If you are planning on wiring recessed lights in series on a standard 120V or 240V AC residential branch circuit, stop immediately. True series wiring for mains-voltage lighting is a functional failure and a severe code violation. Standard recessed can lights must be wired in parallel so each fixture receives the full nominal line voltage. However, series topology is the correct and highly efficient choice for low-voltage (12V/24V) constant-current LED puck lights and specialty architectural lighting. Below, we break down the topology realities, the exact failure modes of both configurations, and provide a complete engineering walkthrough for designing a low-voltage series recessed lighting string.
The Topology Reality: Mains Parallel vs. Low-Voltage Series
The confusion around wiring recessed lights in series usually stems from a misunderstanding of the term 'daisy-chaining.' When you run a single 14/2 NM-B cable from the switch to Light 1, then from Light 1 to Light 2, and so on, you are physically daisy-chaining the cable run, but electrically, the fixtures are wired in parallel.
• L1 (Hot): Connects to the Line-In of Fixture 1, and pigtails to the Line-In of Fixture 2.
• N (Neutral): Connects to the Neutral-In of Fixture 1, and pigtails to the Neutral-In of Fixture 2.
• G (Ground): Bonds to the metal housing of all fixtures and the junction box.
In this parallel layout, the voltage across every fixture remains exactly 120V (nominal), regardless of how many fixtures are on the run.
In a true series topology, the current has only one path. The AC Hot connects to Fixture 1 Line-In. The Fixture 1 Neutral-Out connects to Fixture 2 Line-In. The Fixture 2 Neutral-Out connects to the AC Neutral. If you wire six 120V recessed lights this way, the 120V source is divided equally among them. Each light receives only 20V. They will either glow dimly, flicker, or fail to trigger their internal LED drivers entirely. Furthermore, per NEC Article 410 and general electrical safety standards, fixtures must be rated for the circuit voltage applied to them, making series mains wiring illegal and dangerous.
Where does series wiring actually belong? It is the standard for low-voltage, constant-current (CC) LED arrays. By wiring bare LED chip-on-board (COB) modules or low-voltage puck lights in series, you ensure identical current flows through every diode, guaranteeing uniform brightness and maximizing driver efficiency.
Behavior Table & Failure Extremes: What Breaks When
To understand why parallel is mandatory for mains and how series behaves under stress, we must look at the failure modes. The table below contrasts a 6-light run of 120V 10W LED recessed fixtures (Parallel) against a 6-module string of 3W low-voltage LEDs (Series).
| Parameter | 120V Mains (Parallel Topology) | 24V Low-Voltage (Series Topology) |
|---|---|---|
| Normal Voltage per Element | 120V AC (Full line voltage) | ~3.0V DC (Forward Voltage, Vf) |
| Normal Current per Element | ~0.08A (Draws what it needs) | 350mA (Forced by CC driver) |
| Extreme: One Element Opens (Burns out) | Only that specific light goes dark. The rest stay on at 120V. | The entire string goes dark. Current path is broken. |
| Extreme: One Element Shorts Internally | Massive overcurrent. The 15A/20A branch breaker trips instantly. | The shorted module bypasses. The remaining 5 modules now share the driver voltage. Current remains 350mA, but driver output voltage drops to ~15V. The string stays lit, slightly dimmer. |
| Wire Sizing Requirement | 14 AWG or 12 AWG copper (per NEC ampacity tables) | 18 AWG or 20 AWG stranded (low current, focus on voltage drop) |
The critical takeaway is the short-circuit extreme. In a parallel mains circuit, a short causes a thermal-magnetic breaker trip, protecting the wire. In a constant-current series string, a short simply removes one LED from the circuit, and the driver automatically lowers its output voltage to compensate. This is why series is highly favored in aerospace and automotive lighting, but requires a specialized constant-current driver to work safely in home low-voltage applications.
Design Walkthrough: 24V Constant-Current Series Puck Lights
Let us design a functional series string for recessed architectural lighting, such as under-cabinet pucks or low-profile ceiling accents. We will use real component values to ensure the driver and LED forward voltages match perfectly.
