If you are searching for how to wire standard 120V mains ceiling lights or recessed cans in a series, stop immediately. Under National Electrical Code (NEC) guidelines, 120V branch circuit fixtures must be wired in parallel. Wiring 120V bulbs in series splits the voltage (giving each fixture 60V), causes severe dimming, and creates a single point of failure where one burnt-out bulb kills the entire circuit. It is a direct code violation and a fire hazard.

However, if you are asking how to wire low-voltage DC lights (like 12V or 24V LED strips) in a true electrical series to manage voltage drop over long runs, you are in the right place. True series wiring is exclusively used in low-voltage, constant-current (CC) DC lighting systems. This guide provides the exact bench-tested procedure for wiring a 24V DC LED series string, including the 120V AC mains connection to the power supply.

Series vs. Parallel Lighting: Where Series Actually Belongs

Before stripping any wire, you must understand the electrical topology. Most DIYers confuse "daisy-chaining" (which is electrically parallel) with true series wiring. In a true series circuit, the current remains identical through every component, while the voltage requirement adds up. Below is the data-dense breakdown of where series wiring is applicable and where it is strictly forbidden.

Circuit Topology Voltage Behavior Current Behavior Required Driver Type NEC / Code Status
True Series (Low-Voltage DC) Adds up (e.g., two 12V strips = 24V total) Constant (e.g., 700mA through all) Constant Current (CC) Compliant (NEC Article 411)
Daisy-Chain (Low-Voltage DC) Constant (12V at every strip) Adds up (700mA + 700mA = 1.4A) Constant Voltage (CV) Compliant (NEC Article 411)
Mains Branch Circuit (120V AC) Constant (120V at every fixture) Adds up based on fixture wattage N/A (Grid / Breaker) Mandatory Parallel (NEC Art. 210/410)
Mains Series (120V AC) Splits (60V per fixture) Constant N/A STRICTLY PROHIBITED

Tools, Materials, and Device Ratings

For this installation, we are wiring two 12V, 700mA constant-current LED strips in a true series to create a 24V, 700mA string. This is highly common in under-cabinet and cove lighting to minimize wire thickness and eliminate end-of-run voltage drop.

  • LED Driver: Mean Well APC-35-700 (35W, 700mA Constant Current, 9-48V DC output range).
  • AC Mains Wire: 14 AWG THHN (Black, White, Green) rated for 600V, 90°C. Sized for a standard 15A or 20A mains branch circuit.
  • DC Low-Voltage Wire: 18 AWG stranded copper (Red, Black) rated for 300V. Sized for the 700mA DC load with minimal voltage drop over a 15-foot run.
  • Connectors: Wago 221 3-conductor lever nuts (for mains splices), solderless DC quick-connectors for LED strips.
  • Testing Gear: Klein Tools NCVT-3 non-contact voltage tester, Fluke 117 True-RMS multimeter.

Mains Safety Protocol: De-Energize and Verify

⚠️ CRITICAL MAINS VOLTAGE WARNING: The LED driver requires a 120V AC hardwired connection. Working on live mains can cause fatal arc flashes or electrocution. Never rely solely on a wall switch to isolate power. Local AHJ (Authority Having Jurisdiction) regulations may require a licensed electrician for hardwired junction box connections.
  1. Locate and Kill the Breaker: Identify the 15A or 20A breaker controlling the lighting circuit at your main service panel. Switch it to the OFF position. Apply a breaker lockout/tagout device, or place a piece of electrical tape over the switch with a note reading "DO NOT TOUCH - WORK IN PROGRESS".
  2. Verify Dead with NCV Tester: Turn on your Klein Tools NCVT-3 and test it on a known live outlet first to verify the tool works. Then, scan the wall switch and the junction box where the driver will be installed. The tester must remain completely silent and unlit.
  3. Verify Dead with Multimeter: Remove the junction box cover. Set your Fluke 117 to AC Volts (V~). Place the black probe on the bare copper ground wire and the red probe on the black (hot) wire. The reading must be 0.00V. Repeat between the black and white wires. If you read anything above 0.5V, stop immediately; you have the wrong breaker or a backfed circuit.

