A 240-volt circuit breaker is a twice as thick as a 120-volt breaker--it takes up two spaces in the breaker box. A 30-amp breaker allows 30 amps of current to flow through a circuit before it trips and shuts off the circuit. Before installing additional breakers, you need to determine the maximum number of breakers allowed in the box. Just because there are blank spaces available, it does not mean that you can use them safely. Read the label inside the box to determine the maximum number allowed. If you are at the maximum, you will need to install a sub-panel.
Turn off the main breaker. Remove the screws from the front panel. Hold the cover with one hand while removing the last two screws, then pull the cover straight off.
Hook the breaker on the two clips on the outside edge of the breaker box. Swing the breaker towards the centre of the box and snap it in place.
Strip the end of the green wire to expose 1/2-inch of the conductor and connect it to the ground terminal strip. Strip the end of the white, neutral wire and connect it to the neutral terminal strip. In some cases, both wires will go to the same terminal strip.
Strip the ends of the remaining two wires and connect them to the gold terminals on the breaker. Align the cover with the new breaker and snap out the metal plates that are blocking the breaker.
Replace the cover and turn on the main breaker.
Do not attempt this if you are not comfortable working with electricity. There is a risk of serious injury or death if you make an error. Make sure the equipment connected to the breaker is wired correctly before turning it on. Some places require an electrical permit and inspections to install a breaker. Check with your local building department before starting this project. If you are not comfortable doing electrical work, hire a licensed electrical contractor.