To wire LED lights in true electrical series, you must use a constant-current DC driver where the cathode of one LED connects directly to the anode of the next, creating a single continuous loop. If you are wiring standard 120V AC recessed cans, under-cabinet pucks, or 12V DC LED strips, you are actually looking for a parallel "daisy-chain," not a series circuit. Wiring AC fixtures in true series is a severe fire and shock hazard. This guide provides the exact bench-tested procedure for wiring a true low-voltage DC series loop, while clarifying the AC parallel alternative so you don't botch your rough-in.
The 'Series' Confusion: True Series vs. Daisy-Chain Parallel
In electrical theory, a series circuit forces the exact same current through every component, while the voltage drops across each one. If one LED fails open, the entire string goes dark. This is how old-school incandescent Christmas lights worked.
In modern residential wiring, when DIYers ask "how to wire led lights in series," they almost always mean daisy-chaining. Daisy-chaining 120V AC fixtures means running Line to Line and Neutral to Neutral from one junction box to the next. Electrically, this is a parallel circuit. Every fixture receives the full 120V, and if one burns out, the rest stay on.
True series wiring is exclusively used in low-voltage DC architectural lighting driven by a constant-current power supply. Why use true series for DC? It prevents thermal runaway. In a parallel DC setup, if one LED gets hot, its resistance drops, it draws more current, gets hotter, and eventually burns out. In a constant-current series loop, the driver actively regulates the current (e.g., exactly 700mA). If an LED heats up, the driver simply lowers the voltage to maintain the 700mA limit, protecting the entire string.
Decision Tree: Which LED Wiring Method Do You Actually Need?
Before stripping any wire, identify your load. Use this decision matrix to pick your wiring topology and hardware.
| Your Lighting Load | Electrical Topology | Required Hardware | Verdict |
|---|---|---|---|
| 120V AC Recessed Cans / Puck Lights | Parallel (Daisy-Chain) | Standard 15A/20A Breaker, 14 AWG NM-B | Stop. Search for "how to daisy chain recessed lights". |
| 12V / 24V DC LED Strip Lights | Parallel | Constant-Voltage LED Power Supply | Stop. Wire + to + and - to -. |
| Architectural Constant-Current DC LEDs | True Series | Constant-Current Driver (e.g., Mean Well HLG-48H-C700B) | Proceed with this guide. |
Tools, Materials, and Sizing for a True Series DC Loop
Sizing a constant-current loop requires matching the driver's current output to the LEDs' rated forward current, and ensuring your total forward voltage stays within the driver's compliance window.
- Constant Current Driver: Mean Well HLG-48H-C700B (Output: 700mA, Voltage Compliance: 24V to 48V DC, Max Power: 33.6W).
- LED Modules: Series-compatible constant-current COB modules (e.g., Bridgelux or Cree rated for exactly 700mA). Do not use standard 12V LED strips.
- AC Input Wire: 14 AWG THHN (Black, White, Green) for a standard 120V 15A/20A branch circuit. 14 AWG is rated for 15A at 60°C, which is massive overkill for a 33W driver but required by NEC 240.4(D) for small conductors on standard breakers.
- DC Output Wire: 18 AWG stranded (Red, Black) for the low-voltage series loop. If your DC run exceeds 15 feet, bump to 16 AWG to prevent voltage drop from pushing you out of the driver's 48V compliance window.
- Connectors: Wago 221 3-conductor lever nuts for AC splices; ferrule crimps for the driver's screw terminals.
- Tools: Non-contact voltage tester, digital multimeter, wire strippers (14-18 AWG), precision screwdriver.
Mains Safety and Preparation
Once the circuit is verified dead, strip your 14 AWG AC wires to exactly 11mm (7/16 inch) for the Wago connectors, and crimp ferrules onto the ends of the wires that will land in the driver's screw terminals to prevent stray strands from causing a short.
Step-by-Step: Wiring the Constant Current Series Circuit
Follow these terminations exactly. A constant-current driver will aggressively ramp up its voltage to push 700mA through the circuit; a loose connection will arc and melt.
- AC Line Termination: Connect the 14 AWG Black (Line) wire from your junction box to the driver's L (Line) terminal. Torque the screw to 0.5 Nm.
- AC Neutral Termination: Connect the 14 AWG White (Neutral) wire to the driver's N (Neutral) terminal.
- AC Ground Termination: Connect the 14 AWG Green (or bare copper) ground wire to the driver's PE (Protective Earth) terminal or the metal chassis ground screw. This is non-negotiable for EMI shielding and shock protection.
- DC Output to First LED: Connect an 18 AWG Red wire to the driver's V+ terminal. Route this red wire to your first LED module and terminate it on the module's Anode (+) pad.
- Series Jumper 1: Connect an 18 AWG Black wire to the first LED module's Cathode (-) pad. Route this black wire to the second LED module and terminate it on the second module's Anode (+) pad.
- Series Jumper 2 (and beyond): Repeat the black wire Cathode-to-Anode jumpers for every subsequent LED in your string. Keep the wire lengths between modules as short and uniform as possible.
- Final DC Return: Connect an 18 AWG Black wire to the final LED module's Cathode (-) pad. Route this wire back to the driver and terminate it on the driver's V- terminal.
Verify and Test: Expected Meter Readings
Do not just flip the breaker and hope for the best. Use your digital multimeter to verify the driver is operating within its compliance window.
- AC Input Voltage: With the breaker on, measure across the L and N terminals. You should read between 114V and 126V AC. If it's outside this range, you have a feeder voltage drop issue.
- DC Open-Circuit Voltage: Before connecting the final LED return wire to the V- terminal, turn the system on and measure across the Red and Black DC wires. A constant-current driver will max out its voltage looking for a load. Expect to read 48V to 54V DC (the HLG-48H's maximum open-circuit limit). Do not touch the exposed wire ends; 54V DC can deliver a sharp tingle.
- Closed-Circuit Current: Turn the breaker off. Complete the final V- connection. Turn the breaker back on. Switch your multimeter to the 10A DC current setting, break the circuit at the final return wire, and measure the current in series. You must read exactly 0.70A (700mA) DC, give or take 5%.
- Closed-Circuit Voltage: Measure across the V+ and V- terminals while the LEDs are lit. The reading should be the sum of your LEDs' forward voltages (e.g., if you have four 9V COB modules, you should read roughly 36V DC). If this reading is pegged at 48V+ and the LEDs are dim, you have a high-resistance fault or an open connection in your series chain.
The Most Common Botch and How to Avoid It
The Botch: Wiring constant-voltage LEDs (like standard 12V LED tape light or off-the-shelf 12V puck lights) in series with a constant-current driver.
The Symptom: The lights either glow dimly, flicker violently, or don't turn on at all. In worst-case scenarios, the internal resistors on the 12V strips overheat and melt the silicone casing because the driver is forcefully pushing 700mA through components designed for a fraction of that current.
The Fix: You can only wire bare constant-current LED emitters or specialized series-wired modules in a true series loop. Standard 12V/24V LED strips already have internal parallel resistors and are designed for constant-voltage power supplies. If you bought 12V LED strips, abandon the series plan, buy a constant-voltage power supply (like a Mean Well LRS-150-12), and wire them in parallel.
By strictly matching your LED topology to your driver type and verifying your meter readings at the terminals, your series loop will run cool, flicker-free, and last for tens of thousands of hours.
