Volume 6 Issue 1
July 2002    
What are T5 lamps?

T5 lamps are fluorescent lamps that are 5/8" of an inch in diameter. This report discusses only linear T5 lamps. Differences in length and socket pin design versus conventional fluorescent lamps prevent any problems with electric circuits or human factors. This section focuses on the physical characteristics of T5 systems compared with T8 systems.

What does T5 mean?

The "T" in lamp nomenclature represents the shape of the lamp-tubular. The number following the "T" usually represents the diameter of the lamp in eighths of an inch (1 inch equals 2.5 centimeters). T5 lamps have a diameter equal to 5 times an eighth of an inch, or 5/8". These lamps are approximately 40% smaller than T8 lamps, which are one inch in diameter, and almost 60% smaller than T12 lamps, which are 1˝" in diameter. Figure 1 shows diagrams of lamp ends of T5, T8, and T12 lamps. Figure 1 also shows that pin base type of T5 lamps is different from that of T8 and T12 lamps. T5 lamps have a miniature bi-pin base while T8 and T12 lamps use a medium bi-pin base.

Figure 1. Lamp Ends

Are T5 lamps the same length as T8 lamps?

T5 lamps are slightly shorter than T8 lamps and therefore cannot be used as replacements for the larger lamps. Some luminaires, however, can be made to accept either T5 or T8 lamps by changing the sockets and ballasts. Table 1-1 compares lengths of T5 lamps with T8 and T12 lamps.

Select the measurement(s) to be displayed:  Metric English

Table 1-1. Linear Lamp Lengths

Nominal Length (ft) Actual Length
T5 (mm)
T8 and T12 (mm)

2 549 590
3 849 895
4 1149 1199

Table 1-1. Linear Lamp Lengths

Nominal Length (ft) Actual Length
T5 (in.)
T8 and T12 (in.)

2 21.6 23.3
3 33.4 35.2
4 45.2 47.2






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