You can increase the safety of your old two-prong outlets by installing a new ground fault circuit interrupter receptacle. But just because the GFCI has a hole for a third prong, don’t assume you can plug in three-prong plugs. The National Electrical Code (NEC) states that any appliance equipped with a three-prong plug is required to be grounded, and the installation shown does not provide the necessary equipment ground. Also, some computer equipment won’t work properly if it’s not grounded. The outlet will still be ungrounded, but the GFCI will “trip,” cutting off the current and protecting you from electrocution.
Before you start, make sure the outlet box is large enough to safely hold all of the wires and the new receptacle. Here’s the formula to figure the minimum box size required by the NEC: Add 1 for each hot and neutral wire entering the box, 1 for all of the wire clamps, and 2 for each receptacle. (You’d also add 1 if you had any ground wires.) Multiply this figure by 2 for 14-gauge wire and 2.25 for 12-gauge wire to get the minimum box volume in cubic inches. Our standard metal outlet box is defined by the NEC as having 14 cu. in. Consult the code or call your electrical inspector for the volume of your metal box. Plastic boxes have the volume stamped in them.
Turn off the power to the receptacle at the main electrical panel. Then carefully unscrew the outlet and pull it out of the box. Use a simple neon circuit tester to double-check that the power is off. Place the tester leads across both sets of hot and neutral terminal screws. If the tester lights up, shut off other circuits at the main panel until you find the right one.
Before doing any electrical wiring be sure the power is off to the box or wires you'll be working with.
Match the gauge of these wires to the amperage of the circuit: 14-gauge for 15-amp circuits and 12-gauge for 20-amp circuits. The correct amperage will be marked on the fuse or circuit breaker whose circuit you’re connecting to. Splice the new white wire to the existing neutral white wires, and the black to the existing hot wires. Make sure the wire connectors you’re using can safely connect the three 14- or 12-gauge wires. This information is printed on the package.
Connect the new wires to the GFCI terminals marked “Line.” Connect the black wire to the brass screw and the white wire to the side that’s labeled “white,” “W” or has a silver screw. Do not connect wires to the “Load” terminals. If there’s no equipment ground wire available (copper that’s bare or is covered with green insulation), don’t connect anything to the GFCI ground terminal(green screw).
GFCI receptacles have two sets of hot and neutral terminals, labeled “Line” and “Load.” The “Line” terminals are for incoming power. Connect the hot and neutral wires from the main panel to these “Line” terminals.
The “Load” terminals are for protecting additional receptacles with the GFCI. Don’t use them unless you know where the wiring goes and whether or not you want to protect those receptacles. The “Load” terminals on our GFCI are covered with yellow tape to prevent someone from inadvertently connecting the power leads to them.
Aluminum wiring requires special handling. If you have aluminum wiring, call in a licensed pro who's certified to work with it. This wiring is dull gray, not the dull orange that’s characteristic of copper.