If you are wiring standard 120V or 240V AC mains fixtures, never use series wiring; you must wire in parallel. Wiring lighting in series on a mains circuit divides the voltage across the fixtures, resulting in dim or completely dead lights, and creates a single point of failure where one blown bulb kills the entire circuit. Series wiring is exclusively used in low-voltage DC applications, specifically with constant-current LED drivers powering raw LED COB or SMD chips. This guide breaks down the physics of why series mains wiring fails, the correct way to wire series LED strings, and how to solve the dimmer flicker that plagues modern lighting circuits.
The Physics of Series vs. Parallel Lighting Circuits
To understand why series wiring is forbidden on AC branch circuits, we have to look at Kirchhoff’s Voltage Law. In a series circuit, the supply voltage is divided among the loads. If you wire three identical 120V/60W incandescent bulbs in series on a standard 120V North American branch circuit, each bulb receives only 40V. They will glow a dim, useless orange. If you wire them on a 240V circuit, they receive 80V each—still under-driven and highly inefficient.
Furthermore, standard household breakers and wire sizing rely on parallel current addition. In parallel, a 15A breaker can safely handle roughly 1440W of continuous lighting load (120V × 15A × 0.80 continuous derating). In series, the current remains the same through all loads, but the resistance changes dynamically as filaments heat up or LED drivers attempt to draw power, leading to unpredictable breaker trips.
Circuit Impact Math: Inrush and Power Factor
When transitioning to the correct application of series wiring—low-voltage DC LED strings driven by a constant-current power supply—circuit math becomes critical for sizing your upstream AC components. Consider a 150W constant-current LED driver powering a series string of 36V LED modules.
- Inrush Current: When the AC mains is switched on, the driver’s internal bulk capacitors charge instantly. A 150W Mean Well HLG-150H driver can exhibit an inrush current spike of 45A for 2 milliseconds at 230V AC. While a standard 15A thermal-magnetic breaker will tolerate this brief magnetic spike, chaining too many drivers on a single 15A circuit will cause nuisance tripping. Rule of thumb: limit to four 150W drivers per 15A breaker to manage cumulative inrush.
- Power Factor (PF): A cheap, passive PFC LED driver might have a PF of 0.60. To deliver 150W of real power, it draws 250VA of apparent power (150W / 0.60). A high-quality driver with Active PFC (like the HLG series) boasts a 0.95 PF, drawing only 158VA. Always specify drivers with >0.90 PF to prevent oversizing your branch circuit conductors.
| Characteristic | Series Wiring (Mains AC) | Parallel Wiring (Mains AC) |
|---|---|---|
| Voltage at Fixture | Divided (e.g., 40V per fixture) | Full line voltage (120V/240V) |
| Failure Mode | Open circuit kills all fixtures | Only the failed fixture goes dark |
| NEC Compliance | Violates standard branch circuit design | Standard, code-compliant method |
| Wire Sizing | Unpredictable current draw | Predictable additive current |
Lumens, Watts, and Thermal Constraints in LED Strings
When wiring lighting in series on the DC side of a constant-current driver, you are connecting raw LED emitters. The forward voltage (Vf) of each emitter adds up, while the current remains constant. You must match the total Vf of your series string to the driver’s output voltage range.
Below is a spec-sheet-table of common high-power LED modules used in series strings, including their efficacy context. Efficacy (lumens per watt) drops significantly if the LEDs are overdriven or poorly cooled.
| LED Module | Forward Voltage (Vf) | Drive Current | Wattage | Lumens | Efficacy (lm/W) |
|---|---|---|---|---|---|
| Bridgelux Vero 18 | 36.2V | 1.05A | 38W | 4,200 | 110 lm/W |
| Cree CXB3590 | 36.0V | 1.05A | 38W | 4,800 | 126 lm/W |
| Samsung LM301H (x12 series) | 33.6V (2.8V x 12) | 0.20A | 6.7W | 1,100 | 164 lm/W |
Dimmer Compatibility and Flicker Fixes
While the LED emitters are wired in series on the DC side, the LED driver itself is wired in parallel on the AC mains side. This is where dimmer compatibility becomes a major pain point. Standard incandescent dimmers (leading-edge TRIAC) chop the AC waveform in a way that modern switching LED drivers cannot interpret cleanly.
