Locating live wires behind walls is easy with the sensitive, noncontact voltage detectors that are available at home-improvement and hardware stores. They're relatively inexpensive and they can save you hours of work and hundreds of dollars in material. These probes will detect hot wires behind 2-inch finished walls. Professional electricians carry them to double check a circuit that their expensive digital meters tell them are safe to work on.
Locate your circuit breaker panel or fuse box and check to see what type of wiring system is being used. This will determine which instrument you use to trace the hidden wires. A noncontact voltage probe won't work with armoured cable, which you may well encounter in residential wiring. The probe will indicate the presence of voltage at the receptacle or switch but will lose the signal when you move away from the cover. A two-piece circuit breaker tracer will work with all wiring systems. If you can only afford one or the other, go with the circuit breaker tracer.
Adjust the sensitivity of the noncontact voltage probe by using a receptacle or light switch that you know is live. Place the probe close to the cover plate, turn on the probe and rotate the sensitivity control until it starts to beep and the light emitting diode begins to flash. The probe is now indicating the presence of live voltage.
Adjust the sensitivity of the circuit breaker tracer. Plug the sender unit into a live receptacle on the circuit to be traced. Turn the receiver on, place it next to the sender unit, and adjust the sensitivity control until it beeps.
Move the noncontact voltage probe or the circuit breaker tracer's receiver 12 inches from the device, place it against the wall surface, and readjust the sensitivity control until it starts blinking and beeping again. The probe or circuit breaker tracer receiver is now indicating the live wires inside the wall and you're ready to trace it path to the next device or back to its source, the circuit breaker, or fuse panel.