Universal Lighting Technologies

Appendix F: Ballast Overheating

A fluorescent lamp ballast, like any other electrical device, generates heat during its normal operation. With proper planning for the maximum dissipation of this heat in both fixture design and installation layout, a problem need not arise.

It is imperative that operating temperatures be kept as low as possible. Although excessive temperature may not cause the ballast to burn out immediately, it will definitely shorten ballast life.

According to the Underwriters Laboratories, Inc. requirements, the ballast case temperature should not exceed 90º C and the coil temperature should not exceed 105º C. However, the correlation between coil temperature and ballast case temperature will vary with ballast type and design. It is desirable to minimize the differential between case and coil temperatures to considerably less than 15º C – the lower the coil temperature, the longer the ballast life.

The causes of ballast overheating are many and varied:


  • Incorrect lamp size or type
  • Incorrect number of lamps
  • Incorrect primary voltage or frequency
  • Incorrect fixture

Fixture Design

  • Improper design, resulting in inadequate dissipation of heat from ballast and lamp

Abnormal Conditions

  • Shorted starter
  • Dead or burned-out lamp
  • Rectifying lamp (nearing end of lamp life, blackened ends)
  • Excessive ambient heat


  • Incorrect wiring
  • Excessive line voltage fluctuation
  • Fixture surrounded by heavy insulation
  • Ceiling of low heat conductivity

To prevent damage to the ballast and fixture from overheating and to maintain proper light output, simple precautionary measures can be taken to assure long, trouble-free ballast life.

Universal Lighting Technologies’ Engineering Staff Recommends:

  • Selection of a proper ballast to match the requirements of the lamp, fixture, voltage and installation.
  • Mounting of a ballast within the fixture with as much surface contact as possible between the ballast and metal portions of the fixture.
  • The use of heat-conducting dissipators, if necessary, which increase surface contact or otherwise increase heat conductivity between the ballast case and metal portions of the fixture which are cooler than the ballast.
  • Designing the fixture to attain maximum dissipation of heat by conduction, convection or radiation and, where necessary, allowing space between the fixture and a low-density ceiling.
  • If necessary, remote location of the ballast in a cooler area outside the fixture.
  • Planned lamp maintenance – the organized replacement of dead or burned-out lamps when used with preheat or Slimline ballasts.
  • Use of special LOW HEAT rise, VERY LOW HEAT rise and SUPER LOW HEAT rise ballasts where available and necessary.

Lamps, too, are affected by overheating. A rise in bulb wall temperature beyond its rated operating point generally will result in reduced light output and shortened lamp life.

With the required use of Class P ballasts, it is imperative that all fixtures equipped with that specific ballast be heat-tested under simulated installation conditions.