The NEC does not list a general minimum wire bending radius for <600V single conductors.
It does have bending distances at terminations (as mentioned by Dennis), distances between conduit entrances in boxes, and radii for some cable assemblies. What is your application.
As Jim mentioned, there is no direct Code-required minimum for <600V single conductors. However, the manufacturer may specify a minimum. Code, for listed equipment, calls for installation practices per manufacturer's instructions as part of the listed equipment.