Before you open the box
Turn the breaker off and prove it at the receptacle with a tester or a lamp. If this is your first time inside a box, or the box holds more than two cables, or the wiring is aluminum or two-wire without a ground, this is an electrician's job (two-wire homes have their own rules).
Find the feed
With two cables in the box, one comes from the panel and one goes on to other outlets. Separate them, cap them, restore power briefly, and find which cable is live with a non-contact tester. Power off again. That live cable is LINE.
Land the wires
- Feed cable: black to the brass LINE terminal, white to the silver LINE terminal.
- Onward cable (if you want the downstream outlets protected): black to brass LOAD, white to silver LOAD. Remove the factory tape only for this.
- Bare or green grounds: pigtailed together and to the green screw.
- Fold the wires, seat the device, restore power.
Check it
The indicator should show power (green on most makes). Press TEST: the device trips and any outlet wired from LOAD goes dead. Press RESET: everything returns. If the device will not power up, line and load are swapped — the lock-out doing its job.
Label downstream
Every ordinary receptacle protected through LOAD gets the "GFCI Protected" sticker from the box (NEC 406.4(D)(3)); on an ungrounded circuit it also gets "No Equipment Ground".
Boxes, depth and grounds
A GFCI body is deep; a shallow metal box may need a box extender. The device's green screw must be grounded where a ground exists — the GFCI works without one, but the Code and the plug-in tester both want it.