To properly circuit a LED lighting system, sum the maximum rated wattage of all fixtures, multiply by 1.25 to account for continuous load derating (per NEC 210.20), and select a trailing-edge (ELV) dimmer whose minimum load threshold is explicitly met by the total connected LED wattage. Unlike incandescent bulbs, LEDs rely on switched-mode power supplies (drivers) that introduce power factor losses and high inrush currents, meaning a 150W LED load behaves very differently on a breaker than a 150W resistive load.
Circuit Impact Math: Inrush, Power Factor, and Sizing
When you circuit a LED system, you cannot simply add up the wattages and divide by voltage. LED drivers are capacitive and non-linear loads. Two critical factors dictate your breaker and wire sizing: Power Factor (PF) and Inrush Current.
Power Factor (PF) and Apparent Power
Cheap, residential LED drivers often have a PF of 0.5 to 0.7. High-quality commercial drivers (like Philips Advance Xitanium or Hatch Pro) achieve a PF of 0.9 or higher. Your breaker and wires must be sized for apparent power (Volt-Amps, VA), not just real power (Watts).
- Formula: Apparent Power (VA) = Real Power (W) / Power Factor
- Worked Example: You are wiring 10 commercial downlights, each rated at 15W (150W total). If the drivers have a PF of 0.6, the apparent power is 150W / 0.6 = 250VA. On a 120V circuit, your steady-state current is 2.08A (250VA / 120V), not the 1.25A you would expect from a purely resistive load.
Inrush Current and Breaker Tripping
When an LED driver powers on, its internal input capacitors charge almost instantly, drawing a massive spike of current. This inrush can be 50 to 100 times the steady-state current for 1 to 3 milliseconds. A standard thermal-magnetic breaker's magnetic trip curve handles brief spikes, but if you parallel too many LED drivers on a single pole, the cumulative inrush will exceed the magnetic trip threshold (typically 5x to 10x the breaker rating) and trip the breaker the moment you flip the switch.
Lumens, Watts, and Thermal Constraints
It is a common mistake to look at a lumen table without considering efficacy and thermal derating. LEDs do not burn out like filaments; they experience lumen depreciation (L70/L90) accelerated by heat. If you circuit a LED fixture into an enclosed, insulated space without checking its thermal rating, the junction temperature will exceed 85°C, destroying the driver electrolytic capacitors and dropping light output by 20% or more.
| Fixture Type | Nominal Wattage | Output (Lumens) | Efficacy (lm/W) | Heat & Enclosure Constraints |
|---|---|---|---|---|
| Standard A19 Bulb | 9W - 11W | 800 lm | 72 - 88 lm/W | Must be rated 'Enclosed' if used in fully sealed globes; otherwise, heat traps and kills the base driver. |
| 6-inch IC-Rated Downlight | 12W - 15W | 900 - 1100 lm | 75 - 90 lm/W | Safe for direct insulation contact. Driver must be rated for 90°C ambient if placed in tight, insulated joist bays. |
| 4-foot Linear High Bay | 110W - 150W | 14,000 - 18,000 lm | 120 - 135 lm/W | Requires active heat sinking. Efficacy drops 10-15% if ambient room temp exceeds 45°C (113°F). |
| Under-Cabinet LED Strip | 4W per foot | 200 lm/ft | 50 lm/W | Must be mounted to an aluminum extrusion channel. Bare strips on wood will overheat and delaminate within a year. |
According to the U.S. Department of Energy's Solid-State Lighting program, thermal management remains the primary limiting factor in high-efficacy LED designs. Always verify the fixture's IC (Insulation Contact) rating and the maximum ambient temperature rating of the remote driver enclosure.
