Wire crimpers are tools used to squeeze any of various types of hollow, round connectors onto a piece of wire. A crimper looks like a pair of pliers, except the jaws have a notch cut out of one side and a matching tab extending from the other side. When closed, the tab fits into the notch. Wire crimpers are so simple that they are often built into other electrician tools such as wire strippers. Often they will also come with wire cutters built-in as well. A combination wire stripper/crimper/cutter tool will cover just about all of an electrician's basic needs in one neat package, except for a good screwdriver.

The connectors intended to be used with wire crimpers come in various sizes depending on the gauge of wire that will be used with them. Most have a round, hollow, metal tube on one end and a connector of some sort on the other. It may or may not be insulated (if not, it may have to be wrapped in electrical tape, depending on the specific application).

To crimp the wire to the connector, first strip enough insulation off the end to expose enough bare conductor to ensure a good connection with the connector. Slide the bare conductor into the hollow round portion of the connector, hold it steady, and close the crimper around the connector very tightly, being careful not to pinch the insulation inside. This will compress the connector's fitting around the wire, holding it secure.

    Typical crimp-on connectors
  • butt splice - Used to connect two ends of wire together to make one wire. Has a hollow round opening on both ends.
  • spade terminal - Used to make a temporary connection between two wires. The male end looks like a flat blade and slides into the female end, held in by friction. The ends can be disconnected and reconnected as necessary.
  • ring terminal - Ends in a disc with a hole in the middle, intended to be secured down by a screw or bolt.
  • fork terminal - Ends in two prongs and is similar in use to the ring terminal, except that the screw or bolt does not have to be completely removed to slide it in or out, just loosened.
  • pin terminal - The male end terminates in a short, thin shaft, and the female end in a slightly thicker, hollow shaft. These are usually very small connectors used to make custom serial or parallel port cables for computers.
    Special crimp-on connectors
  • RJ-11 - Used to make custom telephone cables, these are the standard 4-pin plastic ends that snap into the telephone and into the wall connection. The connector actually bites into the wire insulation as it is crimped so no stripping is required. Requires a special-purpose crimping tool which squeezes all four wires simultaneously.
  • RJ-45 - Used to make custom Ethernet cables, they look like thick phone cable ends. The connector actually bites into the wire insulation as it is crimped so no stripping is required. Requires a special-purpose crimping tool which squeezes all eight wires simultaneously. Some tools have slots for both RJ-11 and RJ-45 since they are so similar.
  • RG58 - Standard 50Ω coaxial cable connector for Cable TV and Composite Video connections. Requires a special-purpose crimping tool.

There are hundreds of others, but these are by far the most common.

I have occasionally seen crimps done another way, although the electricians I have spoken to discourage this method. Rather than sliding just the bare conductor into the connector, some people fold the conductor back over the insulation and crimp the insulation into the connector with the conductor. While this may make a good, tight fit initially, the wire insulation isn't designed for this. Over time the insulation can wear or chip down, loosening the connector and eventually causing a short circuit.