The Myth of Fixed Resistance in LED Circuits
If you apply Ohm's Law (R = V / I) to a light-emitting diode, you will quickly hit a wall. Unlike a standard carbon-film resistor, an LED does not possess a fixed electrical resistance. Instead, it exhibits dynamic resistance, a non-linear value that shifts dramatically based on the forward voltage ($V_f$) applied and the junction temperature ($T_j$).
On a workbench, if you sweep voltage across a raw white LED, it draws virtually zero current until it hits its 'knee' voltage (typically around 2.7V to 3.1V). Once it crosses that threshold, the dynamic resistance plummets, and current spikes exponentially. For a typical 1W white LED operating at 350mA, the dynamic resistance ($r_d = \Delta V_f / \Delta I_f$) is often between 1 and 3 ohms. A mere 0.1V increase in supply voltage can force an extra 30mA to 100mA through the die.
This non-linear behavior is exactly why we abandon simple series resistors for general illumination and rely on Constant Current (CC) LED drivers. A CC driver continuously adjusts its output voltage to maintain a precise current, effectively compensating for the LED's shifting dynamic resistance as it heats up or ages. Attempting to drive high-power lighting LEDs with a Constant Voltage (CV) source and a current-limiting resistor wastes power as heat and leaves the circuit vulnerable to thermal runaway.
Lumens, Watts, and Efficacy: Sizing the Load
When designing a lighting branch circuit, you cannot simply add up the nominal wattages printed on the LED boxes. You must account for the driver's Power Factor (PF) to determine the true Apparent Power (VA) the circuit will draw. Furthermore, comparing lumens without looking at efficacy (lumens per watt) hides the true efficiency of the fixture.
The table below maps common incandescent baselines to modern LED equivalents, factoring in real-world efficacy and the VA load imposed on your breaker by a standard 0.7 PF driver.
| Fixture Baseline | LED Wattage | Typical Lumens | Efficacy (lm/W) | Driver VA (at PF 0.7) |
|---|---|---|---|---|
| 60W Incandescent | 9W (Standard A19) | 800 lm | 88 lm/W | 12.8 VA |
| 75W Incandescent | 11W (High-Efficacy A19) | 1100 lm | 100 lm/W | 15.7 VA |
| 100W Incandescent | 15W (Premium A19) | 1600 lm | 106 lm/W | 21.4 VA |
| 2x4 Fluorescent Troffer | 40W (LED Troffer) | 4400 lm | 110 lm/W | 57.1 VA |
| 400W Metal Halide | 150W (LED High Bay) | 21000 lm | 140 lm/W | 214.2 VA |
Dimmer Compatibility, Inrush, and the Flicker Fix
Dimming an LED is not as simple as chopping the AC sine wave. Because LED drivers present a highly capacitive load to the mains, pairing them with the wrong dimmer results in severe flickering, popped breakers, or destroyed driver electronics.
Which Dimmer and Driver for Your Fixture Count?
For standard residential and commercial LED retrofits, you must use a Trailing-Edge (ELV/IGBT) dimmer, such as the Lutron Diva DVCLV or Leviton Decora Smart ELV. Leading-edge (TRIAC) dimmers, designed for incandescent bulbs, require a minimum resistive load to keep the TRIAC latching during the AC cycle.
The Minimum Load Problem: If your dimmer requires a 40W minimum load, and you install three 9W LEDs (27W total), the dimmer will misfire. The result is a strobe-like flicker or the lights simply turning off at low dim levels. The Fix: Either swap to a dimmer with a low minimum load rating (e.g., 10W minimum) or install a dummy load resistor (like the Lutron LUT-MLC) in parallel at the first fixture to provide the missing wattage.
Circuit Impact Math: Inrush and Power Factor
LED drivers contain bulk electrolytic capacitors to smooth the rectified DC. When you flip the switch, these empty capacitors look like a dead short for a fraction of a millisecond, drawing massive inrush current.
- Steady-State Current: A 15W LED driver (PF 0.7) draws roughly 0.18A at 120V.
- Inrush Current: That same driver can pull 30A to 40A for the first 1ms.
- The Breaker Trip Hazard: If you wire 10 of these 15W drivers to a single 15A branch circuit, the combined instantaneous inrush could exceed 300A. A standard US thermal-magnetic breaker (like a Square D QO or Eaton BR) has a magnetic trip threshold typically set between 5x and 10x its rated current (75A to 150A). The 300A inrush spike will nuisance-trip the breaker instantly, even though the steady-state load is only 1.8A.
The Fix: Limit the number of LED drivers per 15A breaker to 4 or 5, or specify breakers with a higher magnetic trip curve (C-curve or D-curve) if permitted by your local Authority Having Jurisdiction (AHJ) and panel manufacturer.
Thermal Constraints and Enclosure Derating
The dynamic resistance of an LED is heavily dependent on temperature. LEDs have a negative temperature coefficient for forward voltage: as the junction temperature ($T_j$) rises, $V_f$ drops (typically by about 2mV per °C).
If an LED is driven by a rigid constant-voltage source, a 60°C rise in junction temperature drops the $V_f$ by 0.12V. Because the dynamic resistance is so low, that 0.12V drop causes the current to surge, which generates more heat, dropping the voltage further. This positive feedback loop is thermal runaway, and it will cook the LED die in minutes. This reinforces why constant-current drivers are non-negotiable for high-power fixtures.
Heat and Enclosure Constraints
While the LED chip gets the spotlight, the driver is usually the component that fails from heat. Electrolytic capacitors inside the driver dry out and lose capacitance when subjected to high ambient temperatures. Most commercial LED drivers are rated for a maximum ambient temperature ($T_a$) of 45°C to 50°C.
When installing LEDs in enclosed fixtures (like globes, recessed cans, or sealed high-bay housings), you must apply thermal derating:
- IC-Rated Recessed Cans: These are sealed to prevent air leakage into the attic. Ambient temperatures inside the can easily reach 55°C. You must use a driver specifically rated for enclosed/IC fixtures, or the driver's internal thermal protection will shut the light down after 20 minutes of operation.
- Derating Math: A standard rule of thumb for driver lifespan is that for every 10°C the ambient temperature exceeds the rated $T_a$, the expected life of the electrolytic capacitors is cut in half. A driver rated for 50,000 hours at 45°C will likely fail in 25,000 hours if trapped in a 55°C enclosure.
Understanding the non-linear resistance, true VA loads, and thermal limits of LED systems separates a frustrating, flickering retrofit from a commercial-grade installation that runs cool and dimms smoothly for a decade. Always verify your dimmer's minimum load against your total fixture wattage, and respect the inrush limits of your branch circuit breakers.
