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Recessing Boxes Behind the Wall Surface
Mistake: Exposed combustible material | Electrical boxes must be flush to the wall surface if the wall surface is a combustible material. Boxes recessed behind combustible materials like wood present a fire hazard because the wood is left exposed to potential heat and sparks.
Solution: Add a box extension | The fix is simply to install a metal or plastic box extension. If you use a metal box extension on a plastic box, connect the metal extension to the ground wire in the box using a grounding clip and a short piece of wire.
7 / 10
Installing Cable Without a Clamp
Mistake: Missing clamp | Cable that’s not secured can strain the connections. In metal boxes, the sharp edges can cut the insulation on the wires. Single plastic boxes do not require internal cable clamps, but the cable must be stapled within 8 in. of the box. Larger plastic boxes are required to have built-in cable clamps and the cable must be stapled within 12 in. of the box. Cables must be connected to metal boxes with an approved cable clamp.
Solution: Install a clamp | Make sure the sheathing on the cable is trapped under the clamp, and that about 1/4 in. of sheathing is visible inside the box. Some metal boxes have built-in cable clamps. If the box you’re using doesn’t include clamps, buy clamps separately and install them when you add the cable to the box.
8 / 10
Overfilling Electrical Boxes
Mistake: Box too small | Too many wires stuffed into a box can cause dangerous overheating, short-circuiting and fire. The National Electrical Code specifies minimum box sizes to reduce this risk.
Solution: Install a larger box | To figure the minimum box size required, add up the items in the box:
- 1 – for each hot wire and neutral wire entering the box
- 1 – for all the ground wires combined
- 1 – for all the cable clamps combined
- 2 – for each device (switch or outlet?but not light fixtures)
Multiply the total by 2.00 for 14-gauge wire and by 2.25 for 12-gauge wire to get the minimum box size required in cubic inches. Then choose a box with at least this much volume. Plastic boxes have the volume stamped inside, usually on the back. Steel box capacities are listed in the electrical code. Steel boxes won’t be labeled, so you’ll have to measure the height, width and depth of the interior. Then multiply to find the volume.
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