“It’s not broken—it’s just breathing electricity.”
—That’s what my friend Sal, a 30-year outdoor lighting installer in San Diego, says every time he sees low-voltage path lights glowing faintly at 2 a.m. He’s not being poetic. He’s diagnosing ghost voltage.
How We Got Here
Twenty years ago, most path light systems were simple: 12V AC transformer → direct burial cable → halogen or early LED fixtures. If a light glowed when off, you blamed the transformer’s residual output or cheap wiring.
Then came smart controls, shared conduits, pool pump circuits running parallel to landscape wire, and NEC updates tightening grounding rules. Suddenly, that soft amber glow wasn’t just annoying—it was a red flag.
I’ve seen it dozens of times: a homeowner installs new path lights along a patio edge, runs everything in one ¾" PVC conduit with their pool pump’s 240V feed, and wonders why the lights look like fireflies at midnight—even with the timer set to “off.”
What’s Really Happening
Ghost voltage isn’t magic. It’s physics sneaking through code loopholes.
- Induced current: A 240V pool pump circuit humming alongside your 12V landscape wire acts like a weak transformer. Even with no physical contact, magnetic fields induce millivolts—enough to excite modern LEDs (especially warm-white 2700K chips).
- Shared neutral or poor grounding: If your transformer’s ground rod is 25 feet from the main panel’s ground—or worse, tied to a separate rod without bonding—you get potential differences. That voltage difference leaks into the low-voltage secondary side.
- Capacitive coupling: Long cable runs buried near hot lines store tiny charges like capacitors. When the system is “off,” that stored energy bleeds slowly into the fixtures.
This falls flat because most DIYers assume “low voltage = safe and simple.” But NEC 411.3(B) exists for a reason: *“Low-voltage lighting systems shall be controlled by a listed disconnecting means located within sight of the transformer…”* That “disconnecting means” isn’t just a switch—it’s a hard break. And if your timer or photocell only interrupts the *primary* side? You’re still feeding induced voltage downstream.
Test It Like a Pro (No Guesswork)
You need a true-RMS multimeter—not the $12 kind with auto-ranging quirks. Set it to AC voltage, lowest range (200 mV or 2V).
- At the fixture: Disconnect both wires from the lamp socket. Measure voltage between them. Anything over 0.3V AC? Ghost voltage is present.
- At the transformer output: With the system “off” at the timer/switch, measure output terminals. Should read <0.1V. If it reads 1–3V, your control device isn’t breaking the circuit fully—or there’s backfeed.
- Ground check: Measure resistance between transformer case and your main service ground rod. Should be ≤25 ohms. If it’s open or >100Ω, your ground is isolated—and dangerous.
I think the biggest mistake I see? People testing at the transformer *before* verifying conduit separation. If your landscape wire shares conduit with any 120V/240V line—even just for 6 feet under the deck—that’s likely your culprit. NEC doesn’t forbid it, but it absolutely invites induction.
Fix It Right (Not Just “Better”)
Here’s what works—and why:
- Relocate the low-voltage cable: Run it in its own dedicated conduit, minimum 12" away from any power line. Not “as far as possible”—12". That’s the sweet spot where induction drops below perceptible levels for most LEDs.
- Install an inline relay switch on the secondary side: NEC 411.3(B) requires it *if* your primary-side switch doesn’t provide visible isolation. Use a 12V DC coil, SPST relay rated for 5A continuous. Mount it right after the transformer, before the first splice. This kills voltage at the source—not just upstream.
- Bond that ground: Run a #6 bare copper wire from the transformer ground lug directly to your main panel’s grounding electrode system. No rods, no clamps on rebar, no “good enough.” Tie it in. Period.
And skip the “LED driver with built-in shunt” gimmicks. They leak current too—and often fail silently. A proper relay switch costs $22 and solves it cleanly.
When to Call a Licensed Electrician
If your voltmeter reads >5V AC at the fixture—even with relay installed—don’t chase it yourself. That’s not ghost voltage. That’s either:
- A damaged transformer winding leaking primary voltage into secondary, or
- A miswired junction box where 120V accidentally contacted low-voltage cable.
Bottom line: Dim glow at night isn’t charm—it’s a symptom. Fix the coupling, fix the ground, break the circuit where it counts. Then your path lights will do exactly what they should: shine when you want them to, and sleep when you don’t.