Key takeaways
- A GFCI trips when current going out and coming back differ by roughly 5 milliamps. That is a small enough number that genuine, harmless leakage from several appliances can add up to it.
- Moisture is the most common cause in exterior, bathroom and garage locations, and it does not need standing water. Humidity in an outdoor box is enough.
- Unplug everything and reset. If it holds, add devices back one at a time. This is slow and it is the method that actually finds the answer.
- GFCIs wear out. A device more than ten to fifteen years old that trips with nothing plugged in is frequently the fault itself, and end-of-life units may fail to reset at all.
A GFCI compares the current going out on the hot with the current coming back on the neutral. In a healthy circuit those match. When they differ by about 5 milliamps, the device opens, because a difference means current is getting back by some other route and a person is one of the routes available.
Five milliamps is a very small number. It is set there deliberately, well below the level that stops a heart, and the consequence is that a GFCI will react to things that are not dangerous as well as things that are.
What is actually causing it
- Moisture. The most common cause on exterior, bathroom, garage and pool circuits, and it does not take standing water. Humidity condensing inside an outdoor box will do it, which is why these trips are often weather-correlated.
- One faulty appliance. Insulation inside a motor or a heating element degrades and leaks current to the frame. Older appliances, and anything with a heating element or a motor, are the usual candidates.
- Cumulative leakage. Every appliance leaks a tiny amount by design. Enough of them on one circuit sum past 5 milliamps without any single one being faulty, which is why a GFCI can trip on a circuit where nothing is broken.
- Shared or crossed neutrals. If the neutral of the protected circuit is tied to another circuit's neutral anywhere, return current bypasses the sensor and the imbalance is permanent. This is a wiring fault and it is common in houses that have been extended.
- Damaged cable. A staple driven too hard, a nail from a picture hook, or rodent damage, any of which lets current find a path to ground.
- A worn-out GFCI. They have a service life and they spend it in damp locations.
The isolation method
It is unglamorous and it works. Unplug everything on the circuit, and be thorough about what that means: a single GFCI can protect outlets in other rooms, so the thing causing the trip may be somewhere you were not looking.
Reset. If it will not reset with the circuit empty, the problem is the wiring or the device, and you can stop looking at appliances.
If it does hold, plug things back one at a time. Give each one a few minutes and, where it matters, a full operating cycle. A freezer that trips the circuit when its defrost heater comes on will pass ten minutes of observation and fail at hour six, which is exactly why intermittent trips are hard to pin down and why patience beats guessing.
When the device itself is the answer
GFCIs wear out, generally somewhere in the ten to fifteen year range, and they are usually installed in the hottest and dampest parts of the house. A unit that trips on an empty circuit, or will not reset, or fails its own test button, is a strong candidate.
Newer devices are required to test themselves periodically and to lock out if they detect their own failure. That is a genuine safety improvement over older units, which could fail in a way that left the outlet powering things while providing no protection at all, with nothing to indicate it. If a GFCI in your house predates self-testing, replacing it on age alone is defensible.
The trip that is doing its job
It is worth saying plainly: a GFCI that trips has usually found something. The instinct to treat repeated tripping as a defective outlet and replace it with an ordinary receptacle is the one genuinely dangerous response to this problem, and it is common enough that it is worth naming.
If a circuit trips its GFCI persistently and the answer turns out to be damaged cable in a wall, the device was the only thing telling you.
This post is about a GFCI that trips. What GFCI and AFCI protection is and which rooms the code requires it in is the neighbouring question, and it has its own post: GFCI and AFCI outlets explained owns that. If your outlet is simply dead and you have not established a GFCI is involved at all, start at outlet not working and the breaker looks fine.
Pool and spa equipment has its own GFCI requirements and its own failure patterns, covered in pool and hot tub wiring rules in San Antonio. When the isolation method runs out and the answer is in the wiring, that is outlet and GFCI work, and we test the device under load rather than resetting it and calling it fixed.