Electricity: Electrical Boxes and Sizing

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Wherever electrical wires are spliced together or connected to the terminals of switches, receptacles or fixtures, they must be enclosed in an electrical box. Made of steel, plastic or fiberglass and available in many sizes and styles, boxes isolate energized connections from flammable building materials and safeguard people, as well. Switches and receptacles are generally housed in rectangular or square boxes, lighting fixtures in octagonal or round boxes. Boxes must be securely supported by the building structure, and unused openings must be effectively closed. They must be installed so the wiring in them can be rendered accessible without removing any of the building finish. In completed installations, all boxes must have a blank cover, a fixture or a device and cover plate.

Are you looking for home electrical items -- such as switches and fuse boxes -- or parts and accessories for ones you already have? Try our dedicated electrical supply pages here:

Basic Boxes

New-work nonmetallic boxes are used with type NM cable only and come in a variety of sizes. They require access to the wall cavity to be nailed in place.

New-work metal boxes can be used with all types of wiring. The mud ring gives the outward appearance of one outlet, while providing more volume for wires, connectors and other fittings in the larger box behind it.

Metal and nonmetallic ceiling boxes on bar hangers allow easy positioning between framing members. Check load rating for heavy fixtures.

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Remodeling Boxes

Spring brackets on some old-work, or remodeling, boxes secure box to the dry wall, plaster or other surface.

Wings and ears on some remodeling boxes are used to secure the box to the wall surface.

Ear, Bracket, Drywall or plaster wall surface Adjustable ears and separate brackets on some remodeling boxes lock the box to the wall surface.


Specialty Boxes

Sealing flanges for attaching vapor barriers meet strict energy code requirements in cold-climate regions.

Floor boxes are common in homes. Adjustable-height floor boxes allow receptacle placements where wall space is limited. Adjustable boxes can be tweaked after installation to ensure they are flush with wall materials such as paneling and tile.

Electrical Box Calculator

Boxes must be of sufficient size to provide free space for all enclosed wires. Nonmetallic boxes are marked with their cubic-inch (or cubic-centimeter) capacity. Official volumes for metal boxes are listed in the codes, but you can measure the inside of the metal boxes to approximate the volume.

Metal Remodeling Box

Metal Remodeling Box (image above): Here are simplified code rules to help you decide what box size you need:

  • Count each hot and neutral wire as one wire each.
  • Count all ground wires combined as one wire.
  • Count all cable clamps combined as one wire.
  • Count all devices that mount in the box each as two wires.
  • Multiply the total wire count by either 2.0 cu. in. (14-gauge wire) or 2.25 cu. in. (12- gauge wire) to determine the minimum box volume required.

Standard Plastic Box

To determine the minimum plastic box volume required in this situation, calculate this way:

  • All conductors entering box (hot and neutral): 2
  • Ground wires: 1
  • Cable clamps: 0
  • Switch or receptacle: 2
Since the total is 5 wires and you should allow 2 cu. in. per wire, the minimum box size in this situation is 10 cu. in.

Last modified: Wednesday, 2016-03-23 15:59 PST