To hook up a GFCI receptacle correctly, you must connect the incoming power (Line) to the LINE terminals and the downstream protected outlets to the LOAD terminals. Specifically, the incoming black (hot) wire goes to the LINE brass screw, the incoming white (neutral) wire goes to the LINE silver screw, and the bare/green ground wire connects to the green grounding screw. Downstream wires connect to the corresponding LOAD screws.
The GFCI Receptacle Wiring Diagram & Symbol Key
Before tracing the physical wires, you need to understand the schematic symbols used in standard GFCI wiring diagrams. Manufacturers like Leviton and Hubbell print these diagrams on the device packaging and the back of the receptacle itself.
- LINE and LOAD Blocks: Represented by two distinct rectangular blocks on the back of the device. The LINE block receives power from the panel; the LOAD block sends protected power downstream.
- The Sensor Core (Toroid): Drawn as a circle or oval with the Line and Load conductors passing through it. This symbolizes the current transformer that monitors the balance between hot and neutral current. If it detects an imbalance of 4-6 milliamps, it triggers the trip mechanism.
- The Trip Solenoid: Usually drawn as a rectangle with a diagonal line or a coil symbol connected to the sensor core. This is the electromechanical latch that physically disconnects the contacts when a fault is detected.
- Ground Symbol: Three horizontal lines of decreasing width (or a circle with a cross inside a box). This indicates the Equipment Grounding Conductor (EGC) path, which bypasses the internal GFCI sensor and bonds directly to the metal yoke and receptacle face.
Node-by-Node Wiring Trace: Source to Load
Let's trace the exact path of the conductors for a standard 20-amp branch circuit using 12 AWG NM-B (Romex) cable and a 20A tamper-resistant GFCI (such as the Leviton GFNT2-W). Polarity is critical here: reversing hot and neutral will allow the receptacle to power devices, but it will fail to trip during a ground fault, creating a severe shock hazard.
- Node 1: Panel to Box (Incoming Line). The 12/2 NM-B cable enters the single-gang box. You have a black (hot), white (neutral), and bare copper (ground) wire.
- Node 2: Ground Path Setup. The bare copper ground wire does not pass through the GFCI's internal circuitry. It must be bonded to the metal box (if applicable) using a green 10-32 ground screw, and a 12 AWG pigtail is run from the box to the GFCI's green grounding screw. Torque the green screw to 14 in-lbs.
- Node 3: Line Neutral Connection. The incoming white neutral wire is stripped to 5/8 inch and wrapped clockwise around the LINE Silver screw. This establishes the return path reference for the internal sensor.
- Node 4: Line Hot Connection. The incoming black hot wire is stripped to 5/8 inch and wrapped clockwise around the LINE Brass screw. Power now reaches the GFCI's internal logic and the line-side contacts.
- Node 5: Load Neutral Connection (Downstream). If you are protecting downstream outlets, the white wire of the outgoing 12/2 cable connects to the LOAD Silver screw.
- Node 6: Load Hot Connection (Downstream). The black wire of the outgoing cable connects to the LOAD Brass screw. The internal relay will only pass voltage to these terminals when the GFCI is reset and no fault is detected.
- Node 7: Downstream Ground. The bare ground wire of the outgoing cable is spliced (wire-nutted) with the incoming ground and the pigtail to the GFCI. Ground continuity must be maintained to downstream outlets regardless of the GFCI's tripped state.
Terminal Mapping & Verification with a Multimeter
Miswiring the LINE and LOAD terminals is the most common reason a newly installed GFCI fails to function or trips immediately. Use the mapping table below to confirm your physical connections, then verify with a digital multimeter (DMM) like a Fluke 117 before energizing the circuit.
| Physical Terminal | Screw Color | Wire Color (120V) | Function & Polarity |
|---|---|---|---|
| LINE Hot | Brass | Black | Incoming ungrounded conductor from breaker |
| LINE Neutral | Silver | White | Incoming grounded conductor from neutral bar |
| LOAD Hot | Brass (Taped) | Black | Outgoing protected hot to downstream devices |
| LOAD Neutral | Silver (Taped) | White | Outgoing protected neutral to downstream devices |
| Ground | Green | Bare / Green | Equipment Grounding Conductor (EGC) bond |
How to Verify Each Connection with a Meter
Once wired, but before pushing the GFCI face into the box, turn the breaker back on and perform these checks:
- Line Voltage Check: Set your DMM to AC Voltage. Place the red probe on the LINE Brass screw and the black probe on the green ground screw. You should read between 114V and 126V (nominal 120V).
- Load Voltage Check (Pre-Reset): Place the red probe on the LOAD Brass screw and the black probe on ground. The reading must be 0V. If you read 120V here before pressing the RESET button, your Line and Load wires are swapped.
- Reset and Re-test: Press the RESET button on the GFCI face. Re-measure the LOAD Brass screw to ground. It should now read 120V.
- Ground Continuity (Power Off): Turn the breaker back off. Set the DMM to Continuity/Ohms. Place one probe on the GFCI's green screw and the other on the bare ground wire of the downstream cable. The reading should be less than 1.0 ohm, confirming a solid equipment ground path.
Frequently Asked Questions
How to hook up a GFCI with no ground wire?
In older homes with 2-prong ungrounded receptacles, you may not have a bare copper or green wire in the box. According to NEC Article 406.4(D)(2), you are permitted to replace an ungrounded receptacle with a GFCI. Connect the incoming hot to LINE Brass and incoming neutral to LINE Silver. Leave the green ground screw empty. You must apply the included "GFCI Protected" and "No Equipment Ground" stickers to the faceplate. The GFCI will still detect current imbalances and protect against shock, even without an equipment ground, though surge protectors plugged into it will not function properly.
How to hook up a GFCI to protect multiple downstream outlets?
To protect standard outlets downstream, you must utilize the LOAD terminals. Connect your incoming power to the LINE terminals as usual. Then, connect the black and white wires leading to the next outlet in the chain to the LOAD Brass and LOAD Silver screws, respectively. Splice all ground wires together with a pigtail to the GFCI and the downstream cable. If the GFCI trips, all outlets wired to the LOAD side will lose power. For detailed code requirements on where this protection is mandated, refer to the Electrical Safety Foundation International (ESFI) guidelines.
How to hook up a GFCI and a standard outlet on the same circuit?
If you want the GFCI to protect itself but not the standard outlet next to it, wire the incoming power to the GFCI's LINE terminals, and wire the standard outlet in parallel to the LINE side using pigtails (wire nuts splitting the hot and neutral to both devices). If you want the standard outlet to be protected by the GFCI, wire the standard outlet to the LOAD side of the GFCI. Never daisy-chain power from the LINE side of the GFCI to the LOAD side of the same GFCI; this will cause an immediate short and trip the device.
Why does my GFCI trip immediately when I hook it up?
Immediate tripping upon resetting usually points to one of three wiring errors: 1) Line and Load reversed: The incoming power is on the LOAD terminals, and the downstream is on the LINE. Swap them. 2) Shared Neutral: A downstream circuit is borrowing the neutral from this GFCI's load side, causing the internal sensor to see an imbalance. 3) Neutral-to-Ground Fault: The downstream neutral wire is touching a ground wire or a metal box. The GFCI sees this as current leaking to ground and trips to protect you. Disconnect the LOAD wires and test the GFCI alone; if it holds, the fault is downstream.
