Extension cords are the least interesting thing on a jobsite and one of the more common causes of electrical fires. The physics is simple enough to explain in a paragraph, and understanding it changes how you buy and use them.

Why cords heat up

Every conductor resists the current flowing through it. That resistance converts a portion of the electrical energy into heat. Thicker wire has less resistance. Longer wire has more.

So heat in a cord comes from three variables working together: how much current the tool draws, how thick the wire is, and how long the run is. Push a heavy load through thin wire over a long distance and the cord warms up, the voltage at the far end drops, and the tool works harder for less output.

That voltage drop is the second problem. A motor fed low voltage draws more current to do the same work, which makes more heat, which makes the situation worse. Tools run on undersized cords run hotter and wear out sooner.

Reading the gauge

Wire gauge runs backwards from intuition. A lower number means thicker wire. A 12-gauge cord carries more than a 14-gauge cord, which carries more than a 16-gauge cord.

Cords are marked with their gauge, conductor count, and rating, usually printed along the jacket. They also carry a rating for indoor or outdoor use and a certification mark from a recognized testing laboratory. Buy cords that have one; unmarked cords are not worth the savings.

The practical guidance: match the cord to the tool's amp draw at the length you actually need, and when you are between choices, take the heavier cord. For long runs with saws, compressors, or heaters, heavier gauge is not optional.

The coiling problem

A cord coiled on a reel or left in a loop while carrying a heavy load is the classic overheating scenario. Coiled wire heats itself and its neighbors, and there is nowhere for the heat to escape. The middle of a tight coil can reach temperatures the jacket was never meant to see.

Uncoil the cord fully before running anything substantial through it. If you are using a reel, pull off all the cord rather than leaving most of it wound.

Habits that prevent problems

  • Use one cord of the right length. Do not chain cords together.
  • Keep connections up off wet ground and out of standing water.
  • Use cords rated for outdoor use outdoors.
  • Protect cords from traffic and from door and window pinch points rather than running them where they will be crushed.
  • Never remove a ground pin or file down a plug to fit an outlet.
  • Do not run cords under carpets, rugs, or through walls and ceilings. Extension cords are for temporary use.
  • Use ground fault protection outdoors, in damp locations, and anywhere required. On construction work, ground fault protection for temporary power has specific regulatory requirements worth knowing.

When to retire a cord

Stop using a cord immediately if it has:

  • Any cut, crack, or abrasion exposing the inner conductors
  • A jacket that has melted, deformed, or discolored from heat
  • A loose, bent, or missing prong
  • A plug or connector that is warm in normal use
  • Repairs made with tape

Tape is not a repair. A damaged cord is either replaced or cut up and thrown away so nobody plugs it back in.

The one habit worth building

Put a hand on the cord partway through heavy work. If it is noticeably warm, the setup is wrong. Shorten the run, get a heavier cord, or reduce the load. That check takes two seconds and is the most reliable warning you will get.

For permanent power needs, the answer is not a better extension cord. It is a properly installed circuit put in by a licensed electrician following local code.