Installing a 30-amp breaker and plug isn't easy, but you can do this without having to hire an electrician. You may need to install a 30-amp breaker and plug because you are adding a laundry room and need to connect your dryer. Welders also run on 30 amps and would need a dedicated circuit for safe operation. Whatever your reason is for installing a new breaker and plug, you can install them safely on your own.
Cut a hole in the wall where you will locate your 30-amp plug by first tracing around the template provided with the single gang "old work" box. Use a keyhole saw to carefully cut around the trace line and remove the drywall.
Turn the power off to your breaker panel. Locate the main breaker panel at the top of your breaker box and turn it off to disconnect electricity to the panel.
Remove the cover to your breaker panel, exposing all wiring and breakers. Use a two-prong electrical tester and test to verify that you have indeed disconnected the power connected to your breaker panel by touching one prong to the screw at the back of any breaker and the other prong to any metal on the panel. If your tester does not light up the power you disconnected the power correctly.
Pull a length of 10/3 NM cable from your 30-amp plug location back to your main breaker panel. Leave at least 8 inches of cable exposed at both the new outlet location and the main breaker panel.
Install the single gang "old work" electrical box into the wall. Pull the wires through the knockout at the back of the electrical box and slide the electrical box into the wall. Secure the "old work" box into the wall by turning the screws at the top and the bottom of the box clockwise. An "old work" electrical box has wings attach to the screws. Tightening the screws lifts the wings and clamps them to the drywall to hold the electrical box in place.
Strip the cable at both the electrical box and the electrical panel. Use dual NM cable cutter/strippers and remove 6 inches of exterior sheath to expose the interior black, red, white and bare copper wires. Strip 1/2 inch of insulation off of the black, white, and red wires.
Install the 30-amp double pole outlet/plug into the single gang "old work" electrical box. On the back of the plug is four screws. Slide the white wire underneath the top screw and tighten the screw to hold the wire in place. Slide the black wire underneath the left screw and the red wire underneath the right screw and tighten both of these screws. Slide the bare copper wire underneath the bottom screw and tighten that screw as well. Carefully push the outlet/plug into the wall and attach the plug with screws onto the box at both the top and the bottom. Cover the outlet/plug with a 30-amp receptacle cover.
Install the 30-amp double pole breaker by first sliding the back of the breaker into the breaker panel at the slot provided to hold the breaker into place. Firmly push the breaker over the center buss bar until it snaps into place. Slide both the red and black wires under the two screws at the back of the breaker and tighten the screws to hold the wires into place. Locate the neutral bar in the breaker panel; it will be a silver bar with white wires attached to it. Slide the white wire underneath one of the screws on this bar and tighten the screw. Locate the grounding bar in the breaker panel. It will be very similar to the neutral bar; however, it only contains green and bare copper wires. Attach the bare copper wire in the same manner you attach the white wire. Replace the cover on the breaker panel and turn on the main breaker.