Residential electric service in the United States (120/240 VAC) is sometimes called two-phase.
Understanding Residential AC Phases. Most people assume that single phase means that both 120v legs in residential application are of the same phase. This is a complete misconception. If the two 120v legs were in phase, the difference between the two would never change, giving you no power between legs: A split phase electricity distribution system is a 3-wire single-phase distribution system, commonly used in North America for single-family residential and light commercial (up to about 100 kVA) applications. Three Phase Four Wire Wye. The most common commercial building electric service in North America is 120/208 volt wye, which is used to power 120 volt plug loads, lighting, and smaller HVAC systems. In larger facilities the voltage is 277/480 volt and used to power single phase 277 volt lighting and larger HVAC loads. Used primarily in industrial facilities to provide power for three-phase motor loads, and in utility power distribution applications. Nominal service voltages of 240, 400, 480, 600, and higher are typical. Residential electric service in the United States (120/240 VAC) is sometimes called two-phase service but this is NOT correct. Two-phase service is an obsolete style of electrical power distribution where two phases are provided that are 90° out of phase with each other. There were two line wires and one neutral, so two-phase service was commonly a two-phase three-wire service. Another variation used four line wires and one neutral for a two-phase five-wire service. This was sometimes incorrectly called four-phase five-wire service. We are not aware of any two-phase service still in use. However, the WattNode should have no trouble monitoring a two-phase three-wire service Compared to a single-phase AC power supply that uses two conductors (phase and neutral), a three-phase supply with no neutral, the same phase-to-ground voltage and current capacity per phase can transmit three times as much power using just 1.5 times as many wires (i.e., three instead of two). Three-phase electric power is a common method of alternating-current electric power generation, transmission, and distribution. It is a type of polyphase system and is the most common method used by electrical grids worldwide to transfer power. It is also used to power large motors and other heavy loads A DC circuit has two wires through which the current in the circuit flows from a source of electricity through a load and back to the source. A single-phase AC circuit also has two wires connected to the source of electricity. However, unlike the DC circuit in which the direction of the electric current does not change, the direction of the current changes many times per second in the AC circuit. The 120 volt electricity supplied to our homes is single phase AC electricity and has two wires-an active and a neutral.  Single phase polemount stepdown transformer (Canadian). In electrical engineering, single-phase electric power is the distribution of alternating current electric power using a system in which all the voltages of the supply vary in unison. Single-phase distribution is used when loads are mostly lighting and heating, with few large electric motors. In contrast, in a three-phase system, the currents in each conductor reach their peak instantaneous values sequentially, not simultaneously; in each cycle of the power frequency, first one, then the second, then the third current reaches its maximum value. Single Phase Three Wire. Also known as an Edison system, split-phase or center-tapped neutral. This is the most common residential service in North America. Line 1 to neutral and Line 2 to neutral are used to power 120 volt lighting and plug loads. Used primarily in industrial facilities to provide power for three-phase motor loads, and in utility power distribution applications. Nominal service voltages of 240, 400, 480, 600, and higher are typical. The actual system used in homes is a split phase electricity distribution system. A split phase electricity distribution system is a 3-wire single-phase distribution system, commonly used in North America for single-family residential and light commercial (up to about 100 kVA) applications.
http://anoldman.com/power_systems/understanding_residential_ac_phases http://www.ccontrolsys.com/w/Electrical_Service_Types_and_Voltages http://www.ccontrolsys.com/w/Two-Phase_Electrical_Service https://en.wikipedia.org/wiki/Three-phase_electric_power http://www.powertogo.ca/phases.htm https://en.wikipedia.org/wiki/Single-phase_electric_power http://www.ccontrolsys.com/w/Electrical_Service_Types_and_Voltages http://anoldman.com/power_systems/understanding_residential_ac_phases
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