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I use a small cutter made for max 14 gauge wires, but I'm bad at removing insulation, it takes me a bit of time to get it right, so I end up cutting and repeating, because I see scratches on copper wire were I cut the insulation. Should I ignore it? How bad are these scratches or marks? These are power works behind sockets.

Blurry photo that doesn't show scratchmarks

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Altoban
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    You know that's not a very useful photo, right? – RedGrittyBrick Dec 05 '17 at 17:40
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    @RedGrittyBrick it's clearly a match melting a straw. Not sure what that has to do with nicked wires. – Brad Dec 05 '17 at 20:22
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    @WayneConrad OK but you need to explain why that's relevant. I doubt the asker is proposing to bend the wire back and forth many times after installing it. – David Richerby Dec 06 '17 at 14:00
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    @DavidRicherby Well, that would turn my comment into an answer :) When installing wiring, it gets pushed, folded, and bent, either now, or later when someone else goes back into the wiring box to do something. It doesn't take many bends to break a nicked wire. – Wayne Conrad Dec 06 '17 at 14:11
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    Unless it's *really* deep, a scratch along the length of the wire is of no consequence. What *is* a potential problem is a fairly deep nick *across* the width of the wire, as that will create a stress point if the wire is subsequently bent (as when you are shoving the outlet back into the box) – Hot Licks Dec 07 '17 at 01:16
  • You forgot to mention what the wire will be used for. It could be simply to carry electricity, or it can be used to carry electrical signals. In the first case, scratches won't matter that much, while in the second case , the signal can be affected. Whether or not it is important not to scratch the copper depends on the use of the copper – Newton fan 01 Dec 07 '17 at 16:20
  • @Newtonfan01 and nowadays electrical cables do carry Ethernet signals via the power line adapters. – Tim Dec 08 '17 at 02:59

6 Answers6

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Minor superficial marking is just that. Unimportant. Deep nicks around the circumference create weaknesses. Those would cause problems. Redo.

With the right tools, careful use and a little practice, you will be able to strip insulation without nicking or scratching the copper.

Blurry photo
Suitable tools, in the same spirit as photo in Q :-)

RedGrittyBrick
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Obligatory X answer: If you have to ask if the nick is bad enough, it's bad enough.

It's not you. It's the piece of junk wire stripper. There are a LOT of really terrible wire strippers out there, either in the $5 tool bin, or they come with variety packs of crimp terminals.

If you only have occasional work to do, take WGroleau's advice and roll an exacto knife around the edge of the wire, pressing just barely enough to penetrate the insulation, not nick the wire appreciably.

Otherwise, go out and buy a really good one. We're talking $12-15. Irwin is the lowest quality I would consider, and not their bottom-end unit either.

It should have a mystery hole about 1/4" from the edge of a side. Bolt shears are nice but not necessary. Having a terminal crimper is usually the mark of a junky cheapie, though it's perfectly possible for a fine tool to have it.

I know it's easy to sit here and spend your money. Rest assured I don't do it lightly. You'll thank me...

