Calculate sag on transmission lines. Does anyone have any sag calculation experience in FME. The formula looks like it should be quite easy to implement in FME but not sure if it is that straight forward to generate the geometry of the transmission

I'm assuming you already have the A and B (mast) locations, you can e.g. use the CoordinateExtractor to retrieve the x,y,z coordinate as numbers. If it is not already the case, you may first want to reproject the coordinates to the same units (e.g. meters or feet) as used by your formula, e.g. using the CsMapReprojector.

If you're moderately fluid with Python, that is probably the easiest and most efficient way to create the three dimensional line between the masts. You can find an example on how to create line geometries in a PythonCaller near the bottom of the page here: http://fme-memorandum-takashi.blogspot.com/2013/11/advanced-geometric-operation-create.html