So it's 2-phase?

From Professor Cardell's web page at

http://www.science.smith.edu/~jcarde...ecPwr_HSW.html

"There are two wires running out of the transformer and three wires running to the house.
The two from the transformer are insulated, and the third one is bare. The bare wire is the ground wire.
The two insulated wires each carry 120 volts, but they are 180 degrees out of phase so the difference between them is 240 volts."