Junction boxes are used to connect one or more electrical branch lines to a particular circuit. Depending on the electrical design, branch wiring from a junction box may extend to a series of lights, outlets or accessory fixtures. This type of installation helps to isolate specific circuits at the breaker panel if necessary.

Typical metal and fiberglass junction boxes are 5 inches square and 1 1/2 inches deep. The boxes have ports for connecting conduit to the sides of the box and a flat cover is held in place with two or more small screws.

Refer to the electrical plan or wiring diagram for a building or the particular floor of a building. Junction boxes are designated with the letter "J" and are clearly defined on the plan. Circle each junction box location if necessary for reference.

Access the area above the ceiling in a home. Follow the path of flexible conduits across the ceiling insulation to a point where two or more conduits appear to intersect. Pull back the insulation at these points to expose junction boxes.

Enter a crawl space or basement door to access the area under the main floor of a building or house. Follow the path of electrical conduits to points where one or more intersect between exposed floor joists to locate junction boxes.

Follow the path of surface-mounted conduits or wire mold on the inside walls of a commercial facility or warehouse. Junction boxes are often located on walls near compressors, pumps or large pieces of equipment. In addition, junction boxes are often used on the roof of commercial buildings to provide access to circuits associated with air conditioning units and ventilation fans.