To wire a lighting circuit for modern LED recessed downlights, you need 14 AWG copper wire on a 15-amp breaker (or 12 AWG on a 20-amp breaker). However, calculating the actual circuit capacity requires accounting for LED driver power factor and inrush current, not just the printed wattage on the box. Always de-energize the panel, lock out the breaker, and verify the wires are dead with a non-contact voltage tester and a multimeter before touching any conductors. Local AHJ (Authority Having Jurisdiction) codes always supersede general NEC-style guidance.
Sizing the Branch Circuit: Wire, Breaker, and Inrush Math
Standard residential lighting circuits use 14 AWG NM-B cable protected by a 15A breaker, which provides an ampacity of 15A at the 60°C temperature column per NFPA National Electrical Code (NEC) Table 310.16. While LED fixtures draw very little steady-state current, the real bottleneck when you wire a lighting circuit for LEDs is inrush current and power factor (PF).
LED drivers are switched-mode power supplies. Residential drivers often have a PF between 0.6 and 0.9, meaning the apparent power (VA) drawn from the circuit is higher than the real power (Watts). Furthermore, the initial charging of the driver's internal capacitors creates an inrush current spike that can be 50 to 100 times the steady-state draw for a few milliseconds.
Imagine wiring 15 recessed LEDs rated at 10W each.
Steady-State: 15 fixtures × 10W = 150W. At 120V, that is just 1.25A.
Apparent Power (PF = 0.7): 150W / 0.7 = 214 VA. The circuit actually sees ~1.78A. Still well under 15A.
Inrush Current: If each driver pulls a 40A inrush spike for 500 microseconds, 15 fixtures switching on simultaneously can create a cumulative spike exceeding 300A. A standard 15A thermal-magnetic breaker has an instantaneous magnetic trip threshold of roughly 5x to 10x its rating (75A–150A). That 300A spike can nuisance-trip the breaker the moment you flip the switch.
The Fix: Limit a single 15A lighting circuit to 12–15 LED downlights. If you must wire more on a single switch leg, stagger the switch legs, use fixtures with integrated soft-start drivers, or upgrade to a 20A breaker with 12 AWG wire to raise the magnetic trip threshold.
Lumens, Watts, and Efficacy Context
When selecting fixtures to wire into your circuit, ignore "incandescent equivalent" marketing claims and look at efficacy (lumens per watt, or lm/W). Higher efficacy means less wasted energy converted into heat inside the ceiling cavity. According to the US Department of Energy Solid-State Lighting program, modern high-efficacy residential LEDs routinely exceed 110 lm/W, while cheaper, standard models hover around 80 lm/W.
| Target Lumens | Incandescent Eq. | High-Efficacy LED (>110 lm/W) | Standard LED (~80 lm/W) | Heat Output (BTU/hr)* |
|---|---|---|---|---|
| 800 lm | 60W | 7.2W | 10.0W | 24 – 34 BTU |
| 1100 lm | 75W | 10.0W | 13.7W | 34 – 47 BTU |
| 1600 lm | 100W | 14.5W | 20.0W | 49 – 68 BTU |
*Heat output calculated based on the wattage delta between the LED draw and standard efficacy baselines, assuming 85% of driver inefficiency is dissipated as heat into the enclosure.
Dimmer Compatibility, Minimum Loads, and Flicker Fixes
Standard incandescent dimmers (leading-edge TRIAC) chop the front half of the AC sine wave. LED drivers require a smooth, trailing-edge (ELV) dimming curve to properly interpret the chopped signal without damaging their internal rectifiers. When you consult the Lutron LED Compatibility Guide, you will see that mismatched dimmers are the root cause of 90% of LED performance issues.
Which Dimmer/Driver for This Fixture Count?
For a standard residential circuit with 10 to 12 downlights (roughly 90W–120W total load), use a trailing-edge ELV dimmer rated for at least 150W LED capacity, such as the Lutron Diva DVCLV or Leviton Decora Smart LED. Ensure the fixture's internal driver is explicitly marked "ELV Dimmable" or "Phase-Cut Dimmable." If you are wiring commercial 0-10V fixtures, you must run a separate 2-wire low-voltage control line alongside your 14 AWG line voltage.
