Installing a switch and a receptacle outlet in the same box is a straightforward upgrade. If you are installing the convenience outlet in a box that already contains a switch, there must be a circuit neutral wire present in the box. The easiest way to tell if you have a neutral in the box is to look at the cables entering the box. If there are two separate cables entering the box with each of the black wires attached to the switch and their white wires spliced together, you have a neutral wire, the white wire.
Installing a Switch and Receptacle in a Single-Gang Box
Turn off the branch-circuit circuit breaker on the service panel. Remove the switch cover plate. Check to make sure that the power is off by turning on the non-contact voltage tester and bringing it near the wires in the box. If there is voltage still present on the wires, the tester will beep loudly. Some boxes have more than one circuit running through them. You may have to turn off more than one circuit breaker before you can work on the circuit safely.
Remove the two #6-32 screws holding the switch in the box. Pull the switch out of the box and disconnect the wires from the switch. Remove the wire nut holding the two white wires together. You may have to use the Lineman's pliers to untwist the white wires once you remove the wire nut to separate them.
Separate all the wires so that they do not touch one another or the box. Turn the branch-circuit circuit breaker on temporarily. Use the non-contact tester to ascertain which black wire is bringing power to the box. Fix the position of that wire firmly in your mind and turn the circuit breaker off again.
Cut the white wire flush with its insulation and remove 3/4 of an inch of insulation from the cut ends with the wire strippers. Cut a 6 to 8 inch length of white wire, removing 3/4 inch of insulation from both ends. Make a 3-way splice with the two white wires in the box. Hold the stripped ends of the wires side by side and twist them tightly together in a clockwise twist with the Lineman's pliers. Complete the splice by screwing on a wire nut. Form a loop, in the free end of the white, pigtail wire.
Connect the black wire bringing power to the box to the side of the switch receptacle with the double brass screws joined together. Connect the black wire going to the light to the single brass screw on the opposite side of the switch. Connect the white wire to the silver screw and the bare copper wire to the green, octagon-shaped screw. Place the loops around the screws in a clockwise direction and tighten the screws.
Install the switch/receptacle in the box, install the cover plate and turn on the circuit breaker.
Installing Separate Switch and Receptacle in a Two-Gang Box
Turn the circuit breaker off and check to make sure the circuit is safe. Cut five 6 to 8 inch pigtail leads—two black, one white and two bare copper. Remove ¾ of an inch of insulation from their ends.
Splice the white pigtail wire to the two white wires in the box. Splice the two black pigtail wires to the black wire bringing the power into the box. Splice the two bare copper pigtails to the two bare copper wires in the box. Make these splices in the manner you made the splices in the first section.
Connect one of the black pigtails to the bottom brass screw on the switch. Connect the other black pigtail to one of the brass screws on the receptacle. Connect the white pigtail to the silver screw on the receptacle. Connect the black wire going to the light to the top brass screw on the switch. Connect the bare copper pigtails to the green screws on the switch and receptacle. Attach the wires to the screws in the same manner as you did with the combination switch/receptacle unit.
Install the switch and the receptacle in the box, install the cover plate and turn on the circuit breaker.