Why must there be a compromise between sag and tension of an overhead transmission line?
Too much sag and the supports fail, too much tension and the integrity of the conductors fail. Consideration for weather, wind, rain and ice, must be taken into account for line placement. In icy conditions, 10' of line coated in ice can mean an extra 100# of pull against the attachments. You allow too much sag and the sideways movements and sway becomes a hazard, as well as sloppy springing in and out in swaying situations can damage supports and stretch cables to failure.