At some time or other, just about everyone wishes they had just another inch or two of cord on a tool or device. While it's simple enough to just plug into an extension cord, there are times when this is less than optimal, especially when you're working in tight spaces. Cable splicing is one answer to the problem of safely extending electrical cables.
Determine the length of the extension you wish to make. Cut the original cable and the new section about an inch apiece longer than what you want.
Strip an inch of insulation from the end of both wires. Use wire strippers instead of a knife, because a knife can cause irreparable damage to the cable.
Twist the two exposed wires together clockwise one and a half to two full rotations. Trim half an inch from the ends' twisted wires, leaving them even.
Wire nut: Twist the wire nut clockwise onto the twisted wires until no bare wire is left exposed and the wires do not come apart when pulled. You may wish to tape the nut in place with electrical tape to prevent slippage, but this is not usually necessary.
Compression sleeve: Slide the sleeve over the twisted wires and use a specialised crimping tool or the jaws of your wire strippers to crimp the sleeve down firmly. Make sure to use plenty of pressure to ensure a firm bond.
Wire nuts are colour-coded according to the gauge of wire they were meant to splice together. They are designed primarily to join two wires of the same gauge, so it's important to match the new wire to the old. Compression sleeves are designed for the long haul, while wire nuts are a temporary fix. If you intend to make permanent use of the spliced cable, you should use a compression sleeve.
Make sure that there is no power to either of the wires you're working with. If possible, unplug both wires; if not, flip the main breaker into the Off position. Improperly joined wires leave you open to fire, electrical shock and possible death. Be very careful when using spliced cables; check the joints before you plug them in. Most areas have building codes regarding the splicing of electrical wires. Check up on local laws to be sure of which type of splice you should use for a given purpose. When joining copper wire to aluminium, you must use a specialised two-compartment connector. Electrician's tape is not a replacement for either wire nuts or compression sleeves. Insulation caps are required in some areas. Check your local building codes for further information.