How do you identify existing wiring in an older building?

If you live in a multi-family dwelling, and you can't figure out this simple of a circuit, hire a licensed electrician. The fire you prevent may save your own life and others living in your building. cheeky anyways buy a circuit tracer fairly inexpensive. it plugs into wall plug and you use a frequency detector at the panel to determine correct breaker very easy Answer Turn off all your breakers. Then turn on all the lights. Turn on all the breakers, but only one at a time, and see which breaker is connected to what. Takes about an hour, usually. Make sure that you don't have two separate breakers that supply the same load; this can create a dangerous situation. The exception is double pole breakers, of course. For the double pole ones, usually the 20A is the air conditioner, the 30A is the water heater or dryer, and the 50A is the stove.