Harper - Reinstate Monica
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    You, in all honesty, do not answer the question. – yo' Dec 05 '17 at 18:23
  • Sorry, but you are comparing apples and oranges. The question you commented on is asking for solutions. This question is asking for analysis. Your analysis is "(a) it is an issue, (b) buy a better tool", which is fine but as you can see, it has two parts. – yo' Dec 05 '17 at 18:52
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    I disagree. You are saying every answer must answer the original question. That is crazy. It results in redundant answers and no added value. Usually people like you add further conditions like "straight up as asked" no matter how crazy or XY the question is. Here, we want answers that are **helpful** and we would rather an answer **complement** another answer than *replicate* it. It's not fine, you made my answer and our collection of answers worse. – Harper - Reinstate Monica Dec 05 '17 at 19:00
  • My snap on wire strippers have a crimp zone and I will put these up as excellent strippers. – Ed Beal Dec 05 '17 at 19:16
  • What about https://www.lowes.com/pd/Southwire-Wire-Strippers/50081506? I sometimes have to strip #10 and #12 Al wire that has been in service 45 years. The insulation is no longer soft. Are there any special strippers for that? I can remember from 45 years ago a recommendation for stripping Al wire to eschew tools that work fine on copper. The recommendation was to use a razor knife and shave the insulation off! – Jim Stewart Dec 05 '17 at 20:05
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    The real problem with nicks is not localised heating, it's that that they concentrate mechanical stresses making it far more likely for the wire to snap. – Peter Green Dec 05 '17 at 20:09
  • That is exactly what the explanation was for the beneficial results of the "pencil sharpening" strip for Al. The claim was that a smooth shallow gouge would not concentrate stresses, but deep nicks would. – Jim Stewart Dec 05 '17 at 20:15
  • @PeterGreen seems to me that would only be a factor if the copper is under mechanical stress. Why would it be? However if it was, the increased heat would not help its physical properties one bit. – Harper - Reinstate Monica Dec 05 '17 at 22:10
  • @JimStewart that's a little industrial for my tastes. I just use a quality-made version that might be mistaken for a cheapie, with cutting surfaces that are actually ground and hardened, not coined. – Harper - Reinstate Monica Dec 05 '17 at 22:12
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    "Nicks in the wire are a constriction to current flow, which creates localized heat, which further weakens an already nicked wire. It can overheat, fail, or potentially start a fire. " No. Just no. Unless you remove a significant portion of the copper (more than %20) for a reasonable distance (2-3mm) there is going to be no measurable impedance difference. – Sam Dec 05 '17 at 23:15
  • Can you cite your sources regarding the localized heating? I, like @Sam, have questions about whether a reasonably small scratch mark could cause a substantial increase in resistance. – Cort Ammon Dec 06 '17 at 01:26
  • @Sam of course it won't affect the *average* impedance much, being only a very, very short distance of wire. However "impedance" misses the point. Instead of using an ohmmeter, look at it with a FLIR. An outsize proportion of total wire heating will occur in that very small spot. This presumes the current is high and the nick not insubstantial. – Harper - Reinstate Monica Dec 06 '17 at 01:33
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    @CortAmmon you and Sam are both setting up a straw-man, where the damage to the wire is insubstantial. For instance Cort, I never said "reasonably small scratch mark". I didn't claim it, so I don't need to prove it. Sam, on the other hand, has claimed that 19% wire damage for 0.3mm will *not* cause heating. THAT is a bold claim, bold proofs are called for. – Harper - Reinstate Monica Dec 06 '17 at 01:41
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    When I didn't have a stripper, I would roll the wire along the edge of a razor blade and then pull that piece of insulation off with fingers. – WGroleau Dec 06 '17 at 03:16
  • @PeterGreen stress concentration is mitigated by copper being a very ductile metal, by the fact that the wire should not be under any significant stress, and in that cable (one core) is normally not used in installations where vibration of the cable is to be expected. With flex, only a few cores get nicked and even if they later break, the installation is still safe and sound. – nigel222 Dec 06 '17 at 13:56
  • @Harper Where'd you get your electrical engineering degree? – Sam Dec 06 '17 at 14:51
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It depends on the frequency. If you nick a wire carrying 60 Hz, no problem. Nick one carrying 10 GHz you've got a big problem.

user78726
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    @HankyPanky These sort of frequencies can come into play on standard network cables. I have manually made CAT7 network cables for 40 Gb/s connections. These operate at frequencies (the higher harmonics at least) well in the 10 GHz range. 5 GHZ is also nothing special on coax cables used to connect an antenna to a Wifi access-point. – Tonny Dec 06 '17 at 11:58
  • @HankyPanky yes, certainly, with hand-tools. You don't get nicks in unfortunate places assembling the connectors though. You just cut the insulated cores to the right length, pop them in the right places, and then crimp on the insulation-displacement connector (or with some sockets, push them into Krone IDC receptacles). 10GBase-T is not 10GHz on the wires: there's some clever multi-voltage-level encoding for multiple bits per clock, and two pairs to transmit in parallel. Caveat: I've never yet had to make a 10GBase-T cable. – nigel222 Dec 06 '17 at 14:12
  • Stress concentrators, work hardening effects etc can cause you grief in DC wiring just as well. – rackandboneman Dec 06 '17 at 16:02
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    @Tonny Isn't cat 7 rated for like 600 mhz? –  Dec 07 '17 at 02:30
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    @Tonny, To my knowledge, there are no or very few transistors that have a gain bandwidth product above ~6GHz, so I *highly* doubt that you are working on cables that carry 10GHz signals directly. – Sam Dec 08 '17 at 16:35
  • @nigel222 _"10GBase-T is not 10GHz on the wires: ... and two pairs to transmit in parallel."_ - Are you sure it's just two pairs in parallel? Even Gigabit Ethernet (1000BASE-T) sends data through 4 pairs in parallel (in both directions, using echo cancellation). I'd expect them to use that trick at 10GbE as well. – marcelm Dec 10 '17 at 01:23
  • @marcelm No, not sure. If so then the frequency on the wires is reduced around 4-fold rather than around 2-fold. – nigel222 Dec 13 '17 at 09:31
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Since it isn't clearly specified what the wire will be used for, i will enumerate a few scenarios when copper wires are used, and analyse the impact of the scratches in each case.

