anyone. There might be a different reason for wire that has been overheated to become brittle, but it hardly matters.

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    \$\begingroup\$ Welcome to stackexchange. This is a Q&A site. Questions and answers are voted on for thoroughness and correctness. Your answer seems like more of a forum post, as it is unorganized and a lot of it has nothing to do with the question. I'm also shocked by your claim: "Excessive current does not cause the wire to melt". As far as I know, excessive current causes excessive heat, which in turn can melt a wire. Please edit your answer by keeping it on topic, and reply with a comment on why you say that current doesn't melt wire. \$\endgroup\$ – Bort Mar 30 '18 at 0:57
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    \$\begingroup\$ Your answer does not answer the main question, and you drift all over the place as if writing a novel. Parse it into paragraphs so they are easy to read. Your statement Excessive current does not cause the wire to melt is flat out wrong. I have seen puddle of copper and burnt insulation in shorted disconnect switch's. -1 \$\endgroup\$ – Sparky256 Mar 30 '18 at 2:59

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