Harold and Susan Vordos wrote:

"Pete C." wrote in message
...
Harold and Susan Vordos wrote:

"DoN. Nichols" wrote in message
...
In article .com,
wrote:
Can you get 120v single phase by picking off one line of 240v 3 phase
as long as you have a neutral to carry the current back to the box?

Not quite. The voltage won't be right for pure three phase. I
think that with Wye connections, you will have something closer to
104V.

I'm at a loss to understand that, DoN. Care to elaborate? I have
wired
three places with delta service, two of which used either the A and C
phase
and the neutral for 120V. All of it was done to code. The third
place
has a single phase panel along with the 3 phase, both of which are fed
from
the same taps from the transformers.

If it had a neutral it wasn't a delta service.

Wrong!! One can have a three, four or five wire delta system.

Huh? You always (unless you've got a 100yr old system) have a ground for
your building system, so once you reach your building distribution you
have a minimum of four wires for a strictly delta system, A, B & C
phases and ground. A Wye system will have five, A, B & C phases, neutral
and ground. The "Wild leg" delta system would also have five wires.

Your "wild leg" delta system is not very popular these days since most
three phase services are provided by three phase pad mount transformers
so the potential cost savings from using two smaller and one larger
single phase transformers to service the load doesn't exist. The "wild
leg" is also a potential safety issue to people and equipment which is
why the NEC requires the orange color coding and placement in the middle
position of the panelboard.


I have a 5 wire system, and it *is* a delta system. It is not a wye, which
does not have the wild leg. Mine does have. Ground is established by
tapping the center of one coil, which results in the longer path to ground
from the other two coils. 208 volts from phase to ground. It's not
conjecture, it's measured.

Ground is not established by the center tap on one transformer, that is
a neutral of sorts. Ground is always established by the ground rod(s)
for your service. The ground and the neutral are always bonded together
at the service entrance panel and never at any sub panels.


The 104V mentioned was a typo, it's really 138V and change. Square root
of three thing for three phase power. 240V Wye service will give you
138V phase to neutral and 208V Wye service will give you 120V phase to
neutral.



And many are delta which *has* no neutral. All may be floating.

In this case, he's already suggested that there would be a neutral, so
it
would be a 5 wire system.

Known as Wye.

Again, wrong. It *is* a delta system. He's talking about 240 volts, not
208. As far as I know, single phase service to the typical house is just
one leg of a three phase delta system. Isn't that how it comes from the
power plants, the primary service? How it's delivered to the customer
depends on the transformers that feed them.

Ok, the "wild leg" configuration is technically a delta configuration.
It is however more often referred to with various derogatory terms due
to it's disadvantages.

While older distribution was often fed in a delta configuration, that is
being phased out for safety reasons.

When the distribution transformers are fed in a delta configuration, in
the event that there is a circuit loss on one of the phases feeding the
transformer due to a cable break of a fuse blow, the line remains hot
due to power feeding from the other phase through the delta wired
transformer(s) and back down the "disconnected" phase.

A Wye connected transformer does not present this risk since only one
leg of the transformer winding is ties to a hot line with the other at
neutral / ground potential.




But a frequent variation has one of the three sides center
tapped (the way the standard residential feed is supplied, 240V center
tapped with the center tap grounded and neutral connected to that.)

The breaker boxes for this have three buses, but only two of
every three positions can be used for 120V single-phase breakers. The
third phase is *way* too high.

I believe this is often referred to as the "wild" leg.

Agreed.



As stated above, I got around that problem in my current shop by having
two
panels, one strictly 3 phase, so none of the positions are lost.

There are / were a lot of strange variations on three phase power, but
most anything new is going to be 208V Wye service. Larger industrial
stuff will get 480V.

My 3 phase delta 240/120 volt service was installed just 4 years ago, at my
request. I did not want a wye service (for obvious reasons), and am
transforming to 480V for one machine.

Huh? What "obvious reasons"? 120/208 Wye service has no disadvantages
that I know of. You can get 120v from any of the phases, allowing you to
balance your single phase loads and single pole, two pole or three pole
breakers breakers can occupy any panel position since all phases are
equal to each other and to the neutral.

I consider there to be considerable disadvantages to a system with a
"wild" leg. You have no way to even come close to balancing your single
phase loads on the three phase feed and you have the "wild" leg which
can cause safety issues and/or equipment damage if people are not paying
attention. From what I recall this configuration was primarily used to
cut transformer costs when serviced from three single phase transformers
and not for any technical advantage.

Pete C.