The variety of National Electrical Code calculations, as you can
tell from the link rack for this portion of the web, are huge. The best approach
is to go through them one after another. When you are sure you understand that
calculation type, move on to the next. Be sure to read the NEC section where the
calculation is required and explained before you start hitting calculator keys.
Also lay out your problem in an organized fashion to keep the details
accurate. Remember the saying, garbage in garbage out. Or, accurate data in
accurate data out.