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How Do I Know If I Need T8 or T12?

Donna McLean
T8 lamps in a parabolic louver troffer provide evenly distributed lighting.

T8 and T12 are two types of fluorescent lamps, which also are called fluorescent tubes. T12 lamps were first used with magnetic ballasts. Newer technology allows them also to be used with electronic ballasts, which weigh less than the old ballasts and create less noticeable light flicker. T8 lamps all use electronic ballasts made especially for this size lamp. The decision to purchase T8 lamps or T12 lamps depends on your intended use or application.

Replace Current Lamp

  1. Read the lettering on the burned-out lamp. You will see either T8 or T12 stamped on one side near the end with the prongs. You also will see the lamp’s rated wattage, usually 32 watts for T8 and 40 watts for T12.

  2. Measure the diameter of the lamp. T12 lamps are larger than T8 lamps, with a 1 ½-inch diameter. The diameter of a T8 lamp is 1 inch.

  3. Look at the troffer, which is the metal fixture that holds the ballast and lamps and is shaped like an upside-down trough. Determine whether it is a lensed troffer, a louvered troffer or an open troffer, in which the lamps are completely exposed.

  4. Look for an acrylic lens, a flat sheet that spans the troffer below the lamps, diffuses light and reduces glare. This is a lensed troffer, which customarily uses T12 lamps.

  5. For T8 lamps, look for a parabolic louver troffer. It has several slanted openings that help shield the lamps from direct view, gather the light and redirect it out into the room.


  1. Remove the old T12 magnetic ballast and install a new, lighter-weight T12 electronic ballast to reduce light flicker and hum. Install the T12 tubes. Choose from warm white or cool white, depending on your lighting preference.

  2. To improve efficiency, remove either type of T12 ballast and wire a T8 electronic ballast in the current troffer. Purchase and install T8 tubes.

  3. To reduce glare and distribute a more uniform light, remove the open troffer or lensed troffer and install in its place a parabolic louver troffer with T8 tubes.

New Installations

  1. Choose T8 fluorescent lamps with parabolic troffers and electronic ballasts for most applications, as they last longer than T12s and contain a triphosphor blend capable of producing a more natural light.

  2. Choose T8 lamps where overheating is a concern, as they produce less heat than T12 lamps

  3. Choose T12 lamps if initial expense is a concern, as they cost less than T8 lamps.

  4. Choose T12 lamps with the older-style magnetic ballasts if you need to install lamps where there will be absolutely no interference to nearby electronic equipment. In the alternative, purchase fixtures with T8 electronic ballasts that have passive filtering.

  5. Tip

    A modern open troffer that is suspended from the ceiling and has rectangular ends often hold T8 lamps. Older open troffers might have the old-style T12 ballasts and lamps. All lamps in an open troffer reflect their light freely in every direction except straight up unless wings are added to direct the light downward. Open troffers allow air to circulate around the tubes, which reduces heat build-up in the fixture. A parabolic louver troffer, which uses T8 lamps, is ideal for applications that require uniform illumination. There are further design details within each category of troffer, as well as a variety of options to consider when choosing a T8 ballast. Talk with a lighting specialist in the supply store. Tell him your requirements and preferences, and ask him to explain the benefits and drawbacks of each type of troffer, ballast and tube. By evaluating your lighting needs and shopping carefully, you will be able to choose the type of fixture, ballast and lamp that fits your needs.


    Electronic ballasts can create interference or line noise. This might result in problems with sensitive electronic equipment as well as overheated transformers, motors and neutral power lines. The problem multiplies when a large number of ballasts are installed and many lights are in use at the same time.

    T8 and T12 lamps are not interchangeable, even though one size will fit into the other size’s ballast.

    Shut off both the electrical breaker and the light switch to your fixture before replacing lamps, re-wiring ballasts or installing new fixtures. Identify your live, neutral and ground wires by tagging them when you remove an old ballast or fixture.