When it comes to ground-fault circuit-interrupter, or GFCI, outlets, there is always a question as to how to connect the wires. This is because GFCIs have two different sets of terminals: the LINE terminals and the LOAD terminals. When you connect only to the line terminals, the outlet provides GFCI protection only for itself. When you connect to both the line and the load terminals, the outlet protects itself and any additional outlets farther down on the same electrical circuit.
The line connection is the point where you will connect the incoming feeder wire (also called the line), which is fed from the home’s electrical service panel. The line connection is used for all GFCI outlet installations. The circuit’s hot wire (typically colored black or red) connects to the black or brass-colored screw terminal marked LINE. The white neutral wire connects to the silver-colored screw terminal marked LINE.
The markings for line and load usually are printed on the back of the outlet’s plastic body. The line terminals are easy to see because they are not covered when you remove the outlet from the manufacturer’s packaging.
The load terminals typically are not visible when you pull the GFCI outlet out of its package because they are covered by a strip of tape (usually yellow in color). The load connection is available to feed additional standard (non-GFCI) outlets downstream from the GFCI location to provide protection from the GFCI. In other words, any regular outlet that is fed from the load side of the GFCI is also protected from a ground fault because of the GFCI outlet.
The benefit of this is to reduce cost because you have to purchase only one GFCI when running additional outlets on the same circuit, instead of buying a GFCI for every outlet. Standard outlets are much cheaper than GFCIs. The downside is that when there is a ground-fault condition with any of these added outlets, the GFCI outlet then trips. If the GFCI is located at quite a distance away or in another room or outside, it can be inconvenient to reset the GFCI and restore power to all of the outlets.
Using the load connection requires two cables in the GFCI’s electrical box. One is the line cable that connects to the GFCI’s LINE terminals. The other cable connects to the LOAD terminals to bring power downstream to additional outlets and other devices on the circuit. As with the line connections, the hot wire of the load cable connects to the black or brass terminal on the outlet. The neutral wire connects to the silver terminal.
Note that when using the load connection, the GFCI protects only the other outlets that are downstream of the GFCI; that is, farther away from the service panel with respect to the circuit wiring. Often outlets (particularly in high-usage or high-moisture rooms, such as kitchens) are protected when the first outlet on the circuit is a GFCI, and the rest are protected from that point on. (In other words, additional breakers past that point on the same circuit are unnecessary.) Additional outlets installed between the GFCI and the service panel are not protected by the GFCI.
Many electricians choose to place a GFCI outlet at the first outlet on a circuit in a kitchen or bathroom to protect all the following outlets on the circuit without having to use all GFCI outlets in the room.
The ground screw on a GFCI is always green and is located on one end of the outlet body. This is where the bare or green-colored ground wire connects. If the GFCI’s electrical box is metal (not plastic), you must join two pigtails (short lengths of wire) to the circuit ground wires and connect one pigtail to the outlet ground screw and one to the metal electrical box. If the circuit wiring does not contain a ground wire, the GFCI will not be affected and will operate as designed, but the outlet will not be grounded. GFCI protection is not the same thing as grounding.
Source: The Spruce