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The minimum size bar used to support a post-tensioned beam is a #4 bar. This size is typically chosen due to its adequate strength and ductility, which are essential for the safe and effective support of the loads applied to a post-tensioned beam.

Post-tensioned beams require reinforcement to handle tensile forces that develop after the tensioning of the cables within the beam. A #4 bar provides sufficient capacity while being manageable for installation. Additionally, it helps to control crack formation in the concrete, ensuring structural integrity and longevity.

In comparison, smaller bars, such as a #2 bar, may lack the required strength and ductility needed for effective reinforcement, while larger bars like #6 and #8, while technically usable, often exceed what's necessary for typical post-tension applications, leading to increased material costs and potential challenges during the installation. Therefore, using a #4 bar strikes an optimal balance between safety, performance, and cost.