Three-prong electrical plugs are standard on all household appliances, and the plugs are used to connect them to three-prong outlets. When a power cord ages, the insulation may begin to crack or peel off, exposing the internal wires which then becomes a fire and safety hazard. If the three-prong plug is molded plastic, the entire cord must be replaced with one of identical power ratings. If the plug is the type that may be disassembled, a new plug may be installed on the outlet-end of the cord. The wiring colors are the same regardless of whether the plug or the cord is being replaced.
Removable Plug Replacement
Remove the screw(s) holding the old three-prong plug together if it is of the type that can be disassembled and replaced.
Unscrew the internal wire mounting screws from the wires in the old plug, paying close attention to which wire colors go to which screw mounts in the plug so that you can easily wire the new plug with minimal difficulty. If necessary, draw a simple diagram on a piece of paper, annotating which color wire went to the upper-left prong, which wire color went to the upper-right prong and which wire color went to the lower third grounding prong.
Remove the wires from the old plug and lay them into the new plug. Connect the wires to the same terminals they were connected to on the old plug. It is important to use this method because wiring colors used in newer homes are different than wiring colors in older homes. The instruction sheet which may have come with the new plug may use modern wire color codes and the colors won't match the older colored wires.
Install the new plug's cover plate and screw it firmly into place.
Replace Entire Cord
Analyze the appliance or device carefully to determine which screws or bolts to remove for accessing the wire ends of the power cord inside of the appliance or device. Remove the necessary screws and panels. If removal of the casing requires the removal of many parts, draw a simple diagram on a piece of paper as each piece is removed and label each part with a marker and on the paper. Replace all parts in reverse order of how they were removed.
Assess the type of connection the wires have inside of the appliance. If the wire ends are soldered into place, heat up a soldering iron and touch to the old solder of each wire to melt it. Hold a length of de-soldering braid between the index finger and thumb of the other hand and briefly touch to the old solder so it absorbs the solder away from the connection. Remove the wire promptly before the joint cools. Use a digital multimeter turned on to measure continuity or set to measure 100 ohms. Touch the black probe to the end of the wire and then touch the red probe to each of the prongs on the cable plug until the meter provides a reading. Mark on a piece of paper which of the prong wires connected to the terminal from which the wire was just removed. Do the same with the other two wires. If the wires are screwed into place, simply remove the screws from the wire terminals and remove the entire cable from the appliance, making note of which wire colors were connected to which terminals.
Remove insulation from the wire ends of the new three-prong power cable. Remove about two inches of the outer insulation jacket and then use the wire strippers to remove 1/4 inch of insulation from each of the three individual wires. If soldering the wire ends into place within the appliance, use the digital multimeter as in Step 2 to find which wires go to which of the prongs on the plug. Connect the correct prong wires to the correct solder terminals within the appliance so the configuration is the same as it was with the wires of the old cable. Proceed to Step 4 below if soldering, or Step 5 below if attaching screw lugs to the wire end for screwing into place.
Hold each wire by the insulation with the left fingers, one wire at a time, and press the stripped wire end to the correct terminal on the appliance. Press the tip of the hot soldering iron against the wire end to hold it into place and to heat the wire up for ten or fifteen seconds. Use the empty hand to grab hold of the solder spool and unroll a three inch section of solder. Touch the solder wire tip to the hot wire end and allow the solder to flow onto the wire end until the wire is covered in solder. Remove the soldering iron tip from the melted solder, holding the wire several inches below where the soldering iron is touching it (the wire is hot within the first inch or two) and hold the wire in place while removing the soldering iron. Allow the new solder bead to cool for a few seconds and then solder the other two wires in the same way, connected to the correct terminals.
Slide a new crimp-on wire terminal onto the stripped wire ends of the new power cable, selecting them from the assortment pack making sure the new ones are the same diameter and hole width as the old ones. Crimp the terminal jackets onto the wire ends firmly so they can't be pulled off. Use the multimeter to determine which wires go to which prongs on the plug of the new cord and then screw each to the correct mount locations as they were connected with the old cable. Replace all panels and screws of the appliance.