How to Change T12 to T8 Lamps


T12 and T8 lamps are two types of fluorescent light bulbs. The “T” in the light designation indicates that the bulb type is a tube, which is common with fluorescents. The number specifies the size of the light in eighths of an inch, so a T12 tube is 1 1/2 inches in diameter, and a T8 tube is 1 inch. When replacing T12 with T8 bulbs, a new ballast must be installed at each end of the fixture to accommodate the difference in size and type.

Things You'll Need

  • Screwdriver
  • Socket wrench
  • Wire cutters
  • T8 ballast
  • T8 tubes
  • Wire nuts
  • Turn the power off at the main breaker. It is essential that you be sure the power is off. The easiest way is to turn the light fixture on, then have an assistant watch it while you turn it off at the breaker box. When the light goes out, you can be certain you have turned off the proper breaker.

  • Remove the cover over the fixture, if it has one. This is typically a plastic panel that lifts or snaps in and out of position. Take the old T12 bulbs out of the fixture.

  • Take the cover off the old ballast. There are many methods for securing covers, so you will need to look closely at yours to see how it is attached. Most likely you will need either a screwdriver or a small wrench.

  • Cut all wires coming from the ballast. Make your cuts several inches from the ballast itself. Unscrew the bolt or bolts holding the old ballast into the fixture. Depending on how it is attached, you may either need a screwdriver or a socket wrench. Remove the old ballast.

  • Set the new ballast in place of the old one, and screw it into the fixture with the bolts you removed previously. Strip the insulation back about 1/2 inch from the end of each wire.

  • Connect the wires from the new ballast to the cut wires in the fixture, being sure to match all of the colors. Secure the wires together by screwing wire nuts onto each connection.

  • Repeat this procedure for the second ballast in the fixture and then replace the ballast cover. Put the new T8 bulbs into place, and put the cover on the fixture. Turn the power back on to complete the job.

Tips & Warnings

  • Switching to T8 bulbs can result in a cost savings of anywhere from 12 to 30 percent. You’ll get the most savings if you replace the complete fixture and not just the ballast. T8 ballasts are also electronic rather than magnetic, resulting in quieter operation and more reliable starting under cold conditions, such as in garages and unheated shops.
  • Working on a light fixture can be dangerous if you don’t turn off the power at the main breaker. Be absolutely certain that the power is off before proceeding to avoid a possible shock hazard.

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  • Photo Credit Felipe Dupouy/Lifesize/Getty Images
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