Electrical boxes are critical components of your home's electrical system. But for many DIYers, the wide variety of boxes is bewildering. There are plastic and metal boxes; "new work" and "old work" boxes; round, square, and octagonal boxes; and boxes with load ratings for ceiling fans and heavy light fixtures.
All of the most commonly used boxes for home wiring projects are available at home centers and large hardware stores, and it's important to know the differences in order to buy the correct boxes. There are a number of options to consider: materials, shape, and size.
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Metal and Plastic Electrical Boxes
Most electrical boxes are either metal or plastic. Metal boxes are generally made of steel, while plastic boxes are either PVC or fiberglass. Weatherpoof metal boxes for outdoor use are generally made of aluminum.
If you are using metal conduit to run wiring to the electrical box, then a metal box is required—both to anchor the conduit and because the conduit and metal box system itself may be used to ground the system. If you are using non-metallic cable, such as Type NM-B (non-metallic sheathed cable), then you can use either plastic boxes or metal boxes, as long as the cable is secured to the box with an appropriate cable clamp.
Modern wiring systems with NM-B cable usually include a ground wire inside the cable, so the box is not part of the grounding system (however, metal boxes must be connected to the system ground, usually with a short length of wire called a pigtail).
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Standard Rectangular Boxes
Single light fixture switches and outlet receptacles typically fit into standard rectangular boxes, also known as "single-gang" or "one-gang" boxes. They are generally 2 x 3 inches in size, with depths ranging from 1 1/2 inches to 3 1/2 inches. Some forms are "gangable"—with detachable sides that can be removed so the boxes can be linked together to form larger boxes for holding two, three, or more devices side-by-side.
Standard rectangular boxes come in various types of "new work" and "old work" designs. Some types have built-in cable clamps for securing NM cables.
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2-Gang, 3-Gang, and 4-Gang Boxes
Like standard rectangular boxes, these are used to hold household switches and electrical outlets, but they are oversized so that two, three, or four devices can be mounted side-by-side. Like other boxes, these come in a variety of "new work" and "old work" designs, some with built-in cable clamps.
The same effect can be created by using standard rectangular boxes with a "gangable" design that allows the sides to be removed so the boxes can be joined together to form larger boxes.
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Round Pan Electrical Boxes
Round pan, or "pancake," boxes typically are only 1/2 inch or 3/4 inch deep. They are used most commonly for ceiling- or wall-mounted light fixtures that weigh no more than 50 pounds. Some types of specially rated metal pan boxes may be used for mounting ceiling fans, but not all pan boxes may be used for this purpose.
It's important that the fixture wires can be properly connected and fitted within the round pan box. These are shallow electrical boxes with only enough space for fitting two or three electrical connections. If there are more wire connections to be made, you will probably need a full-sized octagon or round electrical box with more volume. The number of wires in the box should never exceed its safe fill capacity.Continue to 5 of 11 below.
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Octagon and Round Electrical Boxes
Octagon and standard-size round boxes range from 1 1/2 to 3 inches deep and are the standard box for ceiling- or wall-mounted light fixtures weighing up to 50 pounds. They provide much more room for wiring than shallow round pan boxes and can be used as junction boxes.
Metal boxes are suitable for surface-mounted installations using metal conduit. Round plastic boxes often have "ears" for fastening to the wall or ceiling surface in existing or "old work" applications. These allow you to secure the box to the drywall (or other surface material) rather than cutting a large hole in the drywall to fasten the box to the framing.
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Ceiling Fan-Rated Electrical Box
Ceiling fan boxes come in several different types and sizes, including 1/2-inch-deep "pancake" versions and standard 2 1/8-inch-deep boxes. They are usually round but may be octagonal.
Ceiling fan boxes must be UL-listed for ceiling fan mounting and marked “For Use With Ceiling Fans.” Do not use standard round or octagonal boxes for installing ceiling fans. Ceiling fan boxes require special fastening to withstand the dynamic loading of a rotating fan.
Most ceiling fan boxes are rated for fans or light fixtures weighing up to 75 pounds, depending on the installation method. Boxes can be mounted directly (with four screws) to a ceiling joist or wood blocking, or they can attach to adjustable braces spanning between ceiling joists.
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4-Inch Square Box
Square boxes come in standard depths of 1 1/4 to 2 1/8 inches, but their square corners give them additional interior space, providing maximum volume for multiple conductors and connectors. For this reason, 4-inch square boxes often are used to run multiple conductors in two or more directions. They are also commonly used as junction boxes and can also be installed in ceilings or walls for supporting lighting fixtures or for housing switches or receptacles when matched with the proper cover plates.
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A junction box is not a special type of box but rather a term used to describe any standard electrical box used to enclose wire splices. The standard box used for junctions is a 4-inch square box, which offers plenty of space for making wire connections with multiple wires or cables, but other type of boxes can be also used for this application.
Junction boxes must be installed where they are always accessible; never install a junction box in a concealed wall or ceiling space where the box cannot be accessed in the future. Junction boxes also must be covered with solid covers with no holes.Continue to 9 of 11 below.
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Electrical Box Covers
Electrical box covers come in a wide variety of sizes and shapes to match different types of boxes. A cover is used to enclose the front of the box and is required by code; it is unsafe, and usually illegal, to leave an electrical box uncovered.
Solid, or "blank," covers have no holes and typically are used with junction boxes or for enclosing unused boxes. Covers for 4-inch square boxes may have special cutouts to accommodate switches or receptacles (outlets). These typically include a raised center area that extends the box so the opening for the switch or outlet will be flush with the exposed surface of the drywall or other wall or ceiling finish.
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Outdoor Electrical Box
Weatherproof outdoor boxes are sealed enclosures designed for mounting to the surface of exterior walls, roof overhangs, decks, and other structures. They are used for installing outdoor receptacles (outlets) and light fixtures. Outdoor boxes must have an outdoor cover or fixture rated for damp or wet locations, depending on the application.
Plastic outdoor boxes are usually made from high-impact PVC, while metal outdoor boxes are typically made from aluminum rather than steel.
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Box extenders, or extension rings, come in a variety of sizes and shapes to match standard electrical boxes. They are shaped like boxes but have no back. They are designed to be installed onto the front of standard electrical boxes to increase the box capacity or to bring the box flush with the drywall or other surface material.