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Q: How do I know which dimmer size to use?

Dallas Electrician | Dimmer Sizing | FAQ

Q: How do I know which dimmer size to use?

Finding the Right Size:

  1. Add up all the light fixtures to be controlled by the dimmer. This we'll call "A".
  2. Determine the maximum wattage lamp (light bulb) each light fixture will accept. This we'll call "B". Not the wattage you will install (must be equal or less than the maximum the fixture will allow).
  3. Multiply "A" times "B".
    "A" = 4 light fixtures
    "B" = 100 watts
    Result = 400 watts
  4. For proper dimmer installation dimmer should be rated at 80% of its maximum allowable use. Or less if manufacturer recommends, this does not include any derating for multiple dimmers in the same switch box.
    Take a 600 watt dimmer. 600 x .80 = 480
    A 600 watt dimmer will successfully operate the 4 fixtures at their maximum lamp size.

Dimmer De-Rating Factors

  1. Dimmers will need to be De-rated or have their maximum wattage reduced, for a variety of factors.
    1. Per manufacturers instructions
    2. When using more than one dimer in the same switch box
    3. When the surrounding temperatures exceed the manufacturers limits.

2. How much do I De-rate my dimmers? This will be determined by Manufacturers recommendations. (usually 20% per each additional dimmer.)

3. Example:

If you have 3 dimmers in one box each rated at 600watts.

  • (600 watts x 80% = 480 watts )
  • 480 watts de-rated by two additional dimmers of equal value: 20% + 20% = 40% reduction or a 60% allowable rating of each dimmer
  • 480 watts x 60% = 288 watts per each dimmer.
  • In order to operate any series of lamps that exceed 288 watts, in this set up you will need larger dimmers.

How Do I Install a Dimmer?

  1. Turn off all power in the house.
  2. Turn off switch.
  3. Remove screws from the switch cover.
  4. remove old switch or dimmer.
  5. Install new switch with wire nuts.
  6. Install screws to attach switch or dimmer to switch box.
  7. Re-install cover plate with screws.
  8. Turn on power in house.
  9. Turn on switch or dimmer.

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