A Ground Fault Circuit Interrupter (GFCI) outlet protects against lethal electrical shocks by monitoring the current balance between the hot and neutral conductors. If it detects a leakage as small as 4 to 6 milliamps to ground, it trips the circuit in under 25 milliseconds. But a GFCI can only do its job if it is wired correctly.

The direct answer: A GFCI outlet is wired by connecting the incoming power source to the terminals marked LINE. The black (hot) wire lands on the brass LINE screw, the white (neutral) wire lands on the silver LINE screw, and the bare copper or green (ground) wire lands on the green grounding screw. Downstream outlets that require GFCI protection are wired to the LOAD terminals using the same color-to-screw mapping.

This guide walks through the exact decision path for selecting your device, the mandatory safety protocols, and the precise termination sequence to ensure a code-compliant, arc-free installation.

Device Selection: 15A vs. 20A GFCI Decision Path

Before you touch a screwdriver, you must match the GFCI receptacle rating to your circuit breaker and wire gauge. Installing a 20A GFCI on a 15A breaker is permissible but unnecessary; installing a 15A GFCI on a 20A breaker with 12 AWG wire is a National Electrical Code (NEC) violation if the GFCI is the only receptacle on the circuit.

Circuit Breaker Wire Gauge (Copper) Required GFCI Rating Concrete Part Pick
15 Amp 14 AWG 15A Tamper-Resistant Leviton GFNT2-W (~$16)
20 Amp 12 AWG 20A Tamper-Resistant Leviton GFNL2-W (~$22)
20 Amp 14 AWG CODE VIOLATION Swap breaker to 15A, then use 15A GFCI

Concrete Default Pick: If you are replacing a standard receptacle in a bathroom, kitchen, or garage on a standard 15A residential circuit, buy the Leviton GFNT2-W (15A, 125V, Tamper-Resistant). It fits standard decorator wall plates, accepts 14 AWG solid wire easily, and meets NEC 406.12 tamper-resistant requirements for all dwelling unit 15A and 20A receptacles.

Tools and Materials Checklist

Do not start this job without the proper testing and termination tools. Hand-tightening terminal screws without a torque driver is a leading cause of residential electrical fires due to thermal cycling and arcing.

  • GFCI Receptacle: Leviton GFNT2-W (15A) or GFNL2-W (20A), Tamper-Resistant.
  • Wire Strippers: Klein Tools 11055 (for 14-12 AWG solid copper).
  • Screwdrivers: #2 Phillips and 1/4-inch Flathead (insulated handles preferred).
  • Torque Screwdriver: Calibrated to 14 in-lbs (Leviton's spec for terminal screws).
  • Voltage Tester: Non-Contact Voltage Tester (NCVT) like the Klein NCVT-2.
  • Digital Multimeter (DMM): For verifying exact RMS voltage and neutral-ground bonding.
  • GFCI Receptacle Tester: 3-prong plug-in tester with a physical trip button.
  • Wire Connectors: Ideal wire-nuts or Wago 221 lever nuts (for pigtailing grounds/neutrals).

Mains Safety Protocol: De-Energize and Verify

DANGER: LETHAL VOLTAGE. Working on 120V AC mains can cause ventricular fibrillation. NEC-style guidance requires de-energizing the circuit before exposure. Your local Authority Having Jurisdiction (AHJ) has final authority on permitting and licensed work requirements.

  1. Turn off the breaker: Locate the correct 15A or 20A breaker in your main panel and switch it to OFF.
  2. Lockout/Tagout (LOTO): If you share a panel, place a physical lock or zip-tie and a tag on the breaker so no one accidentally flips it while you are working. (See OSHA's LOTO guidelines for formal procedures).
  3. Verify Dead with NCVT: Insert your Non-Contact Voltage Tester into the top and bottom slots of the existing outlet. It must remain completely dark/silent.
  4. Verify Dead with DMM: Set your multimeter to AC Voltage (V~). Measure Hot-to-Neutral, Hot-to-Ground, and Neutral-to-Ground. All three readings must be 0.0V.

Step-by-Step GFCI Wiring Procedure

New GFCI receptacles ship with a strip of yellow tape covering the LOAD terminals. Do not remove this tape unless you are actively wiring a downstream outlet that requires GFCI protection. For a single-point replacement, all connections land on the LINE side.

