Four brass and silver screws look alike, and that is where the trouble starts. GFCI line and load terminals are not two names for the same pair: line is the incoming supply the device protects from, and load is the outgoing circuit the device extends protection to. Connect them backward and the receptacle powers up while protecting almost nothing.
What each pair is wired to do
The supply from the panel lands on the line pair. Everything the device senses passes through its internal current transformer on the way to the face of the receptacle and on to the load pair. Conductors leaving the load terminals feed downstream outlets, and because their current also passes through the sensor, those outlets inherit the protection. A tape flap or a sticker covers the load screws from the factory precisely because most installations do not use them, and because using them by mistake is the common error. Line and load are a direction, not a preference.
What happens when the two are reversed
Wire the supply to the load terminals and the device usually still energizes, because the internal path passes current in both directions. The face of the receptacle may deliver power and a plug-in tester may show a normal pattern. What has changed is that the sensing element now sits on the wrong side of the supply, and on many designs the reset will not latch at all, while on others it latches and offers no protection to the receptacle face or anything beyond it. The failure mode is quiet either way: the outlet works, so nobody investigates.
Finding the supply conductors before you connect anything
With the breaker off, separate the two cables in the box and cap the conductors so nothing touches. Turn the breaker on and use a non-contact tester or a meter to identify which pair is energized; that is the line. Turn the breaker back off before touching anything again. If the box contains a single cable there is no downstream circuit and the load terminals stay covered. If it contains three or more cables, the circuit may split at this point and you will need to decide deliberately which branches should be protected, then pigtail accordingly rather than stuffing multiple conductors under one screw.
Why modern devices refuse to reset when miswired
Manufacturers responded to this error by adding a lockout feature. A device that has never been correctly powered on the line side will not latch when you press reset, and a device that detects an internal fault during its self-test will trip and stay tripped. Installers read this as a defective part and return it. Almost always the device is reporting a wiring problem accurately. Before you exchange a ground-fault receptacle that will not reset, confirm the supply is on the line pair, that the neutral belongs to the same circuit as the hot, and that no shared neutral from a multiwire circuit is passing through.
The neutral has to keep the same company as the hot
Because the device compares hot and neutral current, those two conductors must be the pair that actually serve the same load. If the neutral on the load side is spliced to a neutral from another circuit, or a multiwire branch circuit has its neutral shared downstream, the currents no longer balance and the device trips immediately or unpredictably. This shows up most often in kitchens and older remodels where neutrals were bundled loosely in a box. Keeping the pairs together is not a matter of tidiness; it is the condition the sensing depends on.
What to check before you act
- Identify the energized cable with the device disconnected, then power down again before wiring.
- Leave the factory tape over the load terminals unless you intend to protect downstream outlets.
- Keep each hot with its own neutral; never share a neutral across the sensing element.
- Press the device test button after installation and confirm downstream outlets lose power.
Common questions
My new GFCI will not reset, is it broken?
Far more often it is wired with the supply on the load terminals, or the neutral does not belong to that circuit. Check the wiring before exchanging the device.
Do I have to use the load terminals at all?
No. If the box has only the incoming cable, leave the load screws covered and the receptacle protects itself and anything plugged into it.
The practical takeaway
Treat line and load as a direction of travel rather than a pair of equivalent screws. Identify the supply with the power on and the device out of the circuit, wire it to line, and use load only when you intend to extend protection. Then press the test button and walk the circuit to confirm what actually went dead.
References and further reading
- OSHA 1910.304: ground-fault protection for personnel requirements
- CPSC: home electrical safety checklist including testing protective devices
- NFPA: electrical safety materials for homeowners and installers
Numerical scenarios are illustrative unless identified otherwise. Follow the exact product instructions; component ratings and local installation requirements can differ.
