How different types of American standard junction boxes look like?

How does a junction box work?

An electrical junction box is an enclosure housing electrical connections, to protect the connections and provide a safety barrier.

A small metal or plastic junction box may form part of an electrical conduit or thermoplastic-sheathed cable (TPS) wiring system in a building. If designed for surface mounting, it is used mostly in ceilings, under floors or concealed behind an access panel—particularly in domestic or commercial buildings. An appropriate type may be buried in the plaster of a wall or cast into concrete—with only the cover visible.

It sometimes includes built-in terminals for the joining of wires.A similar, usually wall mounted, container used mainly to accommodate switches, sockets and the associated connecting wiring is called a pattress.

Three popular types of American standard junction boxes:

The first one is the square box, the size is 4 inch by 4 inch. There are three deeps, 1-1/2”, 1-7/8” and 2-1/8”, and the thickness can be produced from 0.8mm to 1.6mm.  There are two screws on the box for installing the cover. When we install the cover, it is very easy. We only need to put the cover on it, the hole is aligned with the screws, swivel the cover, and tighten the screws.

The second one is the Octagonal box, the size is also 4 inch, There are also three deeps, 1-1/2”, 1-7/8” and 2-1/8”, and the thickness can be produced from 0.8mm to 1.6mm.There are also two screws on the box and the installation method is the same as the square type.

The third one is a Rectangular box, the size is 2 inch by 4 inch. There are also three deeps, 1-1/2”, 1-7/8” and 2-1/8”, and the thickness can be produced from 0.8mm to 1.6mm. There are also many types of covers to fit the box. And the installation method is the same as the square type.

Junction boxes form an integral part of a circuit protection system where circuit integrity has to be provided, as for emergency lighting or emergency power lines, or the wiring between a nuclear reactor and a control room. In such an installation, the fireproofing around the incoming or outgoing cables must also be extended to cover the junction box to prevent short circuits inside the box during an accidental fire.