Always Look on the Bright Side of Life: New dimmer and lamp technology requires mindful installation

Three tulips each growing out of the base of a lamp | Shutterstock / Designs Stock
Shutterstock / Designs Stock
Published On
Jan 15, 2021

Everyone is acquainted with toggle or rocker-style lighting switches, but many installers are not as familiar with the dimmers on the market now. Everyone should become knowledgeable with updates to the design and installation before installing new or replacement dimmer switches.

A dimmer device is connected to a luminaire (what we used to call a lighting fixture) and is used to dim the level of lighting from the luminaire. In the past, many dimmers, known as rheostats” were simply variable resistors that acted to restrict the amount of current to the incandescent lamp installed in the fixture. As more resistance is applied through the dimmer, less current flows from the dimmer to the lamp and less illumination is output by the lamp.

As a brief explanatory aside, people outside the electrical industry call a “lamp” in the luminaire a “bulb.” However, the National Electrical Code does not recognize the term “bulb” and refers to these as “lamps” that screw into lampholders. I always tell people that a bulb is purchased in the plant department of a hardware store and is planted in the ground in the fall and comes up as a flower in the spring. Therefore, it is difficult to screw a bulb into a lampholder, as described in 410.62 of the NEC .

First, we need to determine what load is being supplied from the switch and whether a dimmer can be installed in place of the existing toggle switch or rocker switch. For example, older dimmer switches of the variable resistor type restricted the amount of voltage to the load and caused problems with ceiling-suspended paddle-fan motors. Dropping the voltage to a motor can cause the motor to heat and its insulation to fail.

Although LED lamps are dimmable, dimmers must be compatible with the extremely low-wattage LED, since the dimming range may be much less than a similar range for an incandescent lamp. In addition, with the extremely low wattage of the LED lamps, the dimmer may not shut off the LED lamp at the lowest setting, or the lamps may flicker at some of the settings.

Purchasing a dimmer designed for LED lamps will permit dimmer installation without any possible problems, and these dimmer switches often include a trim-adjustment feature to allow better control and dimming range of the low-wattage lamps.

The next issue is to determine if the box where the dimmer will be installed has an equipment grounding conductor and if the box is large enough to adequately contain the dimmer. The box must have a minimum internal depth to accommodate the “rearward projection” of the dimmer, as stated in the NEC , as well as the conductors that supply the dimmer and any other conductors located within the box.

Since some of these dimmers may be considered “large equipment,” the NEC also states the box that encloses large equipment that projects more than 1⁷/₈-inch rearward from the mounting plane of the box must have a depth of not less than the depth of the equipment (the dimmer), plus ¼ inch, plus the additional space for conductors.

When installing the dimmer, make sure the power is off to the circuit, since an energized circuit with connected load can cause a small arc when the connection is made that often destroys the solid-state electronics in the dimmer. Usually, an electrician only makes this mistake one time.

Dimmers may be a single-pole or a three-way dimmer that will permit control of the dimming circuit at one location. Dimming control from multiple locations is possible, but a special “smart” dimming system or similar design may be purchased and installed for this application. Most dimmers are rated at 600W. For larger loads, however, heavy-duty dimmers or dimmer systems are available in the 1,000W or 1,500W range by special order. With LED lights and lamps, these heavy-duty dimmers are not as readily available now.

Regular fluorescent lamps, compact fluorescent lamps (commonly called CFLs) and high-intensity discharge lamps, such as metal halide lamps, require ballasts for operation, and a special dimmer that works in conjunction with the ballast must be used for these applications so the ballast will not fail. A special dimming ballast can be used to overcome these problems. Of course, these are not typically used in residential settings.

Installing dimmers in new or existing homes can be a challenge, especially with the changes that we see between incandescent and LED lamps. Changing out incandescent lamps to LEDs in existing luminaires can lead to problems with existing dimmers, which may have to be changed to prevent issues with dimming of the luminaires.

About the Author

Mark C. Ode

Fire/Life Safety, Residential and Code Contributor

Mark C. Ode is a lead engineering associate for Energy & Power Technologies at Underwriters Laboratories Inc. and can be reached at 919.949.2576 and

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