How to Wire Lights in Series: The Safe 24V DC Method (And Why Mains Fails)

If you are searching for how to wire lights in series for standard 120V AC home lighting, stop immediately. Standard household lighting is universally wired in parallel. Wiring 120V AC fixtures in series violates the National Electrical Code (NEC), causes severe voltage drop, and creates a fire hazard if mismatched wattages are used.

However, series wiring is a highly practical, code-compliant technique in low-voltage DC systems—specifically when you need to run 12V DC LED lights off a 24V DC battery bank or solar charge controller without using a buck converter. In a series circuit, voltage divides equally across identical loads while current remains constant. By wiring two identical 12V LEDs in series on a 24V source, each light receives exactly 12V.

This guide details the exact procedure for wiring two 12V DC LED fixtures in series on a 24V DC system, including the necessary AC-to-DC power supply mains connections, wire colors at every termination, and multimeter verification steps.

The Physics of Series Wiring (And When to Use It)

Before stripping any wire, you must understand Kirchhoff’s Voltage Law. In a series circuit, the total source voltage equals the sum of the voltage drops across each component ($V_{total} = V_1 + V_2$).

• Parallel (Standard 120V AC Home Wiring): Voltage is constant across all loads (120V). Current adds up. If one bulb burns out, the others stay on.
• Series (24V DC to 12V Loads): Current is constant through the entire loop. Voltage drops across each load. If you put two identical 12V, 3W LEDs in series on a 24V source, each drops 12V. If one bulb fails open, the entire circuit goes dark.

According to the U.S. Department of Energy, low-voltage DC LED systems are highly efficient for off-grid and solar applications, but they require precise voltage matching to prevent thermal runaway.

Tools, Materials, and Device Ratings

Using the correct gauge and device rating is non-negotiable. For this build, we are using a 24V DC power supply fed by 120V AC mains, stepping down to power two 12V DC LEDs.

Item	Specification / Rating	Purpose
Power Supply	Mean Well LRS-35-24 (24V DC, 1.5A)	Converts 120V AC mains to 24V DC
LED Fixtures (x2)	12V DC, 3W LED Puck Lights	Identical loads to ensure equal voltage division
Wire (Mains Side)	14 AWG NM-B (Romex) or THHN	Handles 15A AC mains safely
Wire (DC Side)	14 AWG Stranded Copper (Red & Black)	Low voltage DC routing; 14 AWG provides mechanical strength
Connectors	Wago 221-412 Lever Nuts	Secure, vibration-resistant DC splices
Testing	CAT III Digital Multimeter	Verifying dead mains and DC voltage drops

Mains Safety & Circuit Preparation

Step-by-Step: Wiring Two 12V Lights in Series on a 24V DC System

Follow these exact termination steps. Wire colors are explicitly named for every connection to maintain proper polarity and code compliance.

Phase 1: AC Mains to Power Supply

1. Prepare the Mains Cable: Strip 3/4 inch of the outer NM-B jacket. Strip 1/2 inch of insulation from the Black (Hot), White (Neutral), and Bare (Ground) wires.
2. Ground Termination: Connect the Bare copper wire to the PE (Protective Earth) screw on the Mean Well power supply. This is a green-colored screw directly bonded to the metal chassis.
3. Neutral Termination: Connect the White wire to the N (Neutral) screw on the AC input terminal block.
4. Hot Termination: Connect the Black wire to the L (Line/Hot) screw on the AC input terminal block.
5. Secure: Torque the AC terminal screws to 0.5 Nm (approx. 4.4 in-lbs) and give each wire a firm tug to ensure the connection is solid.

Phase 2: DC Output to First Light

1. Prepare DC Wire: Cut a length of Red 14 AWG stranded wire. Strip exactly 11mm of insulation from both ends (the exact strip length for Wago 221 lever nuts).
2. Power Supply Termination: Insert one end of the Red wire into the V+ (Positive) terminal on the DC output side of the power supply and tighten the set screw.
3. Light 1 Termination: Open the Wago 221 lever nut at the first light fixture. Insert the other end of the Red wire into the lever nut alongside the Red pigtail wire coming from Light 1's positive (+) terminal. Clamp the levers down.

