Doorbell systems with two doorbell switches use six wires. Two wires exit a transformer and two wires travel to each doorbell switch. The transformer supplies a low-voltage power supply for the home's centralized alarm systems, including the doorbell and sometimes fire-safety systems. With a doorbell, the low-voltage electricity flows to each doorbell switch through thin wires. When a visitor presses the doorbell switch, the low-voltage electricity continues to the doorbell ringer. After activating the doorbell ringer the electricity flows back to the transformer.
Identify the transformer's wires. If the wires do not have identification tags, then twist the three white wires together with a wire nut. Place one lead from a voltmeter into the wire nut with the white wires. Touch the end of each red wire with the other voltmeter lead. Mark the red wire that has voltage with masking tape.
Untwist the wire nut and separate the white wires. Twist the red wires together with the wire nut and determine which white wire should connect to the transformer using the same method used to test the red wire. Mark the transformer's white wire with masking tape.
Turn the doorbell's circuit breaker off. Usually the correct circuit breaker will have a "Lighting" or "Hall" label.
Inspect the doorbell's wire terminals. If the doorbell has two or three wire terminals then leave the wire nut connected to the red wires. If the doorbell has six terminals, then remove the wire nut and separate the red wires.
Screw the transformer's wires onto the doorbell's transformer terminal-screw with a flat-head screwdriver. This terminal will have a "C" label when the doorbell uses two or three wire terminals. If the doorbell uses six wire terminals, then connect the transformer's white wire to the "L1" terminal and the transformer's red wire to the "C" terminal.
Screw the white doorbell-switch wires to the doorbell's switch terminals with a flat-head screwdriver. If the doorbell uses two wire terminals then connect both remaining white wires to the remaining terminal. If the doorbell uses three terminals then connect one white wire to each remaining terminal. If the doorbell uses six terminals then connect one white wire to the "F" terminal and one white wire to the "B" terminal.
Secure the red doorbell-switch wires. If the doorbell has six terminals, then screw one red wire to the "RC" terminal and one red wire to the "BC" terminal. For doorbells with two or three terminals, keep the wire nut in place.
Turn the doorbell's circuit breaker on. Test the doorbell switch. If the doorbell does not ring while pressing a switch, then switch the red wires on the "RC" and "BC" terminals.