LED Dimming Compatibility

More callbacks come out of dimming than out of any other part of a lighting package. The fixtures are right, the wiring is right, the electrician is competent, and the lights flicker at the bottom of the slider or glow faintly when they are switched off. Somebody gets blamed, usually the electrician, and usually unfairly.

Almost all of it is predictable at the specification stage. This page is what to check before the job, not after.

The protocols, and why the word dimmable means nothing

A fixture marked dimmable tells you only that somebody believes it can be dimmed by something. The useful question is which control method the driver actually accepts.

• Forward phase, also called leading edge or TRIAC. The dimmer withholds power at the start of each half-cycle and fires partway in. Designed for incandescent filaments and for magnetic low voltage transformers, which need it. It is the method that struggles most with small LED loads.
• Reverse phase, also called trailing edge or ELV. Conducts from the zero crossing and cuts off partway through. Generally the better behaved option with electronic LED drivers. It normally requires a neutral in the wall box, which is exactly what an older building does not have.
• Magnetic low voltage. Requires forward phase. Putting the wrong dimmer on an MLV transformer is not merely a performance problem: DC offset from a generic incandescent dimmer can damage magnetic transformers.
• 0-10 volt. A separate low-voltage control pair alongside the mains. Robust and common in commercial work, but it has a dimming floor, typically around ten percent, and it will not take a luminaire to black on its own. If you need dim-to-off you need a relay or a switching device as well, and that has to be in the design.
• DALI and DALI-2. Digital, addressable, two-way. Right answer where you need individual fixture addressing and feedback, and overkill where you do not.
• Inside the driver, PWM or constant current reduction. This is invisible on a cut sheet and it decides how the fixture behaves at the bottom of its range, including whether the color shifts as it dims.

Why it goes wrong, electrically

• The load collapsed. Six sixty-watt lamps is 360 watts. Six nine-watt LEDs is 54. Dimmers have minimum loads, and below that a triac cannot hold conduction. Note that a dimmer’s watt rating does not transfer between load types at all: a Lutron Maestro LED+ is rated 600 watts incandescent and 150 watts LED, and LED minimums are determined per lamp model rather than as a wattage.
• The dimmer cannot find the zero crossing. A two-wire dimmer with no neutral infers the zero crossing through the load. Through a filament that was rock solid. Through LED drivers the leading edge can wander, which you see as flicker across the whole range. This one cannot be fixed by trying different lamps. It needs a different dimmer, ideally one with a neutral.
• Leakage current became visible. A two-wire dimmer draws a small keep-alive current through the load. A filament swallowed it. An LED driver’s bulk capacitor charges on it, which gives you either a faint glow when off or a slow flash as it charges, fires and discharges.
• Inrush and capacitance. LED drivers present capacitive input. Several on one circuit interact, and the interaction is not linear in the number of fixtures.

What the symptom is telling you

• Flicker only at the bottom of the range — the load has dropped below the dimmer’s minimum, or the driver’s own regulation gives up. Useful test: put a halogen lamp on the same dimmer. If the halogen dims smoothly, the problem is the driver, not the dimmer or the transformer.
• Flicker across the entire range on a two-wire dimmer — unstable zero-cross detection. Replace the dimmer with a neutral-wire model. Do not keep swapping lamps.
• Dead travel at the bottom of the slider — the driver’s cutoff sits above the dimmer’s minimum output. Many dimmers have an adjustable low-end trim, which is the first thing to try and the thing most often left unset.
• Dead travel at the top — the driver is already at full output over the last part of the slider’s travel. Department of Energy testing found this in a substantial minority of lamps tested.
• Pop-on, where the light jumps on at a level and will not fade below it — the driver’s minimum stable operating point is above the dimmer’s minimum.
• Buzz from the dimmer — usually the wrong phase type for the load.
• Buzz or hum from the fixture — transformer laminations, or a saturating toroid, which also runs hot.
• Glow or flashing when switched off — leakage current through a two-wire dimmer.
• Fixtures on the same circuit behaving differently from each other — different drivers. They will not track, and the eye is very good at spotting it in a corridor.
• Shimmer that only shows up on a phone camera — high-frequency flicker the eye does not register directly. It matters in any space that gets filmed, and it is a real consideration for a hospitality or retail client.

What a compatibility list actually promises

Lutron publishes an LED compatibility tool, and it is genuinely useful. It is also more limited than people assume, and Lutron says so themselves. In their own published wording, a listed and compatible bulb may still have less dimming range, dim less smoothly, flicker or shimmer, or buzz. Compatible means it can be dimmed, not that it will dim like a halogen.

The line worth quoting to a client is this one, also Lutron’s own: bulb manufacturers can modify their bulbs at any time, without notice. A listing is a snapshot of a tested sample. The model number on the box can stay the same while the driver inside it changes.

The most honest format in the industry is a per-fixture test report of the kind WAC publishes, which gives a measured low-end percentage for each dimmer rather than a pass or fail. On one of their reports, a single fixture measured anywhere from 0.5 percent to 56 percent depending on which dimmer it was on, with several dimmers marked not recommended. That spread, on one fixture, is the whole argument for specifying the combination rather than the fixture.

What to write into the specification

• Name the protocol. "Forward phase control", "reverse phase control (ELV)", "0-10V", "DALI-2". Never "dimmable" and never "LED dimmer".
• Require a tested combination, and say what evidence you will accept: the dimmer manufacturer’s published tested list, or a per-SKU dimming test report showing measured low-end percentage. The word dimmable in a submittal is not compliance with anything.
• State the minimum dimming level as a number. One percent means something. Fully dimmable does not. If you need dim to black, say so, and remember a 0-10V system needs a relay to get there.
• Specify the driver by model. The driver, not the housing, decides the behavior: the protocol, the floor, and whether the color shifts on the way down.
• Require the same driver throughout any visually continuous space. Different drivers do not track together, and a corridor is exactly where that shows.
• State load minimums and maximums per dimmer in fixture count, not watts. Watt ratings do not transfer between load types.
• Do not mix load types on one dimmer. Where it is unavoidable, get the manufacturer’s mixed-load rating table rather than guessing.

The retrofit trap, which is most of South Florida

The common scenario: a condominium built between the seventies and the nineties, corridors and lobby on incandescent dimmers, and somebody swaps the lamps for LED. Everything is nominally dimmable. It does not work, and the reasons stack.

The existing dimmers are almost certainly two-wire with no neutral in the box, because older switch legs carry hot and switched-hot only. That single fact forecloses reverse-phase dimmers entirely and brings with it both the unstable zero-cross flicker and the leakage-current glow. Meanwhile the load has fallen to a fraction of what it was, so drop-out becomes likely. And where the retrofit lands on old MLV transformers in a lobby cove, the wrong dimmer is not just disappointing, it is potentially damaging.

On low-voltage retrofits specifically, Lutron’s position is worth knowing: it is impossible to predict with certainty how a retrofit LED lamp will perform on an existing transformer. For LED tape they recommend a new dedicated constant-voltage driver rather than reusing an existing transformer. That is a supplier telling you not to reuse the thing you were hoping to reuse, which is usually a sign it is true.

Common questions

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