What additional factors need to be considered when calculating voltage drop?
Two additional factors that should be considered are the power factor of the load and the L/R for the cable. The effect of steel conduit is related to cable inductance. The major electrical equipment manufacturers such as ABB, Allen-Bradley, Eaton, Schneider Electric and Siemens may have helpful information. There are online voltage drop calculators. Some major electric utilities may have information. There may be handbooks published by IEEE or others. NFPA's guide to the NEC may have something.

For 50 for 60 Hz, frequency should not be an issue, but it might be for 400 Hz and would certainly be for higher frequency.

I use the following: