Thanks for the explantion, Don. I've never seen a motor like this in
England. We often get 115/230 V single phase motors which have two
identical windings which can be connected in series or parallel, but not
three phase motors. You just connect them in the star configuration for
400 V or the delta configuration for 230 V. The 230 V can be obtained

from an industrial three phase supply as it is the voltage between any

one phase conductor and neutral. Do you have 115 V, 230 V AND 460 V
supplies in the USA?


Domestic power is a mix of 115 and 230V (or 120/240 depending on
who is saying it) I'll use the 120/240V below.

Picture a transformer winding (at the street), with 240V AC
across the winding, but with a center tap which is grounded. All three
wires are run to the house (with the grounded center tap wire often
uninsulated). It is routed through the KWH (KiloWatt Hours) meter, and
into the breaker panel. There are two buses run vertically in the
panel, with fingers reaching out to left and right (and passing by the
other bus). This means that any single circuit breaker gets 120VAC, but
any two side by side bring out 240V between them. Breakers for 240V are
actually two breakers riveted side by side, with a mechanical linkage
between the arms so if one trips, both go. (Or, sometimes, both breakers
are built within a single housing, with a single toggle to work both at
the same time.)

Wires run from the breaker panel contain one or more hots, an
associated neutral, and a safety ground. (Not all are present in all
cases.)

Most wall outlets in the USA are 120VAC, so one side is
neutral,and the other 120VAC.

Certain appliances need more power, so they run from a double
breaker at 240V. Examples of this include electric kitchen stoves, and
electric clothes dryers. (Electric water heaters are also 240V devices,
but they are normally not fitted with an outlet and plug -- they are
hard-wired to the breaker panel from the water heater.)

Note that while this provides 240VAC for these higher power
loads, still the maximum voltage above ground is no more than 120VAC.
The lines are 180 degrees out of phase (not a useful phase difference
for starting three-phase motors, but useful for getting double the
voltage to a load while keeping the voltage above ground within reason).
This comment about "180 degrees out of phase" will usually fire up some
who believe that it is a mistatement of facts, but it depends on how you
look at it. :-)

Yes, I see what you mean about 180 degrees out of phase. It works the
same way as those 110 V site transformers we get in the UK (used on
building sites for power tools). They have a grounded centre tap so the
maximum voltage to ground on either line is 55 V. I don't know if you
have these transformers in the US? Possibly not as your supply is
already safer.

It is *very* difficult for the typical city home to get three
phase power, and it is charged at industrial rates. This means that your
whole month is charged as though your highest current drain at any time
during that month was being drawn for the whole month. One person who I
know discovered this to his disadvantage when he fired up a large
heat-treat oven which he acquired surplus. He had three phase, because
he was living in what used to be a commercial shop, and it was still a
machine shop -- he was just living in there. :-)

That's a crazy system of billing. Why does anyone put up with that? I
don't think they bill industrial power like that here.

But -- if you *do* have three phase power, the breaker box has
three buses, arranged in a manner similar to the two bus one for 220V,
except that groups of three breakers make up a full three-phase
connection, groups of two make up a 240V connection, and single ones
*may* be 120V, depending on the service. One common one for use where
120V and three phase are both needed is to ground the center tap of one
of the three phases, which allows two out of three breakers to provide
120VAC. The third one (the "wild leg") produces a much higher voltage,
and should *not* be used for 120V loads. I don't know whether the breaker
panels have a provision for preventing a single breaker from entering
the "wild-leg" slots, or whether they depend on the electrician knowing
what he is doing. :-)

Double breakers can be used in any adjacent pair of slots for
240VAC loads, and triple breakers in any adjacent three slots for three
phase output.

Note that all of the above are delta connections. There are
three phase Wye connections where any single phase to neutral/ground is
120VAC, and between any two phases is 208VAC IIRC. This is why some
motors are rated for 208V-204V.

So you don't have 415 V line-to-line voltage three phase supplies?

And also note one other difference from the UK -- the power
frequency is 60 Hz, not 50 Hz. (You can get away with a little less
iron in motors and transformers for 60 Hz compared to 50 Hz.

Thanks for all the information. I now know a lot more about the US power
system.

Best wishes,

Chris