My understanding of A/C is entirely confused being that it's so different from my "happy" understanding of D/C.

That having been said, I know that A/C is oscillating between 0 and the set voltage +/-, depending on where it is in the phase.

My question is with respect to when there's a 240 volt appliance connected to it, like a stove or central air/air conditioning load. Are both of the 120v hot lines exactly in-phase with each other, or are they out-of-phase? My guess is that they are 120 degrees out-of-phase, being that most (if not all) homes receive their power from step-down transformers. And I presume three-phase implies that there are three hot lines coming into (typically) a commercial/industrial building... but then the voltage is at 208 volts (or something like that), due to the out-of-phase math I read within your online e-book. (Very helpful for understanding this, by the way.) Also, if you have two 120 volt lines coming in as 240 volts (say, 100 amps each), the home is actually receiving two lines (out of the three taps possible) from the electric company's transformer. If it's "safer" to use three-phase, why are homes relegated to the 240 volt system... and not the 208 three-phase system. (Of course, everything in our homes would have to be re-purchased to accept 240 volts, OR there'd have to be further electronics within our homes to re-tool the electric current coming in to re-format it to the standard 120 volts A/C at 60Hz.) Single-phase versus "double" phase, or single phase versus three-phase?!? (Inquiring minds want to know!?!)

And my confusion is this: If the two hot leads coming into one's home are 120 degrees out-of-phase, wouldn't it be more accurate to call this 2-phase (or double phase?)... rather than single-phase?

Further confusion: Because I understand the D/C world so much better, if one connects two A/C hot leads together without a legitimate load in between... "bad things happen." (Sparks... more sparks... and the circuit breaker blows.) But, why? If they're both "hot" why is there a problem. Wouldn't this be the same as putting the + connector lead connecting two car batteries in a circuit to the postive connector leads of the car batteries, for example? Why is the D/C circuit okay to do this with, effectively forming a parallel circuit between two batteries (sources), but when you do it from the two A/C hot lines from the transformer... again, "bad things happen?"

 

The correct term is split phase, to prevent confusion with other systems.

http://en.wikipedia.org/wiki/Split_phase

The 240VAC in the USA is actually two legs of 120VAC that are 180° apart. This way we can have 240VAC for appliances and a safer 120VAC for smaller appliances, with a minimum of hardware for the electric company.

 

If the two hot leads coming into one's home are 120 degrees out-of-phase, wouldn't it be more accurate to call this 2-phase (or double phase?)... rather than single-phase?

The phases are 180° out. Think of the the AC as the output of a center tapped transformer. The voltage is 240 from one end to the other, but half that to the center tap.

 

This may be of some help:
http://www.allaboutcircuits.com/vol_2/chpt_10/1.html

Beenthere's center tapped transformer is exactly what gets used in American 120VAC systems. The grounded conductor coming into your home is from the grounded center-tap on the "pole pig" transformer secondary. The hot leads, usually colored red and black in American homes, are from the ends of the transformer. Larger office buildings might have these trasformers in their electrical rooms instead of on a pole outside.

It is important for engineers or installing electricians to try to balance loads across these two halves of the transformer.

Transformer input is either accross one of the three phases or from one of the three phases to ground.