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Electrical Box Basics All Homeowners Should Know

With the variety of different electrical boxes available at home centers, howdo you know what to buy? Dont worry, its not that complicated. Well whittle it down to about a dozen boxes to cover almost every situation.

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New work: 3 types do it all

If you have the walls opened up for remodeling or putting on an addition, these three boxes cover about 99 percent of your needs.

  1. 22-CU.-IN. 4-IN. ROUND BOX for ceiling light fixtures, smoke alarms, carbon monoxide detectors and wall sconce light fixtures.
  2. 22-CU.-IN. SINGLE-GANG BOX for a typical outlet or switch.
  3. 32-CU.-IN. DOUBLE-GANG BOX for two light switches together in the same box, or two duplex receptacle outlets.

The purpose of electrical boxes

Wiring connectionswhere wires join an outlet, switch or other wiresmust be inside an electrical box. Heres why: Connections are the weak link in an electrical system. If they get damaged, loosened or pulled apart, youre left without power, or worse, with a fire. Electrical boxes are simply meant to protect vulnerable connections.

Next, read: The 8 Most Common National Electric Code Violations DIYers Make.

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Box material

Indoor nonmetallic boxes are typically plastic or fiberglass. Cheap PVC boxes like the one shown at far right work fine, but they can move or distort in wood framing as the studs dry. I prefer to spend the extra 20 per box on heavy-duty thermoset plastic or fiberglass boxes. Unlike PVC boxes, theyre super strong and maintain their shape.

Box Size

Wires, receptacles and switches need adequate space. Crowded boxes can damage wires, resulting in a fire or shock hazard. You can use the chart below to calculate the required box size. Add up the numbers for the correspond- ing components in the box to find how many cubic inches youll need. In most cases, I skip the math and just buy the largest volume box available in the style I need. Ive never been frustrated by having a box that was too large.

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Ceiling fan boxes

If youre hanging a ceiling fan, youll need a box designedto support the extra weight. These boxes forceiling paddle fans are sold as kits, with a wide variety of mounting options for new work and oldwork (defined below). Boxes that are thesole support of the fan haveto be rated for up to 70 lbs. If the fan is sup- ported independently ofthe box, you can use a general-purpose box.

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Adjustable boxes

If you know youll be installing ceramic tile or wood paneling or wainscoting, buy adjustable boxes. Theyre mounted to the framing members like any other box;

you just turn a screw to adjust the depth flush with the wall treatment. The adjustment screw is accessible even after the wall treatment is applied.

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Exterior surface-mount boxes

Exterior surface-mount boxesoften molded PVChave either threaded or glued hubs andare used with PVC electrical conduit. I like cast-aluminum boxes for outdoor projects.Theyre extra durable and weatherproof. They often come as a kit, including aground-fault circuit interrupter (GFCI) receptacle and a weatherproof cover, or as a lighting kit with gaskets and lamp holders for floodlights.

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Surface-mount boxes

On concrete or block walls, surface-mount boxes and conduit are the way to go. These boxes can be plastic or metal. Because theyre exposed, they need to be mounted perfectlyplumb and level. They also require conduit to protect the wires.

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Pan boxes

Four-inch round pan boxes (short for pancake) comein handy if a ceiling joist is right where you needto install your dining room light fixture.Cutting a notch in the ceiling joist wouldweaken it. Pan boxes are only 1/2 in. deep and only have a volume of 6 cu. in., but they will safelyaccommodate the three wires you needfor your light fixture.Plus: Check out the Top 10 Tools for DIY Electrical Work

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Gasket boxes

Energy code boxes are for insulated walls and ceilings. They typically have a molded flange with a foam gasket as well as foam- lined cable entries. This helps form an airtight sealto keep out drafts. I like to caulk the box flange right to the vapor barrier.

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Metal boxes

You can wire a wholehome using only non-metallic boxes, butmetal boxes still comein handy for certainsituations. Theyreextremely sturdy andwork well where youneed lots of volume inthe boxfor example,for a welder receptacle in your workshop or fora hub where multiplecables meet. As withnonmetallic boxes, getthe large ones, which are 4 in. square by2-1/8 in. deep. Youllfind cover options for mostconfigurations of switches and receptacles, as well as mud rings.

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Old work boxes

Old work (or remodeling) boxes are for walls that are covered with drywall or other material. They dont need to be fastened to a stud, so you can install them anywhere. There are several types. The one shown has flip-out tabs that squeeze the box flanges against the drywall. Using the box face as a marking template, youll get a nice, close fit.

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Low-voltage brackets

Unlike in the old days when homes had one or two phone jacks, today we havemuch more low-voltage wiring for computers, cable TV, home entertainment systems, and whole-houseaudio, security and temperature control systems. Save time and money by installing low-voltage brackets instead of electricalboxes. Low-voltage wiring doesn’t necessarily need an enclosed box. In fact, you often dont want an enclosed box because it may require the wires to make a sharpbend, which impedes the performance of some cables.

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Brad Holden
Brad Holden, an associate editor at The Family Handyman, has been building cabinets and furniture for 30 years. In that time, he has absorbed so many slivers and ingested so much sawdust that he's practically made of wood.