Phase 3: The Series Jumper (Light 1 to Light 2)

1. Prepare Jumper: Cut a short length of Red 14 AWG stranded wire to bridge the two fixtures. Strip 11mm on both ends. (We use Red here to maintain positive-polarity tracking through the series chain).
2. Light 1 Negative: Insert one end of the Red jumper wire into a Wago lever nut alongside the Black pigtail wire from Light 1's negative (-) terminal.
3. Light 2 Positive: Insert the other end of the Red jumper wire into a Wago lever nut alongside the Red pigtail wire from Light 2's positive (+) terminal.

Phase 4: Completing the Circuit (Light 2 to Power Supply)

1. Prepare Return Wire: Cut a length of Black 14 AWG stranded wire. Strip 11mm from both ends.
2. Light 2 Negative: Insert one end of the Black wire into a Wago lever nut alongside the Black pigtail wire from Light 2's negative (-) terminal.
3. Power Supply Return: Insert the other end of the Black wire into the V- (Negative) terminal on the DC output side of the power supply and tighten the set screw.

Verification and Testing (Don't Skip the Meter)

Never energize a series circuit without verifying the voltage drops. A wiring error here will instantly destroy your 12V LEDs.

1. Energize Mains: Turn the 120V AC breaker back on. The Mean Well power supply fan should spin briefly, and its green LED indicator should illuminate.
2. Set Multimeter: Turn your digital multimeter dial to DC Volts (DCV)
3. Test Source Voltage: Place the Red probe on the V+ terminal and the Black probe on the V- terminal. Expected Reading: 24.0V DC (±0.5V).
4. Test Light 1 Drop: Pierce or touch the probes across the Wago connectors at Light 1 (Red probe on the Red/Red nut, Black probe on the Red/Black nut). Expected Reading: 12.0V DC.
5. Test Light 2 Drop: Move the probes to Light 2 (Red probe on the Red/Red nut, Black probe on the Black/Black nut). Expected Reading: 12.0V DC.

If your readings show 24V across one light and 0V across the other, you have a short circuit or an open connection. De-energize and re-check the Wago lever nuts.

The Most Common Botch (And How to Spot It)

The Botch: Wiring the two 12V lights in parallel instead of series on the 24V DC source. This happens when a DIYer connects both Red pigtails to the V+ terminal and both Black pigtails to the V- terminal.

The Symptom: A loud pop, a distinct ozone/burnt plastic smell, and instant failure of both LED drivers.

The Physics of the Failure: In parallel, both 12V lights are exposed to the full 24V source. Because power equals voltage squared divided by resistance ($P = V^2/R$), doubling the voltage forces the LEDs to draw four times their rated power. A 3W LED will attempt to draw 12W, instantly overheating the internal constant-current driver and popping the LED die. Always verify your series daisy-chain topology with a meter before throwing the breaker.

Frequently Asked Questions

Can I wire 120V AC ceiling lights in series?

No. The National Electrical Code (NFPA 70) requires standard branch circuit lighting to be wired in parallel. If you wire two 120V bulbs in series on a 120V line, they will split the voltage (60V each) and glow dimly. Worse, if you mix wattages (e.g., a 60W bulb and a 100W bulb), the higher-resistance 60W bulb will absorb a disproportionate share of the voltage, overheat, and potentially cause a fire. Always use parallel pigtails for 120V AC fixtures.

What happens if one light burns out in a series circuit?

The entire circuit goes dark. Because a series circuit relies on a single continuous path for current flow, a burned-out LED or broken wire creates an "open circuit." Current drops to zero instantly, and the second light will turn off, even though it is perfectly functional. This is why series wiring is rarely used for critical task lighting, but is acceptable for decorative or non-critical off-grid DC setups.

How do I calculate the wire gauge for a series DC circuit?

In a series circuit, current is constant throughout the entire loop; it does not add up like it does in parallel. Two 3W 12V LEDs draw 0.25 Amps each. The total circuit current is exactly 0.25 Amps. Electrically, an 18 AWG wire (rated for ~14A in chassis wiring) is more than sufficient. However, 14 AWG stranded copper is specified in this guide because it provides superior mechanical strength, resists vibration fatigue in mobile/RV applications, and matches standard 15A DC fuse block terminal sizes without requiring ferrules or pigtails.