Although LED lighting technology provides numerous advantages for the end user, for the installer or specifier, this new technology often creates headaches when it comes to practical implementation.
One of the problems frequently encountered is the question of how many LEDs can be dimmed with one dimmer. Surely a 100 W dimmer for example, should be able to dim 10 x 10 W LEDs, since it is able to dim 2 x 50 W halogen lamps? Unfortunately, LED loading cannot be calculated the same way as a traditional incandescent or a mains halogen light bulb. It is the exception rather than the rule that a dimmer can dim up to its full wattage rating of LED load. While the LED lamp may only draw 10 W continuously, it may have a start-up inrush current or repetitive peak current during every half-cycle that makes it appear much worse, i.e. behaves like a higher wattage load.
For example, when using an LED load, a 500 W incandescent dimmer with a 5 W LED lamp does not necessarily imply that 100 lamps (500 divided by 5) can be used on the dimmer. While the LED lamp may only draw 5 W continuously, it may have a start up inrush current or repetitive peak current during every half-cycle that makes it appear much worse.
The 5 W LED lamp can cause the same stresses on the dimmer as a 70 W incandescent load, depending on its inrush and peak current. A good rule of thumb is to take the maximum rating of the dimmer and divide it by the equivalent or replacement GLS or Halogen rating of the LED. The maximum load of the dimmer divided by the equivalent wattage (HAL/GLS) = the maximum number of lamps.
In the absence of knowing the Halogen or GLS rating, the only way to know for sure what the maximum load will be for a particular LED lamp or fixture and dimmer is to test.
Shuttle Lighting has tested hundreds of different LEDs and posts the results online,
http://shuttlelighting.com/lamps-and-dimmers-that-work-together/ – an excellent resource in determining load and compatibility. However, remember that a key factor in the world of LED lighting is the rate of change. LED modules are being updated continuously, so Shuttle conducts new tests for dimmer maximum loads and compatibility.