A doorbell is powered by low voltage, typically 10 or 16 volts; most modern doorbells are 10. This requires what is known as a step-down transformer to modify the 120-volt household current into a usable source of power. Choose a spot for your chime and transformer that is centrally located so that your doorbells will be heard throughout the house. A chime and transformer that plugs directly into an outlet is the simplest solution for most homeowners.
Locate a suitable plugin for the transformer and chime unit. Choose one that is centrally located so that the bell will be heard. Do not plug into the power yet.
Run 18-gauge wire from both doors where the bells will be installed to the location you selected for the chime and transformer. Staple the wire along the top edge of the baseboard moulding and wall casing along its path, or remove the casing and base shoe to slip the wire into the space behind for a hidden installation. Realign the moulding and nail it back in place with the original nails, touching up the paint with a matching colour.
Drill through the wall where the bell will be installed with a 1/4-inch bit. Feed the wire through from the inside and seal the hole by caulking around the wire with painter's caulk. Allow the caulk to dry. Apply a coat of matching paint to the wire and caulking to blend them in.
Open the back of the bell buttons and locate the screw terminals for the wire. One will have a black lead soldered to it and the other will be white. Match these to the wire coming out of your cable. Strip the ends of the wire back one inch from the end and wrap each wire around the screw of corresponding colour. Tighten the terminal screws and replace the back of the bell and screws.
Mount the bell button to the siding using the screws that came with it. Apply a coat of painter's caulk to the back of the button to adhere it better and seal the wiring. Drive the mounting screws with a drill.
Remove the wiring cover of the transformer and locate the wiring terminal screws. Consult the installation instructions that came with your bell for more specifics. Attach the black wires from both bells to the black, or brass, terminal screw at the bell/transformer. Strip the wires back one inch from the end and twist them together. Wrap the twisted wire around the terminal screw and tighten with a screwdriver. Repeat with the white wires on the white, or nickel, terminal screw.
Replace the cover over the wiring terminals and plug the transformer into the outlet. Test the bell and make any adjustments required.