At 05:20 PM 5/16/2014, Vincent Dow wrote:
>Same for electrical panels. Siemens, Murray and most every residential load
>center is marked single-phase if it has two hot lugs and two bus bars
>producing 240 volts between legs. Pretty much everyone I ever worked with
>called that single-phase. Even the Los Angeles Dept of water and power uses
>this term for a two-conductor, 240V service.

Aye, the transformer on the pole** takes in some thousands of volts
single phase and puts out 220 volts single phase center-tapped; and
all three wires go to the house.  I believe this is usually called a
split-phase (or three-wire single phase) system.

**in US domestic-service practice, not counting parts of Philadelphia
that still have actual two-phase systems with 90-degree difference
between them, and a small group of very specialized electricians who
know how to deal with it.

In the house the center-tap conductor is tied to earth (protective)
ground; and with reference to that conductor the other two conductors
are 180 degrees apart in phase.  However considered with reference to
each other, there is only one phase.  The only advantage I can see is
being able to supply 110V and 220V simply with minimum
wiring.***  There's no phase offset anywhere that would allow a motor
to self-start, and in fact no single device is hooked to more than one phase.

***and something we can throw back at the Brits when they start
talking about ring mains. ;-)

Yours,
David