Wireframe mode creates outlines of the geometric edges of the model. This helps to create models that look like user interface, schematics, or a computer generated motion graphics look.
Go to the Wireframe material settings and check Enable.
By default the wireframe and main geometry will both be on. To render just the wireframe, change the Fill Mode to Lines Only. Line Color will determine whether the diffuse comes from the Line Color picker below or from the current Material settings.
Tip: You can use the Blend Modes in the Advanced material settings with Add or Screen to create an overlapping semi-transparent wireframe look.