Which Dimmer and Driver for Your Fixture Count?
For AC mains circuits controlling constant-current or constant-voltage LED drivers, you must use an ELV (Electronic Low Voltage) trailing-edge dimmer. Trailing-edge dimmers use MOSFETs instead of TRIACs, providing a cleaner cut-off that LED power supplies can process without misfiring.
The Minimum Load Trap: Dimmers require a minimum electrical load to keep their internal circuitry powered. The industry-standard Lutron Diva DVELV-300P requires a minimum of 15W of connected LED load. If you are wiring three 4W LED fixtures (12W total) to this dimmer, it will fail to operate correctly. If your total fixture wattage is below the dimmer's minimum threshold, you must either add more fixtures or choose a dimmer with a lower minimum load, such as the Lutron Skylark SELV-300P (which handles lower LED loads but still requires verification against the specific driver).
Why Flicker Happens and the Fix
The Symptom: Your LED lights strobe, flash, or shimmer when dimmed below 40%.
The Cause: The dimmer chops the AC waveform, but the LED driver’s internal smoothing capacitor holds voltage. This creates a mismatch in the dimmer’s zero-crossing detection circuit. The dimmer thinks the circuit is off, resets, and then turns back on, causing a visible strobe effect. According to the U.S. Department of Energy's Solid-State Lighting guidelines, this impedance mismatch is the leading cause of LED flicker in retrofits.
The Fix: Install a dummy load resistor (such as the Lutron LUT-MLC) in parallel with the lighting load at the first fixture. This device draws a tiny amount of real power (about 2W) purely to provide the minimum holding current the dimmer needs to stay latched on during the zero-crossing phase. It does not affect the brightness of the lights, but it instantly stabilizes the dimmer's MOSFET switching.
Frequently Asked Questions
Can I wire 12V LED landscape lighting in series?
No. 12V LED landscape fixtures are designed to operate in parallel. If you wire two 12V landscape fixtures in series on a 12V transformer, each fixture will only receive 6V and will not illuminate. The confusion arises because landscape wiring is often "daisy-chained" (running a main trunk line and tapping off to each fixture), which is physically a parallel circuit, even though the wire runs sequentially from one fixture location to the next. Always wire 12V/24V landscape fixtures in parallel to ensure each receives the full transformer voltage.
What happens if one bulb burns out when wiring lighting in series?
If a bulb burns out in a true series circuit, the filament breaks (or the LED driver fails open), creating an open circuit. Because there is only one path for current to flow in a series circuit, the current drops to zero instantly, and every single light on that string will go completely dark. This is the exact mechanism used in old-school Christmas tree lights, where finding the single burnt-out bulb was required to restore power to the rest of the string. This single-point-of-failure is why the National Fire Protection Association (NFPA) and NEC mandate parallel wiring for standard building illumination.
Is wiring lighting in series illegal under NEC code?
The National Electrical Code (NEC) does not explicitly use the phrase "series wiring is illegal." However, NEC Article 210 governs branch circuits and requires that equipment be operated at its rated voltage. Wiring 120V fixtures in series on a 120V circuit violates the requirement to provide rated voltage to the utilization equipment. Furthermore, series wiring prevents the proper operation of overcurrent protection devices and grounding schemes as intended by the code. Therefore, while the word "series" might not be explicitly banned in the text, the electrical results of series wiring on mains circuits inherently violate multiple NEC articles regarding voltage drop, equipment listing, and branch circuit design. Always defer to your local Authority Having Jurisdiction (AHJ) for final code compliance determinations.
