The Direct Answer: AC Fixtures vs. DC LED Arrays

If you are wiring 120V AC household fixtures like recessed cans or pendants, never wire them in series; you must wire them in parallel. Series wiring on mains voltage divides the line voltage across fixtures, resulting in dim lights, stalled internal drivers, and eventual failure. However, if you are building low-voltage DC LED arrays (such as custom COB strips, horticulture boards, or architectural linear fixtures), series wiring is mandatory when using a constant current (CC) LED driver. In a DC series string, the current remains identical across every LED chip, ensuring uniform brightness and preventing thermal runaway. The driver adjusts its output voltage automatically to push the set current through the entire series chain.

Circuit Impact Math: Inrush, Power Factor, and Voltage Compliance

When sizing the branch circuit breaker and selecting a driver for a series-wired DC string, you must calculate the total forward voltage ($V_f$) and account for AC-side inrush current and Power Factor (PF).

Calculating DC String Voltage

Suppose you are wiring ten Cree XLamp CXB3070 COB modules in series. Each module has a typical $V_f$ of 36V at 1.4A. The total string voltage requirement is 360V DC. Your constant current driver must have a DC output compliance window that encompasses 360V (e.g., a driver rated for 200V–400V DC output). If your string voltage exceeds the driver's maximum compliance voltage, the LEDs will not light. If it falls below the minimum, the driver will hiccup or shut down.

AC Inrush and Breaker Sizing

LED drivers utilize switching power supplies with large input capacitors, creating massive inrush currents upon startup. A 400W CC driver might draw a continuous current of just 1.75A at 240VAC (assuming a PF of 0.95), but it can pull 40A to 60A of inrush current for 200µs. While a standard 15A thermal-magnetic breaker will tolerate this microsecond spike, chaining too many drivers on one circuit causes cumulative inrush that trips the magnetic trip mechanism instantly.

Breaker Sizing Rule of Thumb: Limit the number of LED drivers on a single 15A branch circuit to a maximum combined continuous wattage of 1,440W (80% of 1,800W at 120V) or 2,880W at 240V. To prevent nuisance magnetic tripping from inrush, do not exceed four 400W drivers or eight 200W drivers per 15A breaker.

Lumens, Watts, and Efficacy in Series Strings

When designing a series string, you must balance drive current against luminous efficacy. Pushing higher current through the series chain increases total lumens but drastically reduces efficacy (lumens per watt) due to thermal droop. According to the U.S. Department of Energy Solid-State Lighting guidelines, operating LEDs below their maximum rated current extends lifespan and maintains high efficacy.

Table 1: Series String LED Module Equivalence and Efficacy (at 50°C Board Temp)
LED Module Type Forward Voltage ($V_f$) Drive Current Wattage Typical Lumens Efficacy (lm/W)
Cree CXB3070 (3000K) 36.0V 1.40A 50.4W 6,800 lm 134 lm/W
Cree CXB3070 (3000K) 37.5V 2.40A (Max) 90.0W 9,500 lm 105 lm/W
Bridgelux Vero 29 36.2V 2.10A 76.0W 9,200 lm 121 lm/W
Samsung LM301H (x100 string) 285.0V 0.15A 42.7W 11,000 lm 257 lm/W

Context: Notice how the Cree CXB3070 drops from 134 lm/W to 105 lm/W when pushed from 1.4A to 2.4A. In a series string, it is almost always more efficient to wire more modules in series and drive them at a lower current than to drive fewer modules at maximum current.

Dimmer Compatibility and the Flicker Fix

Flicker in series-wired LEDs occurs when the driver's internal smoothing capacitors cannot maintain the DC bus voltage during the chopped AC waveform, or when a leading-edge triac dimmer misfires because the LED load fails to meet the dimmer's minimum hold current.

Why Flicker Happens

Standard incandescent dimmers (leading-edge/triac) require a minimum load—often 40W to 60W—to keep the internal triac latched on during the AC cycle. If your series-wired LED string only draws 15W, the triac drops out before the zero-crossing, resulting in a strobe effect. Furthermore, the Lutron LED Dimming FAQ notes that incompatible driver electronics can cause audible buzzing and visible modulation.

The Fix: Trailing Edge and Min-Load Checks

To eliminate flicker on AC-dimmed series strings, follow these criteria:

  • Use a Trailing-Edge (ELV) Dimmer: These use MOSFETs instead of triacs, do not require high hold currents, and chop the trailing edge of the sine wave, which is gentler on LED driver input capacitors.
  • Verify Minimum Load: Check the dimmer's spec sheet. If the dimmer requires a 15W minimum and your string draws 12W, install a wirewound bleeder resistor in parallel with the AC input to make up the 3W difference.
  • Switch to 0-10V or PWM: For commercial or high-end residential builds, bypass AC phase-cut dimming entirely. Use a CC driver with a 0-10V analog dimming input and run a dedicated 16 AWG twisted-pair control wire to a low-voltage dimmer.

Heat and Enclosure Constraints for Constant Current Drivers

Constant current drivers generate heat, and their output capacity derates linearly as ambient temperature rises. A 300W driver mounted inside a sealed wooden valance where ambient temperatures reach 55°C (131°F) may thermally fold back to 60% output (180W) to protect its internal components.

Enclosure Sizing Rule: For every 100W of driver capacity, provide at least 15 square inches of exposed metal surface area for passive cooling, or ensure the enclosure has convective louvers at the bottom and top. Never mount a CC driver directly above the LED heat sink; the driver's ambient sensor will read the LED's waste heat and prematurely dim the circuit.

Additionally, the DC output wires carrying the series current must be sized correctly. While 18 AWG silicone wire is sufficient for a 2A series string over short runs, long architectural runs (over 15 feet) require 16 AWG or 14 AWG wire to prevent voltage drop from eating into the driver's compliance window.

The Decision Path: Sizing Your LED Series String

Use this decision matrix to select the exact driver and dimmer topology for your series-wired LED project. Do not guess; match your total string $V_f$ and wattage to the compliance window below.

Table 2: Series String Driver and Dimmer Selection Matrix
IF your series string has... THEN choose this CC Driver (120/240VAC) AND use this Dimming Topology
1–3 COBs (Max 100W, $V_f$ 36V–108V) Mean Well XLG-100-H-AB (54V-108V window) Lutron Diva DVELV-300P (Trailing Edge)
4–6 COBs (Max 200W, $V_f$ 144V–216V) Mean Well XLG-200-H-AB (142V-284V window) Lutron Diva DVELV-300P (Trailing Edge)
7–10 COBs (Max 300W, $V_f$ 252V–360V) Mean Well XLG-300-H-AB (213V-428V window) 0-10V Analog Dimmer (Run 16/2 shielded)
11+ COBs or High-Voltage Strip (400W+) Mean Well HLG-480H-C1400A (Constant Current) DALI or 0-10V Commercial Controller

Default Recommendation: If you are building a standard residential custom fixture using 4 to 6 high-output COB modules in series, buy the Mean Well XLG-200-H-AB. It features a built-in 3-in-1 dimming function (0-10V, PWM, and resistance), an IP67 rating for damp locations, and a compliance window perfectly suited for 144V to 216V DC series strings. Pair it with a Lutron trailing-edge ELV dimmer on the AC side, and ensure your LED heat sinks are bonded to ground using a green 12 AWG equipment grounding conductor to prevent shock hazards from driver isolation failure.