What signs/markings are required for a buried 4+ feet deep 15kV line: at roads or public areas; every "X" number of feet; at junction boxes: what is required (in the State of Iowa)? Mostly on private property, but does cross public roads and one utility owned transmission line. Thank you.

National Electric Code only requires that a service lateral, from utility to service, have a warning tape 12 inches below grade.

You need to check with the local municipality for the road and the utility for the transmission line for their requirements.