Depends of what system your are modeling. Some railroads used PRR catenary standards EX: Virginian and Reading. PRR catenary trolley wire varies from 19'-0" min to 24'-6" max. The average height is 22'-0"
I have one more question about catenary systems. What is the center to center distance for the support columns for the catenary wires. You had answered my question about the PRR wire height of 22' sometime back.