Walk into a lighting aisle in the USA today and you will see a wall of LED bulbs. The old 60-watt incandescent bulb is mostly gone, replaced by LEDs that use far less power and last far longer. But the boxes now talk about lumens and kelvins instead of watts. This post explains what those numbers mean, how an LED lamp is built, and what the NEC says about home lighting.
What Is an LED?
LED stands for light-emitting diode. It is a semiconductor device that produces light when current flows through it in one direction. There is no filament to heat: the light comes directly from the semiconductor junction, so most of the energy becomes light, not heat.
The Parts of an LED Lamp
The LED chips. One or more semiconductor chips that produce the light, mounted on a circuit board inside the lamp.
The driver. A small power supply that converts 120-volt AC into the low-voltage DC the chips need.
The heat sink. The finned metal base of the lamp. LEDs do not like heat, and the heat sink pulls heat away from the chips.
The diffuser. The frosted plastic dome that spreads the light evenly and hides the individual chips.
Diagram: Cross-section of an LED lamp: diffuser, LED chips, driver and heat sink.
Lumens: The New Way to Measure Brightness
Watts measure how much power a bulb uses, not how much light it gives. Lumens measure the light itself. Because an LED makes far more light per watt than an incandescent bulb, the watt number on the box no longer tells you how bright the lamp is. The lumen number does.
A rough guide: about 450 lumens replaces a 40-watt bulb, 800 lumens replaces a 60-watt bulb, 1100 lumens replaces a 75-watt bulb, and 1600 lumens replaces a 100-watt bulb. An LED giving 800 lumens uses only about 9 to 12 watts. Check the Lighting Facts label on the box for lumens, energy cost, and lamp life.
Colour Temperature: Warm, Neutral or Cool?
Colour temperature is measured in kelvins (K) and describes whether the light looks warm or cool. Low numbers are warm and yellow, high numbers are cool and blue-white. 2700 K is warm white, like an old incandescent bulb, and suits living rooms and bedrooms. 3000 K is a slightly whiter warm light, common in kitchens and bathrooms. 4000 K is neutral white, used in offices and garages. 5000 K and above is daylight, which looks blue-white and suits workshops and task lighting.
Diagram: The colour temperature scale, from 2700 K warm white to 5000 K daylight.
Dimming an LED
Not every LED lamp can be dimmed, and not every dimmer works with LEDs. The box must say dimmable. A non-dimmable LED on a dimmer will flicker, buzz or fail early.
Most home dimmers are TRIAC, or phase-cut, dimmers made for incandescent bulbs. They work by chopping part of each AC cycle. Many dimmable LEDs are designed to work with them, but the pair must be compatible, so check the lamp maker's dimmer compatibility list.
NEC Rules for Lighting Outlets
Under NEC Article 210.70(A)(1), every habitable room, kitchen and bathroom must have at least one wall-switch-controlled lighting outlet, with the switch at the usual point of entry, so you are not walking into a dark room.
Article 410 covers luminaires: they must be listed, installed to the maker's instructions, and kept clear of combustible material. Recessed, or can, lights in a ceiling must be the IC-rated type if they touch insulation; a non-IC can must stay clear of insulation, because trapped heat is a fire risk.
Key Takeaways
An LED is a semiconductor that makes light directly, with a driver, heat sink and diffuser around it. Brightness is measured in lumens, not watts: 800 lumens replaces a 60-watt bulb. Colour temperature in kelvins sets the mood, from 2700 K warm white to 5000 K daylight. Only dimmable LEDs belong on a dimmer. The NEC requires a switch-controlled lighting outlet in every habitable room.
Objective Questions
1. What does the lumen rating of a lamp measure?
a) How much power the lamp uses
b) How much light the lamp gives out
c) How hot the lamp gets
d) How long the lamp will last
Answer: (b) How much light the lamp gives out
Explanation: Lumens measure the light output itself, while watts measure only the power the lamp uses.
2. About how many lumens does an LED need to replace a 60-watt incandescent bulb?
a) 200 lumens
b) 450 lumens
c) 800 lumens
d) 1600 lumens
Answer: (c) 800 lumens
Explanation: About 800 lumens replaces a 60-watt incandescent bulb, and an LED giving 800 lumens uses only about 9 to 12 watts.
3. Which colour temperature gives a warm white light like an old incandescent bulb?
a) 2700 K
b) 4000 K
c) 5000 K
d) 6500 K
Answer: (a) 2700 K
Explanation: 2700 K is warm white, like an old incandescent bulb, while 5000 K and above is daylight.
4. What does the driver inside an LED lamp do?
a) It cools the LED chips with a fan
b) It converts 120-volt AC into the low-voltage DC the chips need
c) It changes the colour temperature of the light
d) It connects the lamp to Wi-Fi
Answer: (b) It converts 120-volt AC into the low-voltage DC the chips need
Explanation: The driver is a small electronic power supply that converts 120 V AC from the wall into low-voltage DC for the LED chips.
5. Under NEC Article 210.70(A)(1), what must every habitable room, kitchen and bathroom have?
a) At least two ceiling fans
b) A dimmer on every light
c) At least one wall-switch-controlled lighting outlet
d) A 240-volt receptacle
Answer: (c) At least one wall-switch-controlled lighting outlet
Explanation: Article 210.70(A)(1) requires at least one wall-switch-controlled lighting outlet in every habitable room, kitchen and bathroom, with the switch at the usual point of entry.
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