The wire passed every test you gave it. It lit the light, it held the load, and the inspector signed off. Eleven years later, on a cold morning, the furnace blower stops and nobody can say why. Inside a junction box, a single solid conductor has parted at a spot about a quarter inch from the screw terminal. The fracture face is clean and bright, and a thin ring runs around the copper like the line on a snapped pencil. That ring is the mark of a stripper that bit a little too deep, one afternoon, when someone was in a hurry.

The nick didn't fail on the day it was made. It waited.

The wrong lesson most people learn

Ask around and you'll hear the explanation that a nick reduces the wire's cross-section, so the wire gets hot there. It sounds right, and it's mostly wrong.

Take the numbers. Resistance goes up as the area goes down, so a nick that removes 20 percent of the copper's cross-section raises the resistance of that tiny slice by 25 percent (1 divided by 0.8). But the nick is only a sliver of the wire's length. The heat from I²R comes from the whole conductor, and a 25 percent bump over a fraction of an inch adds almost nothing to the total. A shallow nick on a 12 AWG conductor carrying 15 amps will not glow.

If heating were the only problem, nicks would be a cosmetic complaint. They aren't, because the danger is mechanical.

Stress goes where the geometry lets it

When a bar of metal is loaded, the stress flows through it like water through a pipe. Where the bar is smooth, the flow is even. Where there's a sharp notch, the flow has to crowd around it, and the stress at the root of the notch climbs well above the average across the wire. Engineers call this stress concentration, and the sharper the notch, the worse it gets. A scratch has a nearly zero radius at its tip, which makes it close to the worst case.

The second ingredient is repetition. Metal that is stressed over and over, even far below the load that would break it in one pull, can develop a crack that grows a little with each cycle. This is fatigue, and it's the same mechanism that makes you snap a paperclip by bending it back and forth. Fatigue cracks start at surface flaws, and a stripper nick is a ready-made flaw.

So a nick has two jobs: it concentrates stress, and it gives a crack somewhere to begin.

Where the cycling comes from

A wire inside a wall box looks as static as anything could be. It isn't, for a few reasons:

  • Thermal cycling. Copper expands when it warms, at roughly 17 millionths per degree Celsius. Every time a load starts and stops, the conductor and the terminal breathe a little. Over thousands of cycles, that adds up.
  • Vibration. Motors, transformers, and fans pass a hum into the box and its contents.
  • Handling. Every time someone pushes a device back into a crowded box, the conductors flex at the point where they leave the terminal. That is exactly where strippers leave their mark.
  • Installation. The wire gets bent into a hook around the screw, straightened, and bent again when the first attempt doesn't look right.

That last one is the cruelest. Solid copper hardens as it's bent, which makes it less forgiving. A hook formed at a nick has already used up some of its life before the cover plate goes on.

Why the ring cut is the worst nick

There are two ways to nick a conductor. One is a scrape along its length, which is mostly cosmetic. The other is a cut that goes around the circumference, the kind made when you score the insulation with a knife or crush it with an ill-fitting stripper slot and twist.

The ring cut is the dangerous one because it's perpendicular to the bending. The crack has a straight run across the entire wire, so one flex after another opens it up. This is also why the failed conductor in our opening scene showed a ring: bending stress in a wire is highest at the surface, and the ring put a notch at every point on that surface.

Once the crack has gone most of the way through, the remaining copper carries all the current through a much smaller neck. Now the heating argument becomes real. The neck gets hot, the oxide on the crack faces thickens, resistance rises further, and the heat builds on itself. The final stage can be an intermittent connection that arcs, which is how a quiet mechanical defect becomes a thermal one. The same runaway logic appears in any loose connection, and it's why the code treats workmanship as a safety requirement. NEC 110.12 asks for installations to be done in a neat and workmanlike manner, and a clean strip is a small, literal piece of that.

Stranded wire has its own version

Stranded conductors don't fail from one clean crack, but they aren't immune. Cutting a few strands while stripping removes a fraction of the cross-section, and the remaining strands then carry a bit more current and take the flexing for the whole bundle. Those cut strands also tend to fray at the end and splay out of a terminal, which is its own hazard. The principle is the same: damage is cheapest to avoid at the moment of stripping.

Your next moves

  • Match the slot to the wire. Strip a scrap of 14, 12, and 10 AWG solid with your tool, and check each one under a light. If you can see a bright line around the copper, use the next size up in the stripper and try again.
  • Never ring-cut with a knife. To open NM cable, use a cable ripper or a knife blade angled away from the conductors. Don't cut around the insulation of an individual wire; use strippers, or if you must use a knife, work like you're sharpening a pencil, with the blade slanting off the copper.
  • Inspect every strip, every time. Take two seconds to look at the exposed copper under good light. A visible score, a flat spot, or a missing strand means you cut it back and strip again. A few inches of scrap is cheaper than a callback.
  • Bend once. Form the terminal hook in one motion with the pliers and place it clockwise under the screw. If it's wrong, cut off the hooked end and re-strip rather than straightening and rebending.
  • Mind the strip length. Use the strip gauge molded on the device so that no bare copper sticks out past the terminal, and so you aren't tempted to re-strip in the box where the cable is short.
  • Replace worn tools. If the stripper's jaws are chipped or the adjustment has slipped, replace it. A $20 tool is cheaper than a repeat visit.

Where the numbers meet the craft

The nick teaches the same lesson as every other quiet failure in this trade: the tables tell you what a conductor can carry when it's whole, and the work is making sure it stays whole. That's the part of the job where calculation and hand skill meet. Voltly, the offline calculator and NEC reference we make, handles the arithmetic side, from ampacity and voltage drop to box fill, so you can spend your attention on the stuff only hands can do, like a clean strip. If you'd like to see it, it's at voltly.lumenlabs.works, no hurry.