People change switches or duplex (two-socket) receptacles, called devices, for a variety of reasons... to replace a faulty or worn out device, to change to a new color or style device, to upgrade from older two-slot receptacle to grounded/polarized three-slot ones, or upgrade a standard switch or receptacle with, for example, a dimmer, GFCI receptacle, timer or programmable switch. Typically, the only tools required are an electrical tester, a slotted screwdriver, a wire stripper and a pair of long nose pliers. You'll also need the new device, and perhaps some solderless wire connectors (Wirenuts), wire and electrical tape.
In most cases, replacing a device is a simple and safe do-it-yourself task. But you can unwittingly create a serious safety hazard if you fail to recognize situations that require special attention. Before attempting any electrical work, you should know how your electrical system works (including how to identify hot, neutral and ground wires, how to shut power off and test a circuit for power and check wires for continuity) and basic wiring techniques. And pay particular attention to applicable cautions mentioned below.
Most complications arise in older homes. Original wiring (or subsequent upgrades) may not have been properly done or inspected. Wire insulation can become brittle with age. If you discover that the insulating cover on any wiring within a box is brittle, it might also be so within walls, presenting a serious fire hazard. Call a licensed electrician to rewire as needed. Some very old houses may not even have a grounded electrical system (see How Your Electrical System Works and look for a neutral/ground busbar in the service panel); and replacing a two-slot receptacle in such a house with a modern three-prong one would deceive users to believe a plugged-in device is grounded when, in fact, it would not be.
From the mid-1960s to the mid-1970s aluminum wire was installed in about 2 million homes in the United States because it was less expensive than copper. When it is connected to devices (receptacles, switches) designed for copper wiring, joints can loosen and dangerous arcing between wires may occur. If you have aluminum wiring without CO/ALR-compatible devices, they should be replaced with compatible ones; and where necessary, wiring should be upgraded with COPALUM connectors.
Anyone with aluminum wiring would be wise to have their system inspected by a professional to determine what if anything needs to be done and to then discuss the pros and cons of replacing/upgrading any devices yourself. And have all work inspected.
Enough Box Space?
If you are replacing a standard switch with a bulky device, such as a dimmer, a GFCI (ground-fault-circuit interrupter) or a timer, box space can become an issue. And it's not just a matter of presenting an awkward task getting everything to fit, it becomes a safety issue. When wires are crammed in a too small box insulation can be damaged, increasing the risk of a short or ground fault. In fact, the National Electrical Code (NEC) spells out minimums. It requires a box to provide 2 cu. in. for every #14 wire (and 2.25 cu. in. for every #12 wire) entering a box; and counts each device (switch or receptacle) as 2 wires and any internal cable clamp as one wire. But multiple ground wires and an unspliced cable passing through the box count only as one.
If an existing box is too small, it must be replaced. Depending on how the existing box was installed and what the wall surface is, removing it without damaging the wall can be relatively easy or quite difficult.
TURN OFF POWER TO THE OUTLET AT THE SERVICE PANEL (trip a breaker or remove a fuse). Remove the outlet cover plate and test the device. For a receptacle, probe the terminals on either side with an electrical tester. For a switch, probe either terminal and the ground wire or a grounded metal box; repeat for all other switch terminals. If the tester does not light, proceed.
Unscrew the device from the box and gently pull it out to access wiring connections. Avoid the temptation to reuse existing looped or stripped ends of wires; instead simply cut off the device with wire strippers.
If numerous wires are involved, code the wires as you remove them or make a diagram, so you know how they must be reconnected to the new device. Devices are rated for particular uses. Make sure your replacement has the same voltage, amp and wire-size rating, and that it has a tester's seal of approval, such as "UL."
Always switch the hot lead (colored black or sometimes red), never the ungrounded neutral (white). Verify that a switch is properly wired by testing for continuity between the terminals and ground; there should not be any is wires attached are truly the "hot" conductors. For single-pole switches, make sure that the toggle points down when the switch is "off". (With three and four-way switches there is no set on or off position.)
Unless a receptacle is the only one on a circuit or at the end of the line, there will be incoming and outgoing lines. You may find each set of wires attached directly to the receptacle terminals. Today most codes call for continuous circuit so that if one receptacle in a run were to fail (or be removed), it would not affect outlets on the circuit. This is achieved by attaching a pigtail to the receptacle and in turn to each of the circuit wires. Take the time to correct such wiring if located: connect like colored wires to pigtails and those to the proper terminals on the device. Some duplex (two-socket) receptacles may have one or both sides switched. If one side only is switched, you see that the link normally present between the two has been removed and that a separate set of wires attached to each socket's terminals.
Some duplex receptacles are fed by two separate circuits. Again, the link is removed and a pair of wires go to each socket. A GFCI receptacle can be wired for single outlet protection or so that downstream outlets are also protected. Detailed wiring instructions are contained with switch.
If a box contains two ground wires, attach a length of green ground wire to both wires under a single solderless wire connector (Wirenut), and to the ground screw on the device. Do this even in a grounded metal box. If the box is metal and does not have the bare (or green) ground wire connected to it, add another pigtail and connect it to the grounding screw on the box.
Strip only as much insulation off wire to make a clockwise loop that fits under the terminal screw so no uninsulated wire is exposed. Wire strippers do the job best without nicking the wire. (Nicks may cause the wire to break and the connection to fail.) Install the loop clockwise under the terminal screw so that tightening the screw will tend to close, not open, the loop. Avoid using plug-in connectors on the rear of some devices. They are not legal everywhere and are generally not as reliable as splices made at screw terminals.
After you make any pigtail connections to ground, hot and neutral conductors, press the bare ground wire into the box first, then the hot and neutral wires, each splice in separate corners of the box. Then connect the device to the pigtails; and carefully press it into the box.
Before returning power, use a continuity tester to verify there are no shorts or ground faults. Probe between a neutral terminal (or splice) and ground; there should be continuity. Then probe between any hot terminal and ground; there should not be any continuity.
Finally, replace the cover plate, and turn the power back on at the service panel and test the outlet with a circuit / voltage tester.