Dimmer Compatibility and Minimum Load Checks
Which dimmer and driver should you use for a specific fixture count? The answer depends entirely on the phase-cut waveform and the minimum load requirements. Most modern LED fixtures require Trailing-Edge (ELV / Electronic Low Voltage) dimmers, not the legacy Leading-Edge (TRIAC / Incandescent) dimmers found in older homes.
| Criteria | Leading-Edge (TRIAC / MLV) | Trailing-Edge (ELV / MOSFET) |
|---|---|---|
| Waveform | Cuts the front of the AC sine wave. | Cuts the back of the AC sine wave. |
| Best For | Magnetic transformers, incandescent, some screw-in LED bulbs. | Integrated LED fixtures, ELV transformers, constant-current drivers. |
| Min Load Requirement | Usually 25W - 40W. | Usually 10W - 25W (varies by model). |
| Noise / Buzzing | High risk of acoustic noise in LED drivers. | Whisper-quiet operation on compatible drivers. |
The Minimum Load Trap
Dimmers need a minimum amount of current to keep their internal semiconductors biased and functioning. If you circuit a single 9W LED bulb to a dimmer with a 15W minimum load, the dimmer's internal circuitry will starve, resulting in strobing, popping, or a failure to turn on.
The Fix: Before buying a dimmer like the Lutron Diva DVELV-300P, check the Lutron LED Compatibility Tool or the manufacturer's spec sheet. If your total connected LED wattage is below the dimmer's minimum, you must either add more fixtures to the circuit, install a dummy load resistor (like the Lutron LUT-MLC) in parallel at the fixture, or upgrade to a smart dimmer with a microprocessor that doesn't rely on load-leakage for power.
Why LED Flicker Happens (And How to Fix It)
Flicker occurs when the dimmer's phase-cut waveform and the LED driver's rectifier fail to communicate properly. The AC mains frequency is 60Hz (120 zero-crossings per second in the US). If the driver's internal smoothing capacitor is too small, or if the dimmer chops the waveform at an incompatible angle, the voltage drops below the driver's threshold on every half-cycle, causing the LEDs to flash at 120Hz.
Diagnostic Steps:
- Check the Dimmer Type: Ensure you aren't using a leading-edge TRIAC dimmer on a fixture that explicitly specifies ELV/Trailing-edge. Flip the dip-switches on the dimmer if it supports both.
- Adjust the Low-End Trim: Many flickering issues happen only at the bottom 10% of the dimmer slider. Use the dimmer's adjustment dial to raise the minimum light level until the flicker stops.
- Inspect the Neutral: A loose or shared neutral in a multi-wire branch circuit (MWBC) can introduce harmonic distortion that confuses the driver's zero-crossing detection. Tighten all neutral terminations to the manufacturer's specified inch-pound torque.
FAQ: Circuiting LED Systems
Can I circuit a LED strip directly to a 120V AC mains line?
No, not without a driver. While there are specialized 'AC-LED' strips that use surface-mount rectifiers directly on the flexible PCB, they are highly dangerous, lack isolation, and flicker terribly at 120Hz. For 99% of projects, you must circuit a LED strip to the low-voltage (12V or 24V DC) output of a Class 2 power supply. The 120V AC mains should only connect to the primary side of the LED driver, which must be housed in an accessible, code-compliant junction box.
How many LED recessed lights can I circuit on a single 15-amp breaker?
Under NEC Article 210.20, a 15A breaker can handle a maximum continuous load of 12A (80% rule). At 120V, that is 1,440VA. If your recessed lights draw 15W each with a PF of 0.9 (16.6VA each), the math allows for up to 86 fixtures. However, due to the cumulative inrush current mentioned earlier, practical jobsite limits cap this at roughly 15 to 20 individual drivers per 15A breaker to prevent the magnetic trip from engaging when the switch is flipped. Always prioritize the inrush limit over the steady-state thermal limit.
Why does my LED circuit glow faintly when the switch is off?
This 'ghosting' happens when a small amount of leakage current bypasses the switch. This is incredibly common if you are using an illuminated switch (which contains a tiny neon or LED indicator that completes a micro-circuit when off) or a smart switch that lacks a dedicated neutral wire. The leakage current slowly charges the LED driver's capacitor until it has enough energy to flash or glow. The fix is to replace the illuminated/smart switch with a standard mechanical toggle, or install a bypass resistor (like the LUT-MLC) at the first fixture to absorb the leakage current.
