On Fri, 07 Sep 2007 02:10:27 GMT, Ken Sterling (Ken Sterling) wrote:

Bruce (or anyone qualified).....
Three phase power ----- isn't each leg 120 degrees from the others?
Single phase power - 220v - isn't each leg 180 degrees from the other?
If the above is correct (and I'm hoping I understand it correctly) it
would stand to reason, that if you needed to hook up a 220v single
phase machine and you had 3 phase available at that location you
could, (and I am not saying it's correct) connect to two of the three
phases and run the machine, even tho the sine waves wouldn't be 180
out, it probably wouldn't hurt anything. Am I correct in this
assumption??? Any and all comments welcome
Thanks.
Ken.

It may help to think of a "phase" as a pair of conductors rather than
a wire. A pair of wires comprises a phase. With threephase, there
are three wires, call them A, B, and C. (Ignoring ground for the
moment). There are therefore three phases: AB, AC and BC. The
sinusoidal voltages on these phases are 120 degrees apart. The
voltages on each of these wires relative to ground will also be 120
degrees apart but line-to-ground voltage in a wye connection is .867
* line-to-line voltage.

Residential single phase power in the US is 120 volts relative to
ground, or 240 volts line-to-line because one line has polarity
opposite (180 deg phase shift) to the other w.r.t. ground. Household
120 volt power has one or the other of these lines on the black wire,
and neutral (grounded somewhere) on the white wire. 240 volt loads
like stoves are connected line-to-line, often do not use the neutral.
Others have treated the various voltages found on each phase of
commonly-found three-phase power distribution setups.