Your Guide to Electrical Switch and Junction Boxes

  • 01 of 09

    Electrical Boxes in the Home

    electrical boxes
    Home-Cost.com

    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. Here's a look at the most common types of boxes for household applications, to help you get the right parts for your next project. 

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  • 02 of 09

    Metal and Plastic Electrical Boxes

    metal and plastic junction boxes
    © Home-Cost.com 2007

    Most electrical boxes are either metal or plastic. If you are using metal conduit to run wiring to the electrical box, then a metal box is required—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 wiring, such as Type NM-B (nonmetallic 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...MORE 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). 

    Metal boxes come in types known as:

    • Drawn steel (1-piece construction)
    • Interlocked/gangable (typically for switch and outlet boxes); these can be joined together to create a larger box

    Plastic boxes can be made from different types of molded plastic and are almost always 1-piece construction. They can be made from:

    • PVC (most common)
    • Phenolic resin
    • Fiberglass
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  • 03 of 09

    Round Pan Electrical Box

    shallow electrical box
    © Home-Cost.com 2007


    Typical Size: 

    1/2-inch and 3/4-inch deep

    Usage:

    • Metal box is typically used for ceiling- or wall-mounted light fixtures up to 50 pounds
    • Plastic box may used for outlet mounting
    • 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 (see Ceiling Fan-Rated Electrical Boxes, below)
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  • 04 of 09

    Octagon and Round Electrical Box

    octoganal and round electrical junction boxes
    © Home-Cost.com 2007

    Typical Size: 

    1-1/2 inch to 2-1/8 inches deep

    Usage:

    • Typically used for ceiling- or wall-mounted light fixtures up to 50 pounds
    • May be used as a junction box
    • Metal box may also be used as a switch or outlet box in surface-mounted installations using metal conduit
    • Round plastic box often has "ears" for fastening to wall or ceiling in existing or "old work" applications
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  • 05 of 09

    Ceiling Fan-Rated Electrical Box

    ceiling fan boxes
    © Home-Cost.com 2007


    Typical Size: 

    1/2 inch to 2-1/8 inches deep

    Usage:

    • Must be UL-listed for ceiling fan mounting and marked “For Use With Ceiling Fans”
    • May be a round or octagon shape
    • Requires special fastening due to dynamic loading of rotating fan
    • Depending on weight rating and installation, may accommodate installation of ceiling fan and light fixture up to 75 pounds or more
    • May be mounted directly (with four screws) to a ceiling joist or wood blocking or may attach to adjustable brace spanning between ceiling...MORE joists

     

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  • 06 of 09

    4-Inch Square Box

    square junction boxes
    © Home-Cost.com 2007


    Typical Size: 

    1-1/4 inches to 2-1/8 inches deep

    Usage:

    • Square boxes provide maximum volume for multiple conductors and connectors; often used to run multiple conductors in two or more directions
    • Commonly used as junction boxes with solid covers
    • Square boxes may be installed in ceilings or walls for lighting fixtures, switches, or receptacles
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  • 07 of 09

    Electrical Box Covers

    different shapes of electrical box covers
    © Home-Cost.com 2007


    Usage:

    • Covers are used to enclose an outlet box and are required by code; never leave a box uncovered
    • Switches or receptacles may be installed into square or octagon boxes by using covers with cutouts
    • Octagon or round covers are used for mounting light fixtures
    • Raised 4-inch square covers are used for mounting switches or receptacles
    • Flat, round closure plates are designed for use in dry locations to enclose unused ceiling boxes
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  • 08 of 09

    Junction Box

    square junction box
    Junction Boxes are Typically Square Boxes. © Home-Cost.com 2007


    Typical Size: 

    1-1/4 inches to 2-1/8 inches deep

    Usage:

    • A junction box is an electrical box used to enclose wire splices 
    • Junction boxes must be installed where they are always accessible; never install a junction box in a concealed wall or ceiling space where is cannot be accessed in the future
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  • 09 of 09

    Box Extenders

    electrical box extension
    Box Extenders Add Wiring Capacity to a Box. © Home-Cost.com 2007

    Usage:

    • Box extenders or extension rings are like boxes with no back; they are 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