Final answer:
The largest interval at which a rigid metal conduit of 1 1/2 inch diameter with threaded couplings set up horizontally in straight runs can be supported depends on the material properties of the conduit and the load it is subjected to.
Step-by-step explanation:
The largest interval at which a rigid metal conduit of 1 1/2 inch diameter with threaded couplings set up horizontally in straight runs can be supported depends on the material properties of the conduit and the load it is subjected to. Without specific information about the material properties and load, it is difficult to determine the exact interval. However, it is important to consider the strength and stiffness of the metal conduit to prevent sagging and ensure structural integrity.