The Reality of Series-Wired Lighting: Theory vs. Modern Practice
A pure light bulb series circuit divides the source voltage across each load according to its resistance or impedance. If you wire two identical 120V/60W incandescent bulbs in series on a 120V line, each receives 60V, drawing roughly 25% of normal power and glowing dimly. In modern electrical practice, series wiring for standard mains branch circuits is obsolete. If one bulb burns out and opens the circuit, the entire string goes dark.
Today, series wiring survives almost exclusively in constant-current (CC) LED driver topologies, airport runway lighting, and specialized low-voltage landscape runs. In a CC LED string, the driver does not output a fixed voltage; it actively adjusts its output voltage (often between 30V and 100V DC) to push a precise, unvarying current (e.g., 1050mA) through every LED emitter in the series chain. Understanding the math, dimming constraints, and thermal limits of these series strings is critical for commercial and high-end residential lighting design.
Circuit Impact Math: Inrush, Power Factor, and Driver Topology
When designing a branch circuit to feed a series-wired LED string, you cannot simply divide total wattage by line voltage to size your breaker. The internal switching power supply (SMPS) of the constant-current driver introduces complex transient behaviors.
Inrush Current Calculations
CC drivers use large bulk electrolytic capacitors on their DC bus. When power is applied, these capacitors act as a dead short until charged. The peak inrush current ($I_{peak}$) is dictated by the peak line voltage ($V_{peak}$) and the equivalent series resistance ($R_{ESR}$) of the circuit:
I_peak = V_peak / R_ESR
For a 150W Mean Well HLG-150H driver on a 120V line, $V_{peak}$ is roughly 170V. With an internal and wiring $R_{ESR}$ of 0.5 ohms, inrush can spike to 340A for a few microseconds. If you wire ten of these fixtures on a single 20A breaker, the cumulative inrush will instantly trip a standard Type B thermal-magnetic breaker. You must specify a Type C or Type D curve breaker with a higher magnetic trip threshold to tolerate this microsecond surge without nuisance tripping.
Power Factor (PF) and Apparent Power
A cheap, non-PFC (Power Factor Correction) LED driver might operate at 0.6 PF. For a 150W real power load, the apparent power is 250VA. The current drawn from a 120V line is 250VA / 120V = 2.08A. Conversely, a high-PF (0.95) driver draws only 150W / (120V × 0.95) = 1.31A. Always size your branch circuit conductors based on the driver's maximum input current rating at the lowest specified input voltage, not the nominal output wattage.
Dimming Series LED Strings: Trailing Edge and Minimum Load Criteria
Dimming a constant-current series LED string requires matching the dimmer's phase-cut method to the driver's input topology. According to the U.S. Department of Energy's Solid-State Lighting guidelines, mismatched dimmers are the leading cause of premature LED driver failure.
Leading-edge dimmers chop the rising edge of the AC sine wave. This sudden voltage step causes the driver's internal EMI filter and bridge rectifier to ring, resulting in audible buzzing, dropped pulses, and eventual capacitor degradation. Always specify a trailing-edge (ELV) dimmer, which cuts the end of the sine wave, allowing the driver's input capacitor to charge smoothly.
Which Dimmer for Your Fixture Count?
The critical metric is the dimmer's minimum LED load, not its maximum incandescent rating. If you are wiring three 9W series-wired downlights (27W total), but your chosen ELV dimmer requires a 40W minimum load to keep its internal MOSFETs properly biased, the lights will strobe or drop out at low dim levels.
For a 27W LED load, select a dimmer like the Lutron Diva DVELV-300P, which has a dedicated low-end trim and a minimum LED load rating of just 15W. Always verify compatibility using the Lighting Facts database or the manufacturer's specific LED compatibility matrix before purchasing.
Lumens, Watts, and Thermal Constraints in Series Enclosures
In a series LED string, every emitter receives the exact same current. However, light output and heat generation are not perfectly linear. As drive current increases, LEDs experience "efficiency droop," where efficacy (lumens per watt) decreases, converting more energy into heat rather than photons.
| Module Wattage | Nominal Lumens | Efficacy (lm/W) | Heat Dissipation (BTU/hr) |
|---|---|---|---|
| 12W | 1,450 | 120.8 | 10.2 |
| 18W | 2,050 | 113.8 | 17.0 |
| 24W | 2,500 | 104.1 | 24.5 |
| 36W | 3,300 | 91.6 | 40.9 |
Heat and Enclosure Constraints
Because the current is identical across the series string, thermal management must be uniform. If one LED module in a poorly ventilated enclosure runs 10°C hotter than the others, its forward voltage ($V_f$) drops. While the constant-current driver maintains the total string current, the localized heat compounds. If the internal enclosure ambient temperature exceeds 45°C, the LED's lumen maintenance (L70 lifespan) plummets, and the phosphor layer degrades rapidly. Always use aluminum-core PCBs (MCPCBs) thermally bonded to an extruded aluminum heatsink, and ensure the enclosure has convective airflow paths.
Frequently Asked Questions
Why do my series-wired LED bulbs flicker and how do I fix it?
Flicker in a series-wired LED circuit usually occurs due to an incompatible dimmer phase-cut or driver dropout. When a leading-edge dimmer is used, the input voltage to the driver falls below its internal bulk capacitor's hold-up threshold during the chopped portion of the AC cycle, causing the driver to reset repeatedly. The fix: Replace the dimmer with an ELV trailing-edge model, ensure the total fixture wattage exceeds the dimmer's minimum LED load requirement, and adjust the dimmer's low-end trim screw to prevent the driver from dropping out at the bottom of the dimming range.
What happens to voltage and brightness if one bulb fails in a series circuit?
In a basic series circuit, an open failure (burnt filament or broken solder joint) breaks the entire loop, and all lights go dark. A short-circuit failure routes full voltage to the remaining bulbs, causing them to overvoltage and fail in rapid succession. Modern commercial series LED strings mitigate this by soldering open-circuit bypass Zener diodes across each LED emitter. If an LED fails open, the voltage across it rises until the Zener diode breaks down and conducts, bypassing the dead LED. The rest of the string stays lit, though the driver will lower its total output voltage slightly to maintain the constant current.
Can I wire standard 120V LED bulbs in series on a 240V line?
No. The internal switching power supply in a standard 120V A19 LED bulb is designed to accept 90-132VAC. Wiring two in series on a 240V line relies on their input impedances matching perfectly to divide the voltage 120V/120V. Because SMPS impedances are highly non-linear and vary dynamically with temperature and component tolerance, one bulb will inevitably hog the voltage. It will quickly exceed its 132V maximum limit, causing its internal metal oxide varistor (MOV) or bulk capacitor to fail catastrophically, often with a loud pop and smoke.
