Four Foot T12 to T8 Conversion
Retrofitting Your Four Foot T12 to T8 Fluorescents
T12 ballasts for 2-4ft 2-pin lamps are rapid start ballasts. They provide voltage to the lamp
cathodes during starting that heats the cathodes prior to the ballast applying an open circuit
voltage (OCV) to start the lamp. This is referred to as a “soft-start” to the lamp and is good
for maintaining longer lamp life in frequent switching applications.
Please see wiring diagram below for a typical T12 rapid start two-lamp ballast. Please note that each lamp base has two wires going to each end of the lamp to provide the rapid start; “soft-start” of the lamp.
Rapid Start Diagram Rapid Start Lamp Holder
T8 lamps have the same 2-pin bases as their T12
counterpart lamps. These lamps can be instant started or rapid/ programmed started. The
difference in wiring between T8 instant start and T12 rapid start is that an instant start only
requires one wire from the ballast to each end of the lamp. However, the lamp holder for T8
instant must be “shunted”.
Shunted is where the lamp holder bridges the contacts of each lamp pin together. There
are two primary ways to achieve the shunted effect. During retrofits of T12 fixtures, cut the two wires coming out of a rapid start lamp holder and wire nut the two wires together to effectively “shunt” the socket. This method does not require the fixture to
have new lamp holders when changing from T12 lamps to T8 lamps with instant start
ballasts. If you do not shunt the wires of the lamp holder together then the ballast
is not able to apply current to both sides of the lamp cathode and the arc within the lamp
most likely will follow the path of least resistant and establish the hot point of the cathode
not at the center of the cathode as intended, but closer to the ends of the cathode where the
cathode support wires crimp on to the cathode. By not shunting the wires of the lamp holder
this effectively results in much shorter lamp life.
Also note that a 4 lamp T12 fixtures has 2 ballast each firing 2 lamps each. You can use one 4 lamp T8 electronic ballast to replace the 2 T12 ballasts.
Get info on 2 Lamp 4' T8 Ballast
Get info on 3 Lamp 4' T8 Ballast
Get info on 4 Lamp 4' T8 Ballast