How to Add Smoke Detectors to Existing Systems

Turn off power to the old smoke alarm and the room where you will be installing the new smoke alarm. Use a circuit tester to make sure there is no power running to the into the room or the alarm.

Select the location for your new smoke detector near a ceiling joist. Cut a small hole into the drywall for the wire to come out. Punch out a hole in an electrical box by placing the tip of a screwdriver on the knock-out and hitting the handle of the screwdriver with a hammer. Secure the electrical box to the ceiling stud with screws.

Lower your old smoke detector down from its location so that its wiring is exposed. Unscrew the wire nuts holding the wiring in place. Remove the insulating cover from the end of the red wire.

Run fish tape through the hole for the new detector to the old detector. Connect black, white and red 12-gauge wire to the end of the fish tape. Reel the wire from the location of the old smoke detector to the new smoke detector.

Strip 1/2 inch off both ends of the newly run white, black and red wires. At the old detector, connect the three white wires and screw a wire nut over them. Repeat this with the three black wires and the two red wires. Mount the old detector back in its location.

Twist the trim ring off the back of the new smoke detector. Knock out the holes in the trim ring, with a screwdriver, which will align with the mounting holes on your electrical box. Secure the trim ring to the electrical box with screws. Make sure the wiring is protruding through the trim ring.

Remove the AC adapter from the back of your smoke detector by squeezing the tabs on either side and pulling up. Remove the insulating caps from all the wires. Hold the AC adapter up to the wiring in your ceiling.

Hold the three white wires together and twist a wire nut over them. Do the same with the three black wires and the two red wires. Push the AC adapter back into the back of the smoke detector.

Line up the marks on the side of the alarm with the marks on the trim ring. Push the alarm onto the trim ring and twist it in the direction of the "ON" arrow.