The Minimum Load Trap
Many smart dimmers and heavy-duty ELV dimmers require a minimum load of 15W to 25W to keep their internal logic circuits powered. If you wire a lighting circuit to a powder room with only two 6W LEDs (12W total), the dimmer will drop out, strobe, or fail to turn off completely. Always verify the dimmer's minimum load spec against your total connected fixture wattage before purchasing.
Why Flicker Happens and the Fix
Symptom: LEDs flicker at the bottom 10% of the dimming range, or exhibit "ghosting" (a faint glow) when the switch is fully off.
Cause: The dimmer is bleeding a tiny amount of leakage current through the circuit to power its internal Wi-Fi radio or nightlight, and the highly sensitive LED driver is attempting to turn on with that micro-current.
The Fix: Install a bypass resistor/capacitor (like the Lutron LUT-MLC) across the line and load terminals at the first fixture in the circuit. This provides a path for the leakage current to bypass the LED driver entirely.
Heat, Enclosures, and IC-Rating Constraints
Unlike incandescent bulbs that radiate heat outward as infrared light, LEDs project heat backward into the fixture's heat sink and driver enclosure. Managing this thermal load is critical when wiring recessed cans.
- IC-Rated vs. Non-IC: If the ceiling cavity contains blown-in insulation or batts, the fixture MUST be IC-rated (Insulation Contact). Non-IC fixtures require a strict 3-inch clearance from all combustible materials and insulation to prevent fire hazards.
- Thermal Throttling: If you wire high-lumen (1000+ lm) fixtures into small, enclosed airtight cans, the driver temperature can exceed 85°C. Quality drivers will thermally throttle (dim themselves) to prevent catastrophic failure, but this shortens the lifespan of the electrolytic capacitors inside.
- Remote Drivers: For high-output architectural lighting in tight enclosures, wire fixtures that utilize remote driver boxes. This allows you to mount the heat-generating driver in an open joist bay while running low-voltage DC to the slim LED puck in the ceiling.
Frequently Asked Questions
Can I wire a lighting circuit using 12 AWG wire on a 15A breaker?
Yes, the NEC allows oversizing wire on a smaller breaker (12 AWG on 15A is perfectly legal and safe). However, 12 AWG is significantly stiffer and harder to fold into a single-gang switch box, especially when adding the bulky wire leads of a smart dimmer. Unless you are running a circuit longer than 75 feet and need to mitigate voltage drop, stick to 14 AWG NM-B for standard 15A lighting circuits to make termination easier.
How many LED recessed lights can I put on one 15 amp breaker?
While the NEC limits continuous loads (on for 3+ hours) to 80% of the breaker rating (12A), residential lighting is rarely classified as a continuous load. Theoretically, you could wire fifty 10W LEDs (500W / 4.1A) on a 15A breaker. Practically, limit the circuit to 12 to 15 fixtures. This keeps the cumulative inrush current below the breaker's magnetic trip threshold and prevents a single short circuit from plunging your entire floor into darkness.
Why do my new LED lights glow faintly when the wall switch is turned off?
This "ghosting" is almost always caused by an illuminated wall switch (a switch with a built-in neon locator light) or a smart switch drawing standby power through the load. The tiny trickle of current passes through the LED driver, charging its capacitors until it has enough energy to flash or glow. To fix it, replace the illuminated switch with a standard toggle, or wire a LUT-MLC bypass capacitor in parallel with the fixture at the ceiling box.
Do I need to use a specific wire color for the switch leg?
When wiring a switch loop, the NEC requires the white wire traveling down to the switch to be permanently re-identified as a hot conductor (usually wrapped in black or red electrical tape). This white wire brings unswitched line voltage to the switch, and the black wire returns the switched hot up to the light fixture. Never use the bare copper ground or green wire as a current-carrying conductor under any circumstances.
