Install a GFCI Outlet Under the Sink

G​FCI outlets—ground fault circuit interrupters—are the safety standard under kitchen sinks, in bathrooms, and anywhere water and electricity might meet. They cut power in milliseconds if they sense a ground fault, which prevents electrocution. If your under-sink outlet isn't GFCI-protected yet, you're working in a code violation and a genuine hazard. The installation itself is straightforward: it's a single outlet swap that takes less than an hour once you've killed the breaker. The real work is understanding the wire connections and making them solid. This guide walks you through it step by step, so you know exactly where each wire goes and why.

  1. Kill the Power First. Locate the breaker panel, find the breaker that controls the under-sink outlet, and flip it to OFF. Test the outlet with a non-contact voltage tester or plug in a lamp to confirm power is dead. Never skip this step. A shock under a sink, where you're wet and cramped, is serious.
  2. Expose the Outlet. Unscrew the single screw holding the outlet cover plate. Set the screw aside in a small container so you don't lose it. Slide the cover plate straight out and set it somewhere safe.
  3. Extract the Old Outlet. Remove the two screws on the top and bottom of the outlet face that hold it to the electrical box. Gently pull the outlet straight out toward you. Don't yank—let it come out smoothly so you don't stress the wires still attached.
  4. Free the Three Wires. You'll see three wires: black (hot), white (neutral), and bare copper (ground). Loosen the terminal screws on the sides and back of the outlet with a screwdriver. The wires should pull free easily. If they're stubborn, don't force—loosen the screw more. Note which wire goes where: hot to the brass screw, neutral to the silver screw, ground to the green screw.
  5. Refresh the Wire Ends. Look at the exposed copper on each wire. You need about half an inch of bare wire. If the copper is corroded or if the insulation is damaged partway down, use a wire stripper to remove the outer casing and expose fresh copper. Twist the strands together tightly so no copper frays out.
  6. Secure the Ground Wire. Find the green terminal screw on the GFCI outlet—it's always the ground. Loosen the screw, insert the bare copper wire into the terminal hole, and tighten firmly. The wire should not slip when you tug on it. Ground connections are critical; this is not the place for a loose fit.
  7. Attach the Neutral Wire. Find the silver terminal screw on the GFCI. This is neutral. Loosen it, insert the white wire, and tighten firmly. Double-check: white always goes to silver, black always goes to brass. If you reverse these, the outlet won't work and you risk damage.
  8. Connect the Hot Wire. Find the brass terminal screw on the GFCI—this is hot. Loosen it, insert the black wire, and tighten firmly. This is the most dangerous wire in the box; make sure the connection is solid and the insulation isn't compressed under the terminal. No bare wire should be visible outside the screw terminal.
  9. Seat the GFCI Outlet. Press the outlet straight back into the electrical box. The wires should fold naturally to the sides. Don't force it; if it binds, stop and check that no wire is twisted or kinked behind the outlet body. Once it's seated flush, screw the top and bottom mounting screws back in snugly. Don't overtighten—you'll crack the outlet.
  10. Restore the Cover Plate. Align the cover plate with the outlet opening and insert the screw. Tighten it finger-tight, then add one more quarter turn. The plate should sit flush against the wall with no gaps. Don't over-tighten; you can crack the outlet or bend the plate.
  11. Verify Power and Protection. Go back to the breaker panel and flip the breaker back ON. Return to the outlet and plug in a lamp or phone charger to confirm it has power. If nothing happens, turn the breaker back off, check all your connections, and try again. Once power is confirmed, use a GFCI outlet tester (a small device you can buy for $10–15) to verify the outlet is protecting you.
  12. Confirm the GFCI Works. Most GFCI outlets have TEST and RESET buttons on the face. Press TEST. Power should cut immediately and anything plugged in should stop. Press RESET. Power should return. If the outlet doesn't trip on TEST, turn the breaker off immediately and recheck your connections. A non-functioning GFCI is worse than no GFCI—it gives false security.