Instructions to electrician about dedicated line

The answer to the, "Why 15A or 20A circuits" is really a code question. The smallest wiring allowed by the NEC for a power circuit in a residence is 14 gauge with 12 gauge being common. Since a 14 gauge wire will carry 15A, it's typical to install a 15A breaker on a 14 gauge circuit. Likewise with 12 gauge and 20A breakers.

Sean's (and others') suggestion to use 10 gauge wire in a 20A circuit is to reduce voltage drop in that circuit. 10 Gauge wire will carry 30A and could have a 30A breaker installed in the circuit. From a power delivery standpoint (up to 20A, of course), this circuit won't be any different with a 20A or 30A breaker installed. However, one might be able to find a higher quality 20A breaker for the same price as a regular 30A breaker.

The thing to remember is that wire is sized to carry a load and the breaker is sized to protect the wire, not the other way around. So, while it's OK to install a 20A breaker in a circuit wired with 10 gauge wire (your electrician will look at you funny, but what does he know?), it is NOT good (or legal) to install a 20A breaker on a circuit wired with 14 Gauge wire.

Sean,

My comments were strictly regarding CODE requirements. The NEC only considers heat generation withing a cable when specifying its ampacity (current carrying ability). Many electrical engineers make their living sizing cable to carry power to voltage sensitive equipment.

I like your idea of isolation transformers on your system's dedicated lines. It would be easy for me to incorporate that addition to my system. One question, though: if cleaning up power is as easy as installing a transformer, then why are the PS Audio Power Plants so popular? It would be interesting to try a Power Plant in the same system using an isolation transformer and not using an isolation transformer. Then you'd know how much regenerating the AC sine wave contributes to the effect of the PP.