When you swap a 60W incandescent bulb for a 9W LED, you aren't just dropping the wattage. You are fundamentally altering the impedance of the branch circuit. Understanding what an LED does to your dimmer and breaker requires looking past the steady-state wattage and examining the microsecond-level behavior of the internal Switch Mode Power Supply (SMPS) driver. This guide breaks down the exact circuit math, thermal constraints, and dimmer compatibility rules you need to specify the right components without blowing a TRIAC or dealing with strobing lights.

The Circuit Impact Math: Inrush and Power Factor

An LED is not a resistive load like a tungsten filament; it is a highly reactive capacitive load. The input stage of an LED driver contains a bulk electrolytic capacitor and a bridge rectifier. When the AC voltage crosses zero and the dimmer's TRIAC fires, that capacitor looks like a dead short for a fraction of a millisecond.

The Inrush Multiplier: A standard 15W LED bulb draws 0.125A at steady state (120V AC). However, its peak inrush current can hit 300A for 200 microseconds. If you put ten of these on a single 15A breaker circuit, the simultaneous inrush can spike to 3000A. This is why standard incandescent dimmers rated for "600W" instantly weld their internal TRIACs shut when loaded with 600W worth of LEDs.

Then there is Power Factor (PF). Cheap, non-PFC-corrected LED drivers often run at 0.5 PF. This means the apparent power (VA) is double the real power (Watts). If you wire 20 bulbs rated at 10W each (200W real power) with a 0.5 PF, the circuit actually draws 400VA. Your breaker sees 3.33A, not the 1.66A you calculated on the back of the box. Always size your dimmer and breaker based on the driver's VA rating, not just the LED chip wattage.

Lumens, Watts, and Efficacy: Sizing the Load

You cannot size a lighting circuit based on lumen output alone; you must account for driver efficacy and power factor. The Department of Energy's Solid-State Lighting guidelines emphasize that system efficacy (including driver losses) dictates the true thermal and electrical load. Below is a sizing reference that factors in real-world driver VA.

Incandescent Eq. LED Watts (Real) Lumens Efficacy (lm/W) Driver VA (at 0.9 PF)
40W 5W 450 90 5.5 VA
60W 9W 800 88 10.0 VA
75W 12W 1100 91 13.3 VA
100W 15W 1600 106 16.6 VA
150W 22W 2600 118 24.4 VA

Note: Never sum the "LED Watts" column to size a dimmer. Always sum the "Driver VA" column. A 300W-rated LED dimmer can typically only handle about 150VA of actual LED load due to inrush derating.

Why Flicker Happens and the Exact Fix

Flicker in LED circuits almost always traces back to a mismatch between the dimmer's minimum holding current and the LED driver's leakage current. Standard leading-edge (TRIAC) dimmers were designed for the massive resistive draw of incandescent bulbs. A TRIAC requires a minimum "holding current" ($I_H$) to stay latched ON during the AC half-cycle.

Because LEDs draw so little current, the current often drops below $I_H$ before the AC sine wave crosses zero. The TRIAC prematurely snaps off, then tries to fire again on the next trigger pulse. This rapid on-off-on cycling manifests as a 120Hz strobe effect. The IEEE 1789 standard explicitly outlines the health and safety risks of this high-frequency flicker.

The Minimum Load Trap: Most legacy dimmers require a 25W to 40W minimum load to operate. If you install three 9W LEDs (27W total), you are below the threshold. The fix is not to buy a bigger dimmer; it is to switch to a trailing-edge (ELV) dimmer or install a parallel bleed resistor (dummy load) to artificially raise the holding current.

The Fix: Trailing-edge (Electronic Low Voltage, or ELV) dimmers use MOSFETs instead of TRIACs. They switch off at the end of the cycle rather than the beginning, completely eliminating the holding current requirement and providing smooth, flicker-free dimming down to 1%.

Heat, Enclosures, and Driver Derating

LEDs do not emit heat forward through the beam; they dump it backward through the heat sink into the fixture enclosure. When you place an LED driver inside an IC-rated (Insulation Contact) recessed can, the ambient temperature ($T_a$) inside the can easily exceeds 45°C (113°F).

Every 10°C rise in operating temperature above the driver's rated $T_c$ (case temperature) cuts the lifespan of the internal electrolytic capacitors in half. If a driver is rated for 50,000 hours at 25°C ambient, running it at 55°C in an insulated ceiling drops its life to roughly 6,250 hours.

Constraints and Fixes:

  • IC-Rated Cans: Derate the driver's maximum load by 20% to compensate for thermal buildup, or use fixtures with remote driver trays that sit outside the insulation envelope.
  • Enclosed Globes: Never put standard non-enclosed-rated LED bulbs in sealed porch fixtures. The trapped heat will trigger the bulb's internal thermal foldback, causing the light to visibly dim as it struggles to survive.
  • Low-Voltage Runs: For 12V/24V constant voltage strips, mount the SMPS driver (like a Mean Well HLG series) in a ventilated junction box or utility space, running only the low-voltage side into the enclosed ceiling space.

Decision Tree: Picking Your Dimmer and Driver

Stop guessing based on the box art. Use this decision matrix to select the exact hardware for your fixture count and wiring topology.

Circuit Condition Required Action / Criteria Concrete Hardware Pick
Retrofitting 3 to 8 standard 120V LED screw-in bulbs on an existing 2-wire (no neutral) switch loop. Must use a trailing-edge ELV dimmer with an adjustable low-end trim to prevent drop-out. Min load check: Ensure total bulb VA exceeds 10VA. Lutron Diva DVELV-300P (Rated 300W ELV, handles low LED loads flawlessly without a neutral).
New construction: 10 to 15 recessed LED downlights with integrated 120V drivers on a 15A breaker. Calculate total inrush. Use a dimmer with a high peak inrush rating (not just steady-state wattage). Use 0-10V or PWM if fixtures support it. Lutron Maestro MACL-153M (Digital, handles high inrush multiplexing, up to 17 bulbs).
Running 24V LED strip lighting (under-cabinet or cove) totaling up to 60W. Do not dim on the 120V AC side. Dim on the 24V DC secondary side using a PWM decoder to prevent transformer hum and DC ripple. Mean Well HLG-80H-24 (Driver) paired with a Zigbee 24V PWM Controller.
Single high-output LED fixture (e.g., 150W shop light) where total load is below dimmer minimum. Install an LUT-MLC (Minimum Load Capacitor) across the fixture's line and load to provide artificial holding current for legacy TRIACs. Lutron LUT-MLC (Bypass capacitor) + any standard CL dimmer.
The Default Recommendation: If you are wiring a standard 120V residential retrofit with 3 to 8 integrated LED recessed cans or screw-in bulbs, and you want a guaranteed flicker-free result without running new neutral wires, buy the Lutron Diva DVELV-300P. Set the low-end trim dial to position 3, and your circuit will operate silently and smoothly. Always verify the manufacturer's LED compatibility matrix before pulling the wire.