T5 and T5HO fluorescent high bay fixtures and other T5-based specialty lights are built around a linear fluorescent tube lamp.  Like other types of lamps, the number in the name of the lamp reflects its size.  The “T5” name reflects the diameter of the tube in eighths of an inch.  A T5 lamp is five-eighths of an inch (5/8”).  Similarly, a T8 lamp is eight-eighths (or one inch) in diameter and a T12 is 12 eighths (or 1-1/2 inches) in diameter.  One of the features that make a T5 bulb and T5-based high bay light fixtures more efficient is the fact that the lamp is 37% narrower tube than a T8 and over 55% narrower than a T12 lamp.

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