To find the steady-state current of LED fixtures on an AC branch circuit, divide the total wattage by the line voltage, then divide by the driver's power factor (PF). The formula is I = P / (V × PF). For a 15W LED downlight on a 120V circuit with a 0.9 PF, the steady-state current is 0.138A. However, sizing breakers, wire, and dimmers requires factoring in inrush current and minimum load thresholds, not just steady-state math. Treating LEDs like simple resistive loads is the most common cause of tripped breakers and flickering lights in modern retrofits.
The Math Behind LED Circuit Current and Inrush
Unlike incandescent bulbs, LEDs require a driver to convert AC mains to DC. This driver introduces reactance, meaning the apparent power (VA) is higher than the real power (Watts). This ratio is the Power Factor (PF). High-quality, DLC-listed commercial drivers maintain a PF above 0.9, but cheap residential drivers can drop to 0.5 or 0.6. If you calculate current using only watts (I = P/V), you will undersize your circuit.
For example, ten 15W downlights with a 0.6 PF on a 120V circuit draw 2.08A steady-state, not the 1.25A you would calculate assuming a PF of 1.0. While 2.08A easily fits on a 15A breaker, steady-state current is only half the story.
The Inrush Current Problem
LED drivers contain large bulk input capacitors to smooth the rectified AC waveform. When you flip the switch, these empty capacitors act like a dead short for a few milliseconds, drawing massive inrush current—often 100 to 300 times the steady-state current.
To prevent nuisance tripping, limit the number of LED fixtures per breaker based on the manufacturer's inrush data. As a rule of thumb for standard thermal-magnetic breakers, keep the total steady-state load below 80% of the breaker rating, and ensure the combined inrush current does not exceed the breaker's magnetic trip threshold (typically 5x to 10x the rated current for standard B/C curves).
Lumens, Watts, and Efficacy: Sizing the Load
When planning a lighting layout, you must translate required lumens into electrical load. Efficacy (measured in lumens per watt, or lm/W) dictates how much current a fixture will draw to produce a specific light output. Higher efficacy means less heat generated and lower current draw on your branch circuit. According to the U.S. Department of Energy's Solid-State Lighting program, modern commercial LEDs routinely exceed 120 lm/W, while budget residential bulbs hover around 70-80 lm/W.
| Fixture Type | Nominal Watts | Lumens | Efficacy (lm/W) | Steady Current (A) |
|---|---|---|---|---|
| 4" Recessed Downlight | 10W | 800 lm | 80 lm/W | 0.092A |
| 6" High-Lumen Downlight | 15W | 1200 lm | 80 lm/W | 0.138A |
| 4ft LED Tube (Type A/B) | 18W | 2000 lm | 111 lm/W | 0.166A |
| High-Bay UFO Fixture | 150W | 21000 lm | 140 lm/W | 1.388A |
Always check the spec sheet for the exact PF and wattage. A '15W' fixture might actually draw 17W at maximum dimming or due to driver inefficiencies. Size your wire and breaker using the nameplate VA or maximum wattage, not the marketing equivalent wattage.
Dimmer Compatibility and Minimum Load Constraints
Dimming LEDs is notoriously finicky because standard dimmers were designed for resistive incandescent loads. To dim LEDs reliably, you must match the dimmer topology to the driver type and respect minimum load requirements.
Trailing-Edge vs. Leading-Edge
Standard TRIAC (leading-edge) dimmers chop the beginning of the AC sine wave. This causes a sudden voltage spike that confuses LED drivers, resulting in audible buzzing and reduced driver lifespan. For phase-cut dimming, always specify trailing-edge (ELV) dimmers, such as the Lutron Diva DVELV-300P or Leviton ATELV-300. These chop the end of the sine wave, providing a smooth voltage ramp-down that solid-state drivers can track cleanly. For commercial 0-10V dimming, you must use a dedicated 0-10V dimmer paired with a 0-10V compatible driver, as outlined in the NEMA SSL 7A standard.
The Minimum Load Trap and Flicker Fixes
Every dimmer requires a minimum wattage to keep its internal MOSFETs or TRIACs properly biased. If you install three 10W LED downlights (30W total) on a dimmer with a 40W minimum load, the circuit will misfire. Symptoms include lights that won't turn on at the lowest setting, strobing, or severe flickering.
The Fix: If your fixture count falls below the dimmer's minimum load, you have two options. First, swap to a low-min-load dimmer (some modern ELV dimmers support loads as low as 5W). Second, install a dummy load resistor (like the Lutron LUT-MLC) in parallel with the first fixture. This resistor draws just enough phantom current to keep the dimmer's internal circuitry stable without generating noticeable heat.
Thermal Derating and Enclosure Constraints
LEDs themselves run cool, but their drivers generate significant heat. The lifespan of the electrolytic capacitors inside an LED driver is governed by the Arrhenius equation: for every 10°C increase in ambient temperature above the rated baseline (usually 25°C), the capacitor lifespan is cut in half.
When installing drivers in enclosed junction boxes, recessed cans, or tight ceiling plenums, ambient temperatures can easily exceed 45°C. This thermal bottleneck causes the driver to throttle current, resulting in dimmer light output, or it leads to premature catastrophic failure.
- IC-Rated Fixtures: Ensure the entire housing is IC (Insulation Contact) rated if it will be buried in attic blown-in insulation. This ensures the thermal management system is designed for zero airflow.
- Remote Drivers: For high-wattage fixtures (like 150W high bays or 60W architectural linear pendants), use remote-mounted drivers. Place the driver in an accessible, ventilated junction box or attic space, and run low-voltage DC or 0-10V control wires to the fixture head.
- Junction Box Sizing: Never cram multiple 15W integrated drivers into a standard 4x4 junction box. Use deep 4x11/16 boxes or dedicated driver enclosures to allow for heat dissipation and NEC box-fill compliance.
Frequently Asked Questions About LED Current
What is the typical current of LED strip lights per meter?
The current draw depends on the strip's voltage and wattage density. A standard 24V, 14.4W/meter high-density strip draws 0.6A per meter (14.4W / 24V). A full 5-meter reel draws 3.0A. When sizing the 12V or 24V DC feed wire and the power supply, always add a 20% overhead buffer. For a 3.0A strip, use a power supply rated for at least 3.6A (roughly 86W) and wire the DC side with minimum 14 AWG to prevent voltage drop, which causes the end of the strip to dim and shift color temperature.
How do you measure the true current of LED bulbs with a multimeter?
Standard AC clamp meters struggle to read the low, non-sinusoidal current drawn by small LED bulbs (often under 0.1A) and will display inaccurate readings due to low power factor. To measure true current and power consumption, plug the fixture into a true-RMS power meter (like the P3 Kill A Watt or a Fluke power quality analyzer). This will give you exact Watts, VA, and PF. If you must use a clamp meter, use a high-precision model with a dedicated low-current scale (like the Fluke 376 FC with the iFlex loop) and ensure you are measuring true-RMS, not average-responding.
Why does the current of LED fixtures spike when turned on?
This spike is inrush current, caused by the rapid charging of the bulk input capacitors inside the LED driver when AC voltage is first applied. High-quality drivers include an NTC (Negative Temperature Coefficient) thermistor or an active inrush-limiting circuit to throttle this spike. If you are experiencing tripped breakers specifically at the moment of switch-on, your fixtures likely lack adequate inrush limiting. The fix is to split the fixtures across multiple breakers or install an inrush current limiter (ICL) relay at the panel.
