The direct answer to LED which leg is positive comes down to physical geometry: on a standard through-hole LED, the longer leg is the positive anode, and the shorter leg is the negative cathode. If the legs have been clipped to the same length, look for the flat edge on the plastic epoxy lens base—that flat side always marks the negative cathode. Inside the bulb, the smaller internal post is the anode, while the larger, anvil-shaped flag is the cathode.
But identifying raw LED polarity is only the first step. When you scale up from a single 5mm component on a breadboard to a 120V/240V architectural lighting circuit, polarity translates into DC driver wiring, inrush current calculations, and dimmer compatibility. Here is the complete circuit-level guide to wiring, driving, and dimming LED fixtures without tripping breakers or destroying solid-state dies.
Identifying LED Polarity: Anode vs. Cathode Legs
LEDs (Light Emitting Diodes) are semiconductor devices that only allow current to flow in one direction. Applying forward voltage across the correct legs illuminates the phosphor; reversing it blocks current until the reverse breakdown voltage is exceeded, which permanently destroys the die.
Never rely solely on leg length for salvaged or clipped LEDs. Set your digital multimeter to the Diode Test mode (usually indicated by a diode symbol). Touch the red probe to one leg and the black probe to the other. If the LED lights up dimly and the meter reads a forward voltage drop between 1.8V (red) and 3.3V (blue/white), the red probe is touching the positive anode. If the meter reads 'OL' (Open Loop), swap the probes.
For Surface-Mount Device (SMD) LEDs (like 2835 or 5050 chips used in strip lights), there are no legs. Instead, look for a green dot, a small notch, or a chamfered corner on the component body—these always indicate the cathode (negative) side. Always verify with a multimeter before soldering down a reel of SMD tape.
Scaling Up: LED Drivers, Inrush Current, and Power Factor
You cannot wire raw LEDs directly to 120V AC mains. Integrated LED fixtures use constant-current DC drivers to step down the AC voltage and regulate the current. When designing the branch circuit for these fixtures, steady-state wattage is only half the story; inrush current and Power Factor (PF) dictate your breaker sizing.
Circuit Impact Math: Inrush Current
When an LED driver powers on, its internal bulk capacitors act like a dead short for the first few milliseconds. This creates an inrush current that can be 100 to 250 times higher than the steady-state operating current.
- Steady-State: A 150W LED high-bay fixture at 120V draws roughly 1.25A.
- Inrush: At a 200x multiplier, the inrush spike is 250A for ~1ms.
If you put ten of these on a standard 15A B-curve breaker, the combined 2,500A inrush spike will instantly trip the breaker's magnetic mechanism, even though the steady-state load is only 12.5A. The fix: Use C-curve or D-curve breakers for commercial LED circuits, which have higher magnetic trip thresholds (5-10x and 10-20x rated current, respectively) to tolerate inrush. Always check the driver datasheet (like those from Mean Well) for the exact 'Maximum Number of Units per Breaker' chart.
Power Factor (PF) and Apparent Power
Cheap, non-corrected LED drivers have a PF of 0.5 to 0.6. This means the driver draws significantly more apparent power (VA) than real power (Watts). A 100W fixture with a 0.5 PF draws 200VA, effectively doubling the current on the branch circuit wires. For commercial and residential main panels, specify drivers with Active Power Factor Correction (PFC) yielding a PF > 0.90 to prevent neutral conductor overloading in 3-phase wye systems.
Dimming LED Circuits: Trailing Edge and Minimum Load Rules
Flickering, strobing, and premature driver failure in dimmable LED circuits almost always trace back to a mismatch between the dimmer topology and the LED driver.
Why Flicker Happens and The Fix
Older leading-edge (TRIAC) dimmers were designed for 300W+ resistive incandescent loads. They chop off the front half of the AC sine wave. LED drivers, being highly capacitive and inductive, misinterpret this chopped wave, causing the internal MOSFETs to oscillate—resulting in visible 120Hz flicker or audible buzzing.
The fix: Use a trailing-edge (ELV / Electronic Low Voltage) dimmer, such as the Lutron Diva DVELV-300P or Leviton IPM06. These use IGBTs or MOSFETs to chop the back half of the sine wave, providing a smooth, zero-crossing turn-off that solid-state LED drivers can process cleanly. For detailed compatibility matrices, consult the Lutron LED Dimming Overview.