Component Selection
- LED Modules: 6x Cree XLamp XP-G3 LED puck modules. At our target drive current of 350mA, each module has a typical Forward Voltage (Vf) of 2.95V.
- Total String Vf: 6 modules × 2.95V = 17.7V.
- LED Driver: Mean Well LCM-25 Constant Current Driver. This driver accepts 120V/240V AC input and outputs a selectable constant current. We will set the DIP switches to 350mA. The LCM-25 has an output voltage range of 2V to 28V DC.
Wiring the Series Nodes
Using 18 AWG stranded silicone wire for flexibility and heat resistance, map your nodes as follows:
- Driver V+ (Red): Connect to the Anode (A) of LED Module 1.
- Module 1 Cathode (K): Connect to the Anode (A) of LED Module 2.
- Repeat: Daisy-chain Cathode to Anode for Modules 3, 4, 5, and 6.
- Module 6 Cathode (K): Connect to the Driver V- (Black).
On the mains side, wire the LCM-25 AC input using standard 14 AWG THHN in conduit or 14/2 NM-B, connecting Line to L, Neutral to N, and Ground to the driver chassis.
How to Breadboard-Test a Low-Voltage Series LED String
Never seal low-voltage series recessed lights into drywall or cabinetry without a bench test. A single reversed polarity connection or cold solder joint will kill the entire string. Follow this step-by-step breadboard verification process.
- Prepare the Bench Supply: If using a benchtop power supply for testing instead of the final Mean Well driver, set it to Constant Current (CC) mode. Dial the current limit to exactly 0.35A (350mA). Set the voltage limit to 30V.
- Wire the Nodes: Using alligator clips or spring-terminals, connect the positive lead to the first Anode. Chain the Cathodes to Anodes. Connect the final Cathode to the negative lead.
- Power On and Observe: Turn on the supply. All 6 modules should illuminate instantly at identical brightness. If they flicker, you have a high-resistance connection (a loose clip) causing voltage instability.
- Measure Individual Vf: Set your multimeter to DC Volts. Probe across the Anode and Cathode of each individual module. You should read between 2.8V and 3.1V per module. If one reads 0.5V and the others read 3.2V, that specific module is defective and internally shorting.
- Thermal Runaway Check: Let the string run for 15 minutes. LED forward voltage drops as temperature rises (roughly -2mV/°C). Because you are using a Constant Current driver, the driver will automatically lower its voltage to maintain 350mA, preventing thermal runaway. If using a Constant Voltage (CV) supply with resistors, this is where the current would spike and burn the LEDs.
Frequently Asked Questions
Can I wire 120V LED recessed lights in series to save wire?
No. While running a single cable from light to light (daisy-chaining) saves wire compared to running a dedicated home-run cable to every single fixture, the electrical connections inside each junction box must be made in parallel. The incoming hot and the outgoing hot must be pigtailed together and connected to the fixture's hot lead. If you wire them in true series, the voltage will divide, the lights will not function, and you will violate NEC wiring standards.
Why do my series-wired 12V recessed lights get dimmer at the end of the run?
If your lights are getting progressively dimmer, they are likely not wired in true series; they are wired in parallel using a Constant Voltage (CV) 12V power supply and undersized wire. This is classic voltage drop. As current travels down the thin wire, resistance causes the voltage to sag, leaving the last fixtures with only 10V or 11V. To fix this, either increase the wire gauge (e.g., step up from 18 AWG to 14 AWG for the main trunk) or switch to a Constant Current (CC) driver with a true series topology, which forces the same current through every LED regardless of wire resistance.
What happens if one recessed light shorts out in a true series circuit?
In a properly designed constant-current series circuit, if one LED module fails short, the driver simply reduces its total output voltage to compensate for the missing forward voltage drop. The remaining lights stay on at the exact same brightness and current. However, if you are using a cheap, unregulated constant-voltage supply with series resistors, a shorted LED will cause the current to spike, potentially overheating the remaining components and causing a cascading thermal failure.