  1. Identify the LINE Cable: If the box has only one cable (one black, one white, one bare), that is your LINE. If the box has two cables, you must identify which is the incoming power. (Temporarily cap all wires, turn the breaker back on, and carefully test with an NCVT to find the hot black wire. Turn the breaker back off and re-verify dead before proceeding).
  2. Prep the Wires: Strip exactly 3/4 inch of insulation from the 14 AWG or 12 AWG solid copper wires using your wire strippers. Do not nick the copper conductor; a nick creates a weak point that will snap when folded into the box.
  3. Terminate the Ground: Form a clockwise hook in the bare copper (or green) ground wire. Hook it around the green grounding screw at the bottom of the GFCI. Tighten to 14 in-lbs. If there are downstream ground wires, pigtail them together with the GFCI's ground using a Wago 221 lever nut.
  4. Terminate the Neutral: Form a clockwise hook in the white (neutral) wire. Hook it around the silver screw marked LINE (usually on the left side of the device). Tighten to 14 in-lbs. Ensure no bare wire is exposed outside the screw head.
  5. Terminate the Hot: Form a clockwise hook in the black (hot) wire. Hook it around the brass screw marked LINE (usually on the right side of the device). Tighten to 14 in-lbs.
  6. Cap Unused LOAD Wires: If you have a second cable in the box that feeds standard downstream outlets (like a bedroom lamp) that do not require GFCI protection, cap those black and white wires individually with wire nuts. Do not connect them to the GFCI.
  7. Fold and Secure: Carefully fold the wires into the back of the electrical box. Push the GFCI flush against the drywall or plaster ring. Drive the two 6-32 mounting screws into the top and bottom yoke ears. Use the line-leveling slots on the yoke to ensure the device sits perfectly plumb before tightening.

Testing and Verification Sequence

Never assume a wired outlet is correct just because the breaker didn't trip when you turned it back on. You must verify the wiring topology and the mechanical trip mechanism.

Post-Installation Meter Readings:
Turn the breaker back ON. Set your Digital Multimeter to AC Voltage (V~) and take the following measurements at the GFCI face or a downstream outlet on the LOAD side:

  • Hot to Neutral: 114V - 126V (Nominal 120V)
  • Hot to Ground: 114V - 126V
  • Neutral to Ground: 0.0V - 2.0V (Anything higher indicates a loose neutral or shared neutral fault).

The GFCI Trip Test:
Plug a 3-prong GFCI receptacle tester into the outlet. The tester should display two green lights (or one yellow/one green, depending on the manufacturer's legend indicating 'Correct'). Press the black 'TEST' button on the tester. You must hear a distinct, sharp click from the GFCI receptacle, the power to the outlet must drop to zero (tester lights go out), and the red flag on the GFCI face should pop out. Press the 'RESET' button on the GFCI face until it clicks and locks back into place. The tester lights should return to normal.

The Most Common GFCI Wiring Botch (And How to Avoid It)

The single most frequent mistake DIYers make when asking 'how is a GFCI outlet wired' is reversing the LINE and LOAD connections.

The Botch: The installer connects the incoming power from the breaker panel to the LOAD terminals, and the downstream outlets to the LINE terminals. Sometimes, they remove the yellow tape and wire incoming power to LINE, but also wire incoming power to LOAD, creating a direct parallel short or a backfeed.

The Symptom: If incoming power is wired exclusively to LOAD, the GFCI will either refuse to reset (the button won't stay depressed), or it will appear to work but will not trip during a ground fault. This is a lethal failure mode because the outlet provides power, giving the user a false sense of security, but the internal sensing toroid is completely bypassed. If you drop a hairdryer in the sink, the breaker will not trip until the thermal 15A limit is reached—long after a fatal shock has occurred.

The Fix: Always trace the incoming cable from the breaker panel. In a retrofit, if you aren't sure which cable is which, use a non-contact voltage tester with the breaker ON and the wires separated and capped. The cable that makes the tester beep is your LINE. Connect it only to the brass and silver screws aligned with the 'LINE' printed text on the plastic housing. Leave the LOAD tape intact if no downstream GFCI protection is needed.

By strictly following the LINE-to-source mapping, torquing your terminals to 14 in-lbs, and verifying the trip mechanism with a dedicated tester, your GFCI installation will meet NEC Article 210.8 requirements and provide reliable, life-saving protection for decades.