100 and 70 Volt line amps and what not to do

[Gwyllym]

Hey Dave,
One of the things that i constantly run into in the real world is a total lack of knowledge this subject.  I do realise that it isnt a complex subject but it would be of benefit to those playing with electronics as well as those who install the gear.
If this could include simple transformer theory and phasing issues it would be great.
 

[ColinA]

I would like to build an impedance meter so the watts being drawn from 25v or 70.7v distrubited systems can be read more easily. The couple that are on the market are so expensive.