I'll tackle a few....
Edit: what icewolf said, its been a long day....
First... West Verginia... Watts*
Volts=
amps
Basically, amperage is a result of how many watts of electrcity is being pulled multiplied by the amount of voltage being supplied. A traditional (north american) household outlet is either 15 or 20 amps @ 120 volts, so they can supply a total of 2400 or 1800 watts.
Now the question for neutrals on 3 phase. You will hear the term "load balancing" thrown around by people. When a
dimmer rack is hooked up to three phase power each dimmer is supplied by a different set of phases (so 8 8 and 8 in a 24
channel rack). Say you have 20 fixture on at full on the first 8 dimmers and nothing on the other 2 sets. You are pulling in a ton of power on two phases but nearly nothing on the third. That power has to go somewhere, so it gets put on the neutral to go back out in the world. You will notice that 3 phase motors have no neutral because they use all 3 phases equally all the time. You will also see company switches that have three phase, and ground, and two neutrals. This is extremely common for sound tie ins where the load will very rarely be balanced. If you put too much power accross the neutral you run the risk of overheating it and starting a fire, hence the reason that double neutrals are becoming more and more common.