Jon Elson wrote:

On 10/25/2010 02:58 PM, Karl Townsend wrote:
"The Kid" is putting in underground wiring to his outbuildings. For
now, power will come from the house to the shop.

The shop sits twenty feet from a three phase line. future plans is to
install a three phase service to the shop, cut the ordinal transformer
out, and then feed one phase back to the house from the wiring being
installed now.

OK, a one phase house service has two 110 legs on the same phase
opposite polarity to get 220 across the two hot wires, neutral is the
center tap. As I understand it you in effect get three 110 hot wires
120 degrees apart in the phasing for three phase with the center tap
for neutral.

So, can you run single phase 220 off this? Need any special provisions
installed now? He's putting in conduit - four wires - two hot, neutral
and ground.
There are essentially 4 ways to wire a 3-phase 2xx Volt service.
There is 240 V delta, and 208 V Wye service. 240 V delta is not what
you want here. 208 Wye service will work, but if you hook the house
to 2 of the 3-phase hots, you get 208 V on the appliances that expect
240. Check the air conditioners, etc. carefully for their ability to
run off 208.

Then, there are two variants of the above systems, you may not be able
to get these installed by your local electric company. One is
corner-grounded delta, that gives you 2 240 V hots, good for running machine
tool motors, the advantage is you can use normal 2-pole electrical
panels and 2-pule breakers. No 120 V in that service, so not suitable
for the house.

The last form is center-grounded delta. It gives you both a 3-phase
service, as well as one phase that is grounded at the center-tap. So,
those two hots and neutral look exactly like single-phase home service.
There is a third leg to provide the 3rd phase.

IF!! you can get your electric utility to provide this service, it
probably is the best choice, as you get real 240 V single phase
service PLUS 3-phase with only 5 wires (3 phases, neutral and ground).

Jon

That last one is also known as "wild leg" delta service since the third
phase is not at 120V relative to the ground/neutral, and that third
phase is normally color coded orange to identify it. It wouldn't be a
problem for the house, since that wild leg wouldn't be fed to the house,
but it has been known to cause confusion and blown up 120V stuff in many
installations. Another issue with this type of service is that some
three phase machines may want 120/208V Wye service with it's neutral to
phase voltage at 120V for all phases.

Generally, if you ask the utility for three phase service, you will be
getting 120/208V Wye service unless you specifically ask for something
different. Expect a lot of questions if you ask for three phase service
and a lot more if you ask for anything but the common commercial
120/208V Wye service.

As noted, most appliances these days are 208/240V rated since 120/208V
service is very common in apartment buildings. Check everything to be
sure, but you're not likely to find much that isn't ok with 208V.