LED lights glow when switched off because micro-ampere leakage currents charge the LED driver’s input capacitor. Once the capacitor voltage reaches the driver’s internal strike threshold, the LED emits a continuous low-level ghost glow or periodic flash. This leakage typically originates from illuminated wall switches, smart switches lacking a neutral wire, or capacitive coupling between parallel conductors in long cable runs.
Fixing this requires identifying the leakage path and either providing a bypass for the micro-current or upgrading the switch hardware. Below is a complete circuit-level breakdown of why this happens, how to calculate your true lighting loads, and how to spec dimmers and drivers to prevent both ghosting and flicker.
The Physics of Ghosting: Leakage Paths and Thresholds
Unlike incandescent bulbs, which require hundreds of milliamps to heat a filament and emit light, modern LED drivers can turn on with less than 1 milliamp (1000 µA) of current. When you run a 3-wire cable (like 14/3 NM-B) to a multi-way switch setup, the unswitched 'hot' wire and the switched 'traveler' wires run parallel for dozens of feet. This creates a parasitic capacitor. The AC voltage induces a small current across this capacitance, which flows through the LED driver, rectifies, and slowly charges the bulk input capacitor.
If your ghosting is caused by a smart switch without a neutral (which leaks ~200 µA to power its internal WiFi/Zigbee radio), install a bypass resistor across the load and neutral at the fixture. A Lutron LUT-MLC or a standard 100kΩ 1/2W metal film resistor provides a lower-resistance path for the leakage current, preventing it from charging the LED driver's capacitor.
Table 1: Common Leakage Sources and Ghosting Thresholds
| Leakage Source | Typical Leakage (µA) | Ghosting Behavior | Required Fix / Hardware |
|---|---|---|---|
| Illuminated Toggle Switch (Neon) | 50 - 150 µA | Continuous faint glow | Replace with non-illuminated switch |
| Smart Switch (No Neutral Wire) | 150 - 300 µA | Slow charge, bright flash every 10-30s | Install LUT-MLC or 100kΩ bypass resistor |
| Capacitive Coupling (Long 14/3 Runs) | 10 - 40 µA | Very faint glow, mostly visible in dark | Upgrade to higher-quality driver with bleeder resistor |
| Induced Voltage (Parallel Feeders) | 5 - 20 µA | Intermittent micro-flashing | Separate low-voltage and line-voltage runs |
Lumens, Watts, and Circuit Impact Math
When designing a lighting circuit, looking only at the nominal wattage of an LED fixture will lead to undersized breakers and nuisance tripping. You must account for Power Factor (PF) and Inrush Current. The LED driver's internal switching power supply draws current in sharp spikes rather than a smooth sine wave, meaning the apparent power (VA) is higher than the real power (Watts).
According to the US Department of Energy Solid-State Lighting guidelines, commercial and high-end residential drivers should target a PF > 0.9, but cheap retrofit bulbs often sit at 0.5 to 0.6. Furthermore, the initial charging of the driver's capacitors causes an inrush current that can be 200x the steady-state draw.
Table 2: LED Equivalence, Efficacy, and Circuit Impact Data
| Fixture Type | Nominal Watts | Lumens | Efficacy (lm/W) | Power Factor (PF) | Inrush Current (A) @ 120V |
|---|---|---|---|---|---|
| Standard A19 Retrofit Bulb | 9W | 800 lm | 88 lm/W | 0.55 | 15A (for <100µs) |
| High-Output Downlight (IC-Rated) | 15W | 1200 lm | 80 lm/W | 0.92 | 35A (for <200µs) |
| Linear High Bay (Commercial) | 150W | 21,000 lm | 140 lm/W | 0.98 | 120A (for <500µs) |
| Low-Voltage MR16 (with Transformer) | 7W | 500 lm | 71 lm/W | 0.65 | 25A (for <150µs) |
The Circuit Math: If you wire ten 15W high-output downlights to a single 15A breaker, the continuous real load is only 150W (1.25A). However, the apparent power is 150W / 0.92 PF = 163 VA (1.36A). More critically, the combined instantaneous inrush current is 350A. While a standard thermal-magnetic breaker's magnetic trip is usually set at 5x to 10x the rated current (75A - 150A for a 15A breaker), a 350A inrush spike lasting 200µs will often fall under the breaker's trip curve time-delay threshold. However, if you mix these with motor loads or use a GFCI/AFCI breaker with sensitive electronics, that inrush spike can cause nuisance trips. Always calculate total inrush against the breaker manufacturer's let-through curves.
