An outlet box is the termination point of wires or connections for electrical wiring or fitting. One basic principle that should already be observed is that wire connections should always be contained inside an outlet or electrical box. The same is true for switches and receptacles. Doing so prevents accidents resulting from damage to the wires or connections, and it also helps contain the danger if there should be any sparks and unintended heat resulting from a loose connection or loose circuit. Not using an enclosure is extremely dangerous, and it is also illegal and a violation of code standards.
In this article, we take a look at some of the code standards for the installation requirements of outlet boxes.
- Use appropriate box size
The NEC provides tables of electrical box sizes in dimensions and cubic inches. The actual size that you will be using is dependent on the number of wires that will be enclosed in the box.
The importance of using the appropriate size in the electric box you will use is to prevent unwanted electric hazards and accidents. If you are using a box that is too small, the wires will be crowded and bent within the box, which could lead to damaged wiring and loose connections, both of which could cause an electric arc. Keeping your electrical equipment and tools well maintained is as important as doing baseline testing.
Refer to this NEC guide to electrical box sizes concerning the number of wires used, which is located in Section 314. The same provision also provides a guide to the measurements of the appropriate free space within the box, concerning the conductor or wire size that is used.
- Use a box extension if installing an electrical box to a combustible wall surface
If a box is recessed behind a combustible material like wood, for instance, this installation can potentially become a fire hazard if the wood is exposed to possible heat and sparks. A metal or plastic box extension can be installed to address this issue. Otherwise, the electrical box must be flush to the wall surface, not recessed.
Always remember that if you are using a metal box extension on a plastic box, the metal extension should be grounded, or connected to the ground wire using a grounding clip and a short wire.
- Secure cables with a clamp or clip
Leaving wires or cables loose, even inside a box, is potentially dangerous. The sharp edges of a metal box can damage the wires, and even loose wire inside a plastic box can become a potential fire hazard if not secured correctly.
Thankfully, most boxes nowadays are also equipped with built-in cable clamps. Or you can also purchase approved cable clamps. For single plastic boxes, you can staple the wires within 8 inches of the box.
- Boxes must always be covered
The Code requires outlet boxes to be covered. Outlet boxes for mounting light fixtures should be octagon or round; boxes for switches or receptacles are to covered with raised 4-inch square covers; flat, round closure plates approved for dry locations are to be used for unused ceiling boxes; and for switches and receptacles, use a cover with cutouts.
- For ceiling fans, use code-approved boxes marked with “for use with ceiling ”
Because of the dynamic nature of ceiling fans, outlet boxes used for these types of installations must be code-approved. They come with weight ratings to accommodate the weight and expected pull of the fan, and may be mounted either directly to a ceiling joist, with wood blocking, or to adjustable brace that spans the distance between two ceiling joists. These types of boxes can range from 1/2 inch to 2 1/8 inches deep.
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