Several points have been raised in this discussion to
which I would like to respond.
First, the normal 3-wire 120/240V North American service is most
definitely single phase, NOT two phase. There are two "hot" legs on
the supply, 180 degrees out of phase with respect to neutral/ground,
but it is still a single secondary winding on the transformer. The
180-degree difference comes about only due to the center-tap position
of the neutral connection. Note that the primary of the transformer
is still also just a single winding with only two connections, and
whether that primary is connected between phase-and-neutral or between
two phases of the HV line, it still has only a single sinewave applied
to it. You need some sort of third reference point to even be able to
measure any sort of phase difference.
Seombody mentioned a delta supply which has one phase grounded and
whether that would be considered two phase. Such a system is commonly
known as a "corner-grounded" delta, and it is still three phase.
There are some 3-phase delta supplies which are ungrounded. Once
again, the application of a ground connection to one phase to turn
that into a corner-grounded delta doesn't alter the 3-phase nature of
the supply -- it merely provides a reference point to ground.
One arrangement which seems to be uniquely North American is the
FOUR-wire delta arrangement, also known by various names such as
high-leg delta, wild-leg delta, red-leg delta, and so on. To
visualize this system, start with a basic ungrounded 240-volt delta
secondary, drawn with phase B at the top and phases A and C at either
end of the horizontal winding at the bottom. Now, instead of
connecting a ground to one phase as you would for a corner-grounded
delta, put a center-tap on the A-C winding. Ground that center-tap,
and extend it to the building as a neutral, giving the four wires.
You can now connect a 3-phase 240V delta load A-B-C as you would with
the basic delta system, and you can connect a single-phase 240V load
A-B, A-C, or B-C, as before. Thanks to that center-tap, you can now
also connect 120V loads phase A to neutral -OR- phase C to
neutral. The result of that ground placement, however, is that phase B
will be at 208 volts with respect to neutral/ground, hence the various
"high-leg" et al descriptions.
I understand that the 4-wire delta was once common for light
commercial services, as it allows for 3-phase 240V delta equipment
while also providing 120V for lighting and general small loads without
having to resort to adding extra transformers or installing a separate
Turning to the east side of the Pond, we have several differences in
the way distribution is handled. Speaking for the U.K., we tend to
use one very large transformer to feed a whole section of a
neighborhood rather than the much greater number of smaller
transformers that would provide power to an American neighborhood with
a similar number of houses.
Anywhere where there are more than just a few houses, you'll find a
3-phase transformer with its secondaries feeding a 415Y/240V 3-phase
4-wire wye distribution network. Individual houses then get a 2-wire
240-volt single-phase service tapped from one phase and neutral, the
load being distributed between the phases as evenly as possible. In
towns, that same wye network can also provide 3-phase 4-wire service
for commercial power.
It's possible for a VERY heavy domestic load to be provided with two
or three phases, but extremely rare, and only likely to be found in a
VERY large house which is all-electric. The standard 240V 2-wire
residential service these days is 100 amps, which provides 24kW and is
ample for most purposes. There are plenty of older services rated 80,
60, and even 40A still in service. I've even seen an old 30A service
as recently as two or three years ago, although they're pretty rare
We do also have single-phase transformers, most commonly found on
poles in rural areas to serve one or two isolated houses or farms.
These have a straight 240V secondary with one end grounded to provide
a single-phase 240V service.
Finally, out here in the boondocks you can also find 3-wire
single-phase 240/480V distribution where there are a dozen or two
houses scattered along a road. The system is similar to the American
120/240V arrangement with a center-tap neutral, except that each house
still gets just a 2-wire connection to provide 240V and a single
transformer will feed the whole lot. Although you won't find this
system in towns, it's a convenient "halfway house" for some rural
On the primary side of these transformers, everything here is
connected between phases. In fact NONE of our HV lines have a neutral
run with them, so three-phase primaries are always delta-connected,
and the primary on a single-phase transformer is connected across two
phases of the HV. That primary supply to the final transformers is
almost always 11kV (measured phase to phase), although there are still
a very few local distribution networks operating at 6.6kV in a couple
of areas. Thus a single-phase HV spur line has to be run as two "hot"
As noted before, in Continental Europe 3-phase supplies into homes are
very common, however, and to British and American minds they seem to
take 3-phase to extremes. In France, for example, it's not at all
uncommon to find a small house which has a full 3-phase 4-wire
380Y/220V service, with the main breaker set to just 15 amps per
phase! Arranging heating and cooking loads on a service like that can
be quite a juggling act.
I've referred to U.K. supplies as 240V, but in fact we only finally
standardized on 415Y/240V in the early 1970s. Prior to that, the
nominal declared voltage varied from region to region, anything
between 200 and 250V with corresponding 3-phase voltages (380Y/220,
400Y/230, 433Y/250V, etc.).
As noted, we're OFFICIALLY now 400Y/230V to align with European
standards, although in practice most areas haven't actually changed
anything yet. They just juggled the old +/-6% tolerance to an
assymetric +10/-6% so that we could declare ourselves to be 230V
without changing anything!
We had several different frequencies in use across Britain in the
early days. The push to standardize on the 50Hz that we use today
came about with the plans for our "National Grid" in the 1930s.
Some parts of the American west coast also had 50Hz power in earlier
times. I understand that the Los Angeles area was on 50Hz originally,
and converted over to 60Hz in the early 1930s, complete with publicity
programs to help get clocks and other synchronous devices
changed. I've seen other references which suggest that some parts of
California (and maybe Oregon and Washington too) had patches of 50Hz
power until the 1950s, presumably in those places which generated
locally and were not connected to any sort of statewide grid.
Other places have also used "oddball" frequencies. I was looking at
some service manuals for old Garrard turntables (early 1950s) a couple
of months ago, and noticed that they offered not only 50 and 60Hz
motor pulleys but also 40Hz versions at that time. I found out from a
contact "down under" that some parts of western Australia were using
40Hz power at that time.
In Britain, DC supplies survived well into the 1950s and early 1960s
in some urban areas (those older parts of cities which had been the
first to get power). These DC systems used an Edison-type 3-wire
distribution system, running at anything between 200/400 and 250/500V
(this was before there was nationwide standardization of
voltage). Normal domestic services were just 2-wire 200 to 250V, some
houses tapped from the positive "outer," others from the negative.
Commercial service could then get the full 3-wire supply so that they
had 400-500V available as well.
One other odd DC system which we still have is that on the London
Underground (subway) system. LU uses a 4-rail arrangement, at a
nominal 630V DC. However the ground reference is set on a
one-third/two-thirds arrangement, so that the positive conductor rail
(located outside the running rails in the usual "3rd rail" position)
is at +420 volts and the negative rail (placed centrally between the
running rails) is at -210 volts with respect to ground.