a) carry electricity (for the purpose of powering an electrical device)

b) carry electrical signal (which is the same as carrying electricity, but this time, either the intensity/voltage/frequency of the current vary in time, as to encode some digital/analogue signal)

c) generate magnetic field

d) generate heat

e) generate light

Analysis of the impact:

a) as long as there is physical contact between the wire and the next metal part in the circuit, electricity will flow. Scratches do not affect conductivity (this aspect has been previously mentioned in Jeffrey Boettger's answer). The fact that the current will flow is not even influenced by the contact surface between the two metal parts - so scratches don't have the least influence

c) this is the case when the wire is used to make a coil, which is a component widely used in electronics (load speakers are just an example where we use coils). This time, scratches will affect the magnetic field, making in irregular. If the coil has many turns, scratches over a short segment might not be critical, but, from a theoretical point of view, every geometrical irregularity of the wire will affect the magnetic field.

b) this is the case when the wire is used for internet/television/telephone. As i mentioned above, the signal is encoded as variations in the current's voltage/intensity/frequency. None are influenced by scratches in the wire. However, in telecommunications, it is often the case that a pair of cables are twisted together, for the purpose of cancelling each other's magnetic field. That's because magnetic field represents an interference that affects the signal. As i stated at point c) above, if the wire has any irregularities, its magnetic field will not be uniform, meaning that it can affect the neighboring wire's signal.

d) this is the case when the wire is used in electrical heaters. The heat property is only influenced by the current intensity. Scratches will not affect that

e) this is the case when the wire is used for a light bulb. Light, just the same as heat, is only influenced by the current's intensity

Considering the photo provided in the question, i am aware this wire will not be used for either heating or lighting. I just thought it was worth mentioning points d)and e) along with the other points.

There is also a mechanical aspect involved - how resistant will be the wire to repeated bending. Scratches can be of two types: longitudinal (along the wire) or transverse (across the wire). Bending has the potential to enlarge transverse scratches, but it will not influence longitudinal scratches. (This aspect has also been mentioned in a comment by Hot Licks).

Newton fan 01
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    Just a small nitpick: Why would you define an arbitrary ordering for your bullet points, then discuss them in an alternate order? If you wanted to discuss point c before point b, then you should have called it b. – Tim Seguine Dec 09 '17 at 11:06
  • i thought someone would notice that; from a logical point of view, point b comes after point a, because it's a particular case of it. however, when i detailed the explanation, i needed to use at point b something from point c, that's why i chose to discuss point c before point b – Newton fan 01 Dec 11 '17 at 08:52
  • That much of your intent was clear from the text. What I am really saying is that it is unnecessarily confusing to read. In terms of electrodynamics, an electric field is in no way more fundamental than a magnetic field, so you could have just as easily put point c first, and kept the ordering linear, logical, and consistent. I consider it a loss, because this contains relevant information that the accepted answer lacks. – Tim Seguine Dec 17 '17 at 15:01
  • My opinion here is different. I do regard the electric field as more fundamental, perhaps because of the order i have studied these subjects in school (the physics curricula in my country introduces the magnetic field 3 or 4 years after the electric field). I specifically put the electric field before the magnetic field. However, i must agree that for a professional physicist these topics are equally difficult; moreover, they are strongly correlated. But the average individual has higher chances of having heard about the electric field, and not about the magnetic field – Newton fan 01 Dec 18 '17 at 09:11
  • You essentially counted 1, 2, 3, 4, 5, then immediately counted 1, 3, 2, 4, 5. That is confusing. Period. My concern is completely editorial. By countering my suggestion for an alternative way of ordering it, all you did is demonstrate you are missing the point. – Tim Seguine Dec 19 '17 at 17:38
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Small scratches should not affect conductivity of the copper wire at all. This is common when stripping wires bare with knives, side cutters and other misc tools. When one is preparing copper wire they should get into the habit of using wire strippersthis particular tool strips wire to gauge minimizing damage.

Don't be to worried about little nicks and scratches. Just be mindful of the plastic coating. Exposed wire could be a potential shock or fire hazard.

Jeffrey Boettger
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Like @RedGrittyBrick already mentioned, if it's superficial surface markings, then no worries, they're only markings. Use a flashlight and magnifying glass, so you can try see to see if you would call them surface markings vs. a real nick. If you see something you'd call a "nick", then just redo the stripping.

I only occasionally do electrical work, but have broken wires at the insulation just by moving them slightly in the box. Since it's after the fact, I can only guess it was due to nicks. And if so, no way to measure how deep the nick was before the break. So better safe than sorry.

Also like @Harper says, make sure you have good wire strippers. Try about a dozen times on some extra wire, if you are not getting the hang of it, then maybe you should consider the tool then. If it's the red cutter tool with screw, make sure that the screw has been set properly for 14 gauge. If it's the yellow tool with multiple positions, then make sure a) you're actually doing 14 gauge wire and b) the wire is going into the 14 gauge position and not its 16 gauge neighbor. It is possible to strip with the 16 gauge position but will usually lead to bad results.

Chris O
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