Why Is My LED Light Glowing When Off? Circuit Causes and Fixes

An LED light glowing when off is almost always caused by micro-amp leakage currents charging the driver's internal bulk capacitor, or induced "ghost" voltage from parallel cable runs. Because LEDs require incredibly little current to produce visible photons (sometimes less than 1 mA), stray voltage that would never light an incandescent filament will cause an LED array to emit a faint, eerie glow. To fix it, you must either provide a dedicated neutral for your smart switch, install a bypass resistor, or upgrade to a true trailing-edge ELV dimmer.

The Physics of Phantom Glow: Why LEDs Stay Lit

Switch-mode power supplies (SMPS) inside modern LED drivers use a bridge rectifier and a bulk electrolytic capacitor to smooth the DC output to the LED chips. When you install a smart switch (like a Lutron Caséta or Leviton Decora Smart) that lacks a neutral wire, the switch must power its internal Wi-Fi or Zigbee radio by leaking a tiny current—typically 1 to 5 mA—through the bulb itself to complete the circuit.

This micro-current slowly charges the driver's bulk capacitor. Once the voltage across the capacitor hits the LED array's forward voltage threshold (usually around 2.5V to 3.0V per chip, meaning ~30V for a 10-chip series string), the LEDs flash or settle into a continuous faint glow. Similarly, long parallel runs of 14/2 or 12/2 NM-B cable can induce "ghost voltage" via capacitive coupling between the always-hot wire and the switched-leg wire, feeding enough micro-amps to keep high-efficacy LEDs illuminated.

Dimmer Compatibility and Circuit Impact Math

Standard leading-edge (TRIAC) dimmers were designed for resistive incandescent loads. They chop the leading edge of the AC sine wave, which causes severe electromagnetic interference (EMI) and misfiring when paired with the capacitive input stages of LED drivers. For LED circuits, you must use a trailing-edge (ELV/IGBT) dimmer, which chops the trailing edge of the wave and provides a softer turn-on that prevents driver damage.

Circuit Impact Math: Inrush and Power Factor

When sizing a breaker and dimmer for a multi-fixture circuit, you cannot rely on steady-state wattage. You must account for inrush current and Power Factor (PF).

• Inrush Current: A typical 11W BR30 LED has an inrush current of 35A for the first 200 microseconds as the bulk capacitor charges from zero. If you wire 15 of these to a single 15A breaker: 15 × 35A = 525A instantaneous inrush. This can cause magnetic trip nuisance tripping on standard thermal-magnetic breakers or permanently weld the TRIAC inside a cheap dimmer.
• Power Factor (PF): A 15W downlight with a cheap driver and a 0.6 PF draws 15W / 0.6 = 25VA of apparent power. At 120V nominal, that is 0.208A per fixture. On a 15A circuit (derated to 12A for continuous lighting loads per NEC 210.20), you can safely run 12A / 0.208A = 57 fixtures. If you incorrectly calculated using pure wattage (15W / 120V = 0.125A), you would mistakenly think you could run 96 fixtures, potentially overloading the neutral conductor.

Which Dimmer and Driver for Your Fixture Count?

For a standard residential 120V circuit running up to 17 LED fixtures, the Lutron Diva DVCL-153P is the industry benchmark trailing-edge dimmer. It handles up to 150W of LED load. However, you must perform a minimum load check: this dimmer requires a minimum of 15W of connected LED load to operate correctly. If you are only dimming a single 9W bulb, the dimmer will misfire and flicker. You must either add a second bulb or install a dummy load resistor to meet the 15W floor.

Lumens, Watts, and Efficacy: Sizing the Right Driver

When replacing older fixtures or designing a new layout, matching lumens to watts requires understanding efficacy (lumens per watt, or lm/W). Efficacy is not linear; it suffers from thermal droop as wattage and junction temperatures increase.

Heat and Enclosure Constraints

Enclosed fixtures (like flush-mount glass globes or airtight recessed cans) trap heat. Standard LED drivers use 105°C rated electrolytic capacitors. The Arrhenius equation dictates that for every 10°C the ambient temperature exceeds the capacitor's rating, its lifespan halves. If you install a standard driver in an enclosed fixture where ambient temps reach 75°C, the electrolyte will boil off, causing the driver to fail, flicker, or short out. Always look for drivers explicitly labeled "Suitable for Enclosed Fixtures" (often potted in thermal epoxy) and ensure recessed housings are IC-rated (Insulation Contact) if they will be buried in attic blown-in insulation.

Troubleshooting Flicker and Ghosting

Flickering on a dimmed circuit usually happens when the driver's undervoltage lockout (UVLO) circuit cycles. The dimmer chops the sine wave so aggressively that the driver's input voltage drops below its operational threshold. The driver shuts down, the input capacitor recharges, the driver fires up, and the cycle repeats at 5 to 10 Hz, creating a visible strobe effect.

The Fix: If you are using a smart switch without a neutral wire, or a trailing-edge dimmer at the very bottom of its travel range, install a bypass resistor (such as the Lutron LUT-MLU) across the line and load terminals of the first fixture in the circuit. This resistor provides a dedicated path for the smart switch's leakage current and stabilizes the minimum load for the dimmer, eliminating both the phantom glow and the low-end flicker.