A GFCI that keeps tripping on a landscape lighting transformer is doing its job. It has detected a current imbalance somewhere downstream and is shutting off power to prevent a shock hazard. The diagnostic question is where the imbalance is. Work through the checks in order. Each one isolates a section of the system.
Safety First
Before any of this, kill power at the breaker. Reset the GFCI only after each diagnostic step. If the GFCI trips the instant you reset it with nothing plugged in, the GFCI itself is bad. Replace it before doing any other work.
The Pattern Tells You a Lot
Note when the GFCI trips:
- Instantly when reset, even with the transformer unplugged: GFCI is bad or the outlet is wet
- Instantly when the transformer is plugged in: line-side fault, water in the transformer, or a damaged transformer winding
- After the transformer has been running for some minutes: low-voltage side fault, water in a fixture or splice, or a damaged buried wire
- Only during or after rain: water intrusion somewhere on the low-voltage side
- Only at specific times of day: photocell or timer-driven load change exposes a marginal fault
Write down the pattern. It saves an hour of testing.
Step 1: Verify the GFCI Outlet
Unplug the transformer. Reset the GFCI. Plug a known-good lamp or appliance into the outlet. If the GFCI trips, the GFCI or the outlet is the problem, not the lighting system.
Replace the GFCI outlet. Outdoor GFCIs have a finite lifespan, especially in regions with high humidity or temperature swings. A 10-year-old outdoor GFCI is a candidate for replacement on its age alone.
If the new GFCI holds with the lamp plugged in, reconnect the transformer and continue.
Step 2: Inspect the Transformer for Water
Open the transformer enclosure. Look for:
- Standing water at the bottom
- Corrosion on tap terminals
- A wet primary wire entry point
- Insects or debris that may have created a moisture path
If you find water, dry the enclosure completely. Check the gasket and the cable gland on the line-side conduit. Reseal anything that has failed. Allow the unit to dry for several hours – ideally overnight – before testing.
Resetting a GFCI on a still-wet transformer will just trip again.
Step 3: Isolate the Low-Voltage Side
Disconnect every wire from the transformer's COM and tap terminals. Reset the GFCI with the transformer plugged in but no load connected.
- GFCI holds with no load: the fault is somewhere on the low-voltage side, not in the transformer
- GFCI trips even with no load: the transformer has an internal fault and should be replaced
Step 4: Walk the Low-Voltage Runs
This is the longest step. Reconnect one run at a time. After each reconnection, reset the GFCI and run the system. If the GFCI holds for several minutes, that run is clean. Move on.
When you reconnect the run that contains the fault, the GFCI will trip again. That tells you which run to inspect.
Step 5: Find the Fault on the Bad Run
On the failing run, look for the same suspects you would for any low-voltage failure:
- Cracked fixture housings letting water into the bulb socket
- Splices that are not buried in waterproof connectors
- Wire damage from yard work, root intrusion, or animals
- Junction boxes with failed gaskets
Open each fixture along the run. Pull each splice. Replace any waterproof connector that looks discolored or feels loose. If you find a fixture that has water inside the housing, replace the gasket or replace the fixture.
Step 6: Test Sections
If walking the run does not find the obvious fault, divide and conquer. Cut the run at a midpoint and cap both ends. Connect only the first half to the transformer.
- GFCI holds: the fault is in the second half
- GFCI trips: the fault is in the first half
Keep halving until the fault is in a section short enough to dig up or visually inspect.
Common Hidden Causes
- A fixture stake with a screw that has worked its way into the buried wire
- A buried splice in a low spot where water collects
- A path light that fills with water through a cracked lens and shorts when the bulb gets warm
- Rodent damage to insulation in a mulch bed
- A connector that was never buried properly and has been intermittently wet for months
When to Bring in a Pro
If you have walked the runs, replaced the GFCI, dried the transformer, and the GFCI still trips, the fault is buried and not obvious. A wire tracer with a ground-fault locating function can find the exact spot in under an hour. Most professional landscape lighting service calls for this kind of fault take one to two hours total.
Prevention
After the repair, take a few minutes to prevent the next call:
- Replace any wire connector that is more than 8-10 years old
- Make sure every splice is in a waterproof connector rated for direct burial
- Elevate splices above low spots where water collects
- Check the gasket on the transformer enclosure and replace if it is cracked or compressed
Next Step
Start with the pattern. The timing of the trip is the single most useful piece of diagnostic information. If the GFCI trips only after rain, you are looking for water intrusion. If it trips instantly, the fault is line-side. Match the pattern to the step in this guide and most of the troubleshooting time is already saved.
