Turn off the power at the main panel when you're doing electrical work.
If you have two-slot outlets, it's tempting to replace them with three-slot outlets so you can plug in three-prong plugs. But don't do this unless you're sure there's a ground available. Use a tester to see if your outlet is grounded. A series of lights indicates whether the outlet is wired correctly or what fault exists. These testers are readily available at home centers and hardware stores.
If you discover a three-slot outlet in an ungrounded box, the easiest fix is to simply replace it with a two-slot outlet as shown.
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.
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.
Connecting the black hot wire to the neutral terminal of an outlet creates the potential for a lethal shock. The trouble is that you may not realize the mistake until someone gets shocked, because lights and most other plug-in devices will still work; they just won't work safely.
Always connect the white wire to the neutral terminal of outlets and light fixtures. The neutral terminal is always marked. It's usually identified by a silver or light-colored screw. Connect the hot wire to the other terminal. If there's a green or bare copper wire, that's the ground. Connect the ground to the green grounding screw or to a ground wire or grounded box.
GFCI (ground fault circuit interrupter) outlets protect you from a lethal shock by shutting off the power when they sense slight differences in current. They have two pairs of terminals. One pair, labeled “line,” is for incoming power for the GFCI outlet itself. The other set is labeled “load” and provides protection for downstream outlets. You'll lose the shock protection if you mix up the line and load connections.