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Increase the safety of your old two-prong outlets by installing a new ground fault circuit interrupter receptacle. The GFCI will protect against a deadly shock even if it's not connected to a ground. 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 cubic inches. Consult the code or call your electrical inspector for the volume of your metal box. Plastic boxes have the volume stamped in them.
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.
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