The Minimum Load Trap
Every dimmer requires a minimum electrical load to keep its internal semiconductors biased and functioning. A standard trailing-edge dimmer might require a 15W minimum load. If you install three 4W LED recessed downlights (12W total), the dimmer will starve, causing the lights to strobe or fail to turn off completely. Always sum the wattage of your fixtures and verify it exceeds the dimmer's minimum load rating. If it falls short, add a dummy load resistor (like a Lutron LUT-MLC) in parallel at the first fixture.
| Feature | Leading-Edge (TRIAC) | Trailing-Edge (ELV) |
|---|---|---|
| Waveform Chop | Front half of sine wave | Back half of sine wave |
| Best Load Type | Incandescent, Halogen, MLV | LED, CFL, ELV transformers |
| Typical Min Load | 25W - 40W | 10W - 15W |
| LED Flicker Risk | High (without dummy load) | Low (smooth zero-crossing) |
Thermal Constraints and Lumens-to-Watts Efficacy
LEDs do not emit heat in their light beam (radiant heat), but the semiconductor junction generates massive conductive heat. If the junction temperature exceeds 85°C, lumen output drops permanently, and the phosphor layer degrades. This makes enclosure constraints critical.
According to the U.S. Department of Energy Solid-State Lighting guidelines, modern commercial LEDs have pushed efficacy far beyond legacy sources. However, cramming a high-efficacy LED into a non-IC (Insulation Contact) rated airtight can without thermal relief will cook the driver.
| Fixture Type | Wattage | Lumens | Efficacy (lm/W) | Enclosure Constraint |
|---|---|---|---|---|
| Legacy Incandescent | 60W | 800 | 13 lm/W | Tolerates high ambient heat |
| Standard LED A19 | 9W | 800 | 88 lm/W | Do NOT use in enclosed glass globes |
| Commercial LED Panel | 40W | 4,400 | 110 lm/W | Requires open T-bar ceiling plenum |
| High-Bay LED (2026 Spec) | 150W | 27,000 | 180 lm/W | Heavy finned heatsink required |
Heat Fix: If you must install LEDs in fully enclosed, airtight, or IC-rated recessed fixtures, you must buy bulbs specifically rated for 'Enclosed Fixtures'. These use specialized high-temperature capacitors and derated phosphors that survive 60°C+ ambient environments inside the can.
Frequently Asked Questions
How do I tell which leg is positive on a surface-mount (SMD) LED?
SMD LEDs (like 5050 or 2835 packages) do not have legs. Instead, look for a visual indicator on the component body. A small green dot, a chamfered (cut) corner, or a silkscreen notch on the PCB footprint always indicates the cathode (negative) side. The opposite side is the positive anode. Because visual markers vary by manufacturer, always verify with a multimeter's diode test mode before reflow soldering.
What happens if I wire an LED circuit with reverse polarity?
If you wire a raw DC LED backward, it simply will not light up because the diode blocks reverse current. However, if the reverse voltage exceeds the LED's Reverse Breakdown Voltage (typically around 5V for standard indicator LEDs), the semiconductor junction will experience avalanche breakdown, permanently shorting or destroying the die. Integrated 120V AC LED fixtures have internal bridge rectifiers, so swapping the AC Line and Neutral wires won't destroy the fixture, but it creates a shock hazard at the socket shell and violates electrical code.
Why does my LED fixture glow faintly when the switch is off?
Ghosting or faint glowing occurs when a tiny amount of leakage current bypasses the switch and reaches the highly efficient LED driver. This is common when using smart switches (which require a neutral wire or trickle current to keep their internal WiFi/Zigbee radios powered) or when using older illuminated wall switches. The fix is to install a LUT-MLC (Minimum Load Capacitor) across the Line and Load at the fixture, or upgrade to a smart switch that utilizes a dedicated neutral wire to complete its control circuit without passing current through the load.
How many LED fixtures can I put on a 15-amp breaker?
Do not calculate this using steady-state wattage alone. While a 15A breaker at 120V can theoretically handle 1,800W (or 1,440W at the 80% continuous load rule), LED drivers have massive inrush currents. A standard 40W LED flat panel might draw 15A of inrush for 500 microseconds. If your 15A breaker is a standard B-curve, its magnetic trip threshold might be as low as 45A. Therefore, you can only safely wire three of those panels before the combined inrush trips the breaker at startup. Always check the driver manufacturer's 'Max Units per Breaker' chart and use C-curve breakers for commercial LED lighting panels.
