Things You'll Need
Tandem circuit breaker
A tandem breaker fits two individual breakers into the size of one. That allows you to add new breakers when your breaker box doesn't have any new slots open for it. Assuming your circuit panel is cleared to accept a tandem breaker, you can install it the same way you install a regular breaker. It requires a healthy respect for electricity and an awareness of the dangers it poses but otherwise can be conducted without the services of an electrician.
Open the panel in the circuit box and the check to see the number of circuit poles it can safely carry. A tandem breaker only uses one slot in the row of circuits, but it uses two poles. Never install more poles in your box than it can handle.
Shut the main power switch in the circuit box off, and turn the tandem breaker to the off position as well. Put on a pair of rubber shows or stand on a rubber mat before you proceed.
Unscrew the metal panel surrounding the circuit breakers and pull it away.
Locate the wiring you wish to connect the tandem circuit breaker to. There should be two "hot" wires, usually insulated in black, and two grounding wires, usually insulated in green. Test them all with a multimeter set to the "Volts AC" position to make sure there is no electricity running through them.
Use a set of wire strippers to remove the last 1/2 inch or so of insulation.
Connect the hot wires to the corresponding terminal screws on the tandem circuit breaker, being careful to note which wire goes to which terminal. Then, tighten the screws with a screwdriver.
Snap the tandem circuit breaker into position on the circuit panel. In rare instances, you may need to screw it into place with a screwdriver instead.
Connect the ground wires to the corresponding screws on the grounding bar, then tighten the screws just as you did with the hot wires.
Turn the main power back on--returning electricity to your house--then turn both switches on the tandem circuit breaker on. Test all of the wiring with the multimeter to confirm that the power is flowing.
Return the metal panel to its former position and screw it into place. If the knockout tab corresponding to the position of the tandem breaker is still in place, snap it off with a pair of pliers before you screw the panel back in.
If you are replacing one or more old circuit breakers with a tandem circuit breaker, compare the amperage levels. The tandem breaker needs to precisely match the amps of the breaker it is replacing.
Standing water is a huge hazard when working with electricity. Make sure your work area is dry before you install the tandem circuit breaker, especially if the circuit box is in a basement or similarly damp area.