Step-by-Step: Wiring the Mains Feed and DC Series Run

With the circuit verified dead, proceed with the terminations. We will wire the AC mains to the driver, and then wire the DC output in a true series topology across the two LED strips.

Phase 1: AC Mains to the LED Driver

  1. Strip the Mains Wires: Strip 1/2 inch of insulation from the 14 AWG Black (Hot), White (Neutral), and Bare/Green (Ground) wires coming from the junction box.
  2. Terminate Ground: Connect the bare/green mains ground wire to the driver’s green ground pigtail using a Wago 221 lever nut. If the driver has a metal chassis and a ground screw (marked ⏚), land a 14 AWG green pigtail directly onto that screw and torque it firmly.
  3. Terminate Neutral: Connect the 14 AWG White mains wire to the driver’s Blue AC Neutral pigtail (or the terminal marked N) using a Wago lever nut.
  4. Terminate Hot: Connect the 14 AWG Black mains wire to the driver’s Brown AC Live pigtail (or the terminal marked L) using a Wago lever nut.

Phase 2: True DC Series Wiring (Constant Current)

In a true series circuit, the current flows out of the driver, through the first light, directly into the second light, and back to the driver. The voltage requirement stacks.

  1. Driver to Strip 1 (Anode): Take an 18 AWG Red wire. Connect one end to the driver’s DC output terminal marked V+. Connect the other end to the positive (+ / Anode) pad of the first 12V LED strip.
  2. Strip 1 to Strip 2 (The Series Link): Take an 18 AWG Black wire. Connect one end to the negative (- / Cathode) pad of the first LED strip. Connect the other end to the positive (+ / Anode) pad of the second LED strip. This is the critical series bridge.
  3. Strip 2 to Driver (Cathode): Take a final 18 AWG Black wire. Connect one end to the negative (- / Cathode) pad of the second LED strip. Connect the other end to the driver’s DC output terminal marked V-.

Verification, Meter Readings, and Common Botches

Before closing up the junction box or applying adhesive to the LED strips, you must verify the circuit behavior. Re-energize the breaker and perform the following tests.

Expected Meter Readings

  • AC Input Voltage: Set multimeter to V~. Measure across the L and N terminals at the driver. Expected: 114V to 126V AC.
  • Total DC String Voltage: Set multimeter to V⎓. Place the red probe on the V+ terminal and the black probe on the V- terminal at the driver. Because you wired two 12V strips in series, the driver will automatically push enough voltage to overcome the forward voltage drop of both. Expected: 22V to 26V DC (depending on the exact forward voltage of the LED diodes at 700mA).
  • DC Current Verification: If your multimeter has a micro-amp/milli-amp loop, or if you use a DC clamp meter over the V+ wire, the reading must be exactly 700mA (0.70A). In a series circuit, current is identical at every point.

The Most Common Botch: Mismatching the Driver Topology

The single most frequent mistake DIYers make when attempting series wiring is using a Constant Voltage (CV) power supply instead of a Constant Current (CC) driver.

The Symptom: If you wire two 12V LED strips in true series but power them with a standard 12V Constant Voltage power brick, the lights will either fail to turn on entirely or glow incredibly dim.

The Physics: A 12V CV driver is physically incapable of pushing current through a series string that requires ~24V of forward voltage to operate. The driver hits its maximum voltage limit and current drops to near zero. Conversely, if you wire the strips in parallel but use a 700mA CC driver, the driver will attempt to force 700mA through a parallel topology, resulting in overvoltage faults, flickering, and thermal shutdown. Always match the driver topology (CC for series, CV for parallel) to your wiring method.

By strictly adhering to constant-current series topology for low-voltage runs, and maintaining standard parallel topology for 120V mains branch circuits, you ensure a safe, highly efficient, and code-compliant lighting installation.