Wiring Lights in Series: DC LED Design vs AC Home Wiring Rules

The Short Answer: When to Wire Lights in Series (And When to Avoid It)

If you are wiring standard 120V or 240V AC home lighting (recessed cans, chandeliers, or wall sconces), never wire lights in series. The National Electrical Code (NEC) and basic AC circuit theory dictate that home branch circuits must be wired in parallel. Wiring AC mains lights in series will cause severe voltage drops, uneven illumination, and a total circuit failure if a single bulb burns out.

However, if you are designing low-voltage DC applications—such as 12V/24V landscape lighting, DIY LED fixtures, or internal LED strip strings—wiring lights in series is the correct and most efficient topology. Series wiring ensures identical current flows through every LED, preventing thermal runaway and eliminating the need for massive wire gauges.

Series Topology Explained: Nodes, Voltage Drops, and Current Flow

In a series circuit, there is only one path for current to flow. The current (I) is identical at every point in the loop, while the supply voltage (V) is divided among the components based on their individual resistance or forward voltage drop. For a deeper theoretical breakdown, All About Circuits provides an excellent primer on series DC networks.

Topology Node Map

Imagine a 24V DC string with three LEDs and one current-limiting resistor. Here is the exact node-to-node path:

• Node A (Source +): 24V DC positive terminal.
• Node B (Post-LED1): Voltage drops by LED1's forward voltage (e.g., 24V - 3.2V = 20.8V).
• Node C (Post-LED2): Voltage drops again (20.8V - 3.2V = 17.6V).
• Node D (Post-LED3): Voltage drops again (17.6V - 3.2V = 14.4V).
• Node E (Post-Resistor): The resistor burns off the remaining headroom (14.4V - 14.4V = 0V).
• Node F (Source -): Returns to the 24V DC negative terminal (Ground).

Behavior Table: What Changes When You Alter One Element?

Failure Modes: What Happens at the Extremes?

Understanding failure modes is why we don't use series wiring for critical home lighting. The Cree LED component datasheets detail how solid-state lighting fails, which dictates your circuit protection strategy.

The Open Circuit (Burned Out LED or Broken Wire)

If one LED in a series string fails open (the internal wire bond snaps), the single path for current is broken. Current drops to exactly 0A. Result: The entire string goes completely dark. This is the exact reason old-school Christmas lights were frustrating to troubleshoot, and why NEC-style home wiring mandates parallel paths for 120V fixtures.

The Short Circuit (LED Fails Short)

High-power LEDs often fail by melting internally and creating a short circuit across their anode and cathode. If LED2 fails short, its 3.2V voltage drop disappears. That 3.2V is now pushed across the remaining LEDs and the current-limiting resistor. Result: Current spikes. The remaining LEDs are overdriven, generating excess heat, which accelerates their degradation (thermal runaway). The resistor must now dissipate more power and may burn out if not sized with adequate overhead.

Design Walkthrough: Building a 24V DC Series LED String

Let's design a real-world 24V DC series string for a custom under-cabinet lighting fixture. We will use actual component values to ensure safe, reliable operation.

Component Selection

• Power Supply: Mean Well LRS-35-24 (24V DC, 1.46A max output).
• LEDs: 6x Cree XLamp XP-E2 (Cool White, 350mA nominal).
• Target Current: 350mA (0.35A) for optimal luminous efficacy and thermal management.

The Math

1. Calculate Total Forward Voltage (Vf): The Cree XP-E2 has a typical Vf of 3.2V at 350mA. For 6 LEDs: 6 × 3.2V = 19.2V.
2. Calculate Voltage Headroom: Supply Voltage - Total Vf = 24V - 19.2V = 4.8V. (This is the voltage the resistor must drop).
3. Calculate Resistor Value (Ohm's Law): R = V / I = 4.8V / 0.35A = 13.71 Ω. We select the next standard E12 value up: 15 Ω to slightly underdrive the LEDs for longer lifespan.
4. Calculate Resistor Power Dissipation: P = I² × R = (0.35)² × 15 = 0.1225 × 15 = 1.83W.

Final Bill of Materials (BOM)

Never run a resistor at its absolute maximum rating. A 2W resistor running at 1.83W will run too hot to touch and may scorch the PCB. Always double the wattage rating.

• Resistor Pick: Ohmite 5W 15Ω Wirewound Resistor (Part # 50F15RE). Mount it with leads elevated for airflow.
• Wire Gauge: 22 AWG stranded copper is rated for ~5A in free air, which provides a massive safety margin for our 350mA load.

Step-by-Step Breadboard and Bench Testing

Before soldering this into a permanent fixture, verify the behavior on the bench. Never trust theoretical math without physical verification.

1. Prep the Breadboard: Insert the 6 Cree LEDs (mounted on star PCBs) and the 15Ω 5W resistor in a single continuous loop. Ensure LED polarity (anode to cathode) is strictly observed.
2. Set the Multimeter to Current Mode: Move your DMM probe to the 10A (or mA) port. Break the circuit at Node A and insert the DMM in series to act as a temporary bridge.
3. Power On and Measure Current: Turn on the Mean Well 24V supply. The DMM should read between 0.32A and 0.36A. If it reads higher, your headroom is too low; power down and increase the resistor value.
4. Switch to Voltage Mode: Remove the DMM from the current path and reconnect the wire. Switch the DMM to DC Volts.
5. Probe the Nodes: Place the black probe on Node F (Ground) and the red probe on Node B. You should read ~20.8V. Move to Node C (~17.6V), and so on. Verify the resistor is dropping exactly the remaining voltage.
6. Thermal Check: Let the circuit run for 15 minutes. Carefully touch the body of the 5W resistor. It should be warm, but not hot enough to burn your skin (keep it under 60°C). If it's scorching, upgrade to a 10W chassis-mount resistor bolted to a heat sink.

Final Decision Matrix: Series vs. Parallel for Your Project

Stop guessing. Use this decision tree to lock in your topology based on your exact hardware constraints.

By matching the topology to the voltage source and load characteristics, you eliminate uneven dimming, prevent thermal failures, and ensure your wiring passes both bench testing and real-world longevity requirements.