GFCI Trip: Test Line vs Load vs Ground First

Why this matters

A GFCI trips because current returning on the neutral does not match current going out on the hot - a leakage path to ground somewhere in the protected zone. The leak can sit on the line side feeding the device, on the load side feeding downstream receptacles, in the wiring between, or in a connected appliance. Pulling and replacing the GFCI first is the common wrong move: a new device trips identically because the fault is downstream. Separating line, load, and the ground path in sequence isolates the leak to one segment and prevents condemning a healthy device.

Symptom presentation

The customer reports a GFCI that will not reset, trips immediately, or trips intermittently. Downstream outlets fed through the device are also dead. The trip may be tied to a condition: only when an appliance is plugged in, only after rain on an exterior circuit, only in cold or damp weather. That conditional pattern is a strong pointer to where the leak lives.

Quick checks

• Press TEST then RESET. If it resets and holds with nothing plugged in, the device is functional and the leak is load-driven.
• Unplug everything on the circuit, including downstream receptacles, then reset. A hold means an appliance or downstream load is the leak.
• Identify line versus load terminals. A GFCI miswired with feed on the LOAD pair will not reset; this is the top cause on a freshly installed device.
• On exterior or wet-location circuits, check covers, gaskets, and for water intrusion at boxes before deeper testing.
• Note the trip behavior: instant on reset (hard leak or miswire), trips after a delay or under load (intermittent or moisture leak), or trips only with a specific appliance (appliance leak). A standard Class A device responds to leakage around 4 to 6 mA, so even a small wet-path or insulation leak will trip it.
• Distinguish a true ground-fault trip from a worn or end-of-life device. A unit that fails its own self-test or will not pass the on-board TEST is defective; replace it before chasing the wiring.

Isolation tree

Isolate the wiring side by side, starting at the device.

1. Device alone. De-energize, disconnect all conductors from the GFCI, restore power to the LINE feed only, and reset with nothing on the load terminals.
   • Holds: the device and its line feed are good. The leak is on the load side or in a connected appliance. Go to step 2.
   • Will not hold with only line connected: the leak is on the line feed itself or the device is faulty. Confirm line wiring; if clean, replace the device.
2. Load side, unloaded. Reconnect the load conductors that feed downstream receptacles, with all downstream loads unplugged. Reset.
   • Holds: the downstream wiring is clean. Reintroduce loads one at a time; the load that trips it is the leak.
   • Trips: leakage is in the downstream wiring - a damaged cable, a neutral touching ground or box, or a wet box. Walk the downstream chain.
3. Neutral-to-ground integrity. A common load-side fault is a shared or crossed neutral. De-energized, ohm each circuit neutral to ground and to other circuit neutrals.
   • Continuity neutral-to-ground or between two circuits' neutrals: a bootleg or shared neutral is bleeding current outside the GFCI's sensing window. Separate the neutrals so each returns only through its own GFCI.
4. Ground path itself. Confirm the equipment grounding conductor is bonded and continuous. A GFCI does not require a ground to function, but a hot conductor contacting a grounded box or raceway is the leak; locate the contact point along the run. A staple driven through a cable, a screw into a buried conductor, or insulation pinched under a clamp are common contact points - the leak often appears only when that section flexes or wets.
5. Conditional / environmental leaks. If the device holds dry and trips only after rain or in cold weather, the leak is moisture-driven. Water in an exterior box, a degraded gasket, condensation in a conduit, or wet insulation in a buried run bridges hot to ground enough to trip. Open the wet-location boxes, check the in-use cover and gasket, and look for a low point in the conduit collecting water. These leaks measure as intermittent neutral-to-ground or hot-to-ground resistance that drops when wet.

Confirming diagnosis

You have isolated the fault when the GFCI holds with one segment disconnected and trips when that segment is restored. Confirm by:

• Reintroducing the suspect segment or load and reproducing the trip on demand.
• Reading neutral-to-ground resistance on the suspect run; a low reading where it should be open confirms a leakage bond.
• For appliance leaks, testing the appliance on a separate known-good GFCI; if it trips there too, the appliance is the fault, not the wiring.

Remediation

• Miswire (feed on load): move the supply conductors to the LINE terminals; downstream feed goes on LOAD. Reset and verify.
• Downstream wiring leak: locate and repair the damaged cable, crossed neutral, or wet box; reseal exterior boxes with proper gaskets and in-use covers.
• Shared/bootleg neutral: re-separate neutrals so each branch returns only through its own protective device; correct any neutral-to-ground bond outside the service.
• Appliance leak: advise repair or replacement of the leaking appliance; the circuit is healthy.
• Failed device: replace with a listed GFCI; verify the on-board TEST and RESET both function after install.

References

• NFPA 70 (NEC) Article 210 - GFCI protection requirements for branch circuits.
• NFPA 70 (NEC) Article 250 - Grounding and bonding, neutral and equipment grounding conductors.
• NFPA 70 (NEC) Article 406 - Receptacle installation and wet-location requirements.
• UL 943 - Standard for Ground-Fault Circuit-Interrupters.
• NFPA 70E - Energized testing practices and PPE.