Dimmer Compatibility, Fixture Counts, and Flicker Fixes
Flickering is distinctly different from ghosting. While ghosting happens when the circuit is off, flicker happens when the circuit is on and dimmed. Flicker occurs when the dimmer's internal timing circuit misfires because the connected LED load is too low to sustain the dimmer's internal power supply, or when the dimmer's phase-cut waveform clashes with the LED driver's active rectification.
For modern LED circuits, you must use a trailing-edge (ELV - Electronic Low Voltage) dimmer rather than a leading-edge (TRIAC) dimmer. Trailing-edge dimmers use MOSFETs or IGBTs to cut the back half of the AC sine wave, which aligns much better with the capacitive input of LED drivers and eliminates the harsh 'snap' on/off that causes acoustic buzzing and LED strobing.
Every trailing-edge dimmer has a minimum load requirement, typically 10W to 20W. If you install a Lutron Diva DVCL-153P (min load 10W for LEDs) on a circuit with only two 4W LED step lights (8W total), the dimmer will starve for current. The result is a 60Hz strobe effect. Always sum your nominal LED watts and ensure they exceed the dimmer's minimum load by at least 20%.
Table 3: Dimmer and Driver Matching Matrix
| Driver Type | Dimmer Technology | Min Load Check | Best Use Case |
|---|---|---|---|
| Constant Voltage (12V/24V DC) | Trailing Edge (ELV) | Sum of all fixture watts > Dimmer min load | LED strip lighting, under-cabinet pucks |
| Constant Current (350mA/700mA) | 0-10V Analog or PWM | N/A (Control signal, not phase-cut) | Commercial downlights, high-bay panels |
| AC Retrofit (Internal Driver) | Trailing Edge (ELV) with adjustable low-end trim | Total bulb wattage > 15W recommended | Residential recessed cans, A19 lamps |
The Fix for Flicker: If your fixture count is too low to meet the dimmer's minimum load, you have two options. First, add a dummy load resistor (like the Lutron LUT-MLC) in parallel with the fixtures to draw the extra required current. Second, swap the switch for a specialized low-load LED dimmer, such as the Lutron Skylark SCL-153P, which is engineered to operate stably with as little as one 5W LED bulb.
Thermal Constraints and Enclosure Derating
Heat is the primary killer of LED drivers, and degraded drivers are significantly more prone to both ghosting and flickering. The electrolytic capacitors inside the driver smooth the rectified AC voltage. As ambient heat rises, the electrolyte inside these capacitors slowly evaporates, increasing their Equivalent Series Resistance (ESR). A high-ESR capacitor cannot filter leakage currents effectively, making the fixture hyper-sensitive to the micro-amp ghosting currents mentioned in Table 1.
According to Lutron's LED compatibility and thermal guidelines, enclosing a driver in a poorly ventilated space drastically reduces its lifespan. The Arrhenius equation dictates that for every 10°C increase in operating temperature, the lifespan of an electrolytic capacitor is cut in half. A driver rated for 50,000 hours at 85°C will fail in roughly 12,500 hours if the enclosure traps heat and pushes the internal ambient to 105°C.
Enclosure Rules for Recessed Fixtures:
- IC-Rated (Insulation Contact): The fixture's thermal management is designed to handle direct contact with ceiling insulation. The driver is usually potted in thermal epoxy or separated from the heat-generating LED COB (Chip-on-Board) array. Use these anywhere insulation is present.
- Non-IC Rated: Requires a minimum 3-inch clearance from all combustible materials and insulation. If you install these in an insulated ceiling, the heat will pool in the metal can, bake the driver, and cause premature capacitor failure (leading to severe flicker and eventual dead-shorts).
- Remote Drivers: For high-wattage fixtures (75W+), mount the LED driver in an accessible, ventilated junction box outside the ceiling canopy. This removes the primary heat source from the fixture enclosure and allows for easy replacement if the driver fails.
By matching your trailing-edge dimmers to the correct minimum load, calculating inrush currents against your breaker curves, and respecting thermal derating limits, you will eliminate both the off-state ghosting and the on-state flicker that plague improperly spec'd LED circuits.
