Locating and tracing concealed electrical wiring can be a challenge for even an experienced professional electrician. For the do-it-yourself electrician, nothing is more frustrating than trying to trace concealed electrical wiring through a house. Tracing energised electrical wiring can be made less frustrating by using a test set called a circuit breaker tracer, but there isn't any simple way to trace wiring that can't be energised.
Plug the circuit breaker tracer's transmitter in a live receptacle on the circuit to be traced. The transmitter generates a tone that can be traced back to the circuit breaker panel or fuse box.
Place the receiver next to the transmitter and adjust the sensitivity control until its light emitting diode (LED) starts to blink and the sonic alert starts to beep. Slowly move the receiver along the wall surface; it will continue to beep as long as you are over the current carrying conductors for that circuit. You may have to readjust the sensitivity control from time to time, as the conductors move further from the wall's surface.
Trace an incandescent lighting branch circuit by removing the light bulb and screwing the receptacle adaptor into the lamp socket. Plug the transmitter into the receptacle adaptor and adjust its sensitivity as you did in Step 2 above. Follow the same steps for tracing out the lighting outlet's wiring as you did for the receptacle circuit.
Use the alligator clip test lead adaptor to trace out fluorescent lighting circuits. Remove the lamp tubes and the ballast cover. With the lamp switch in the off position, remove the wire nuts from the black and white ballast leads. Clip the alligator clips to the stripped ends of these wires. Position them so that they don't touch one another or the metal parts of the light fixture. Plug the transmitter into the test adaptor and turn the light switch on. Adjust the receiver's sensitivity and trace out the circuit as you did the incandescent lighting circuit.
Knob and tube wiring is especially difficult to trace; there is no rhyme or reason to the way this wiring system is installed. Knob and tube wiring is run parallel to framing members, perpendicular to framing members and diagonally to framing members. The electrician used whichever method provided the shortest most direct path from source to load.
Many people use a non-contact voltage probe to trace hidden wires. Non-contact voltage probes will work equally well as the circuit breaker test set when tracing knob and tube wiring or when tracing nonmetallic sheathed cables, but they will not work if you have metallic sheathed cable or wiring installed in metal conduit; whereas, the two-piece circuit breaker tracer will work equally well with all wiring methods.