# Unusual electrical service wiring

Discussion in 'Electrical Engineering' started by Michael Moroney, Sep 18, 2003.

1. ### Michael MoroneyGuest

I was looking at a webpage that showed typical power transformer
connections for different types of services. In addition to
the usual 120V/240V single phase and 120V/208V three phase
configurations, they showed a few odd ones One was 3 240V
transformers in a delta configuration with the center tap
of one of them grounded. The leg not connected to the
grounded transformer was referred to as the 'wild' leg
and apparently carries a voltage of 208V to ground.
What sort of devices would one have that would cause
one to request such a service from the electric company?
3 phase motors with a requirement for 240V delta seem
like they'd be kind of oddball, esp with 208V equipment
being common. A motor connected in a Y would be even
odder, it would get 138V (if my math is correct) and its
neutral would be 70 volts above ground.

Also in the examples shown sometimes one of the
3 transformers (not the one with the ground) was
omitted.

Even weirder was a "Scott" arrangement, two
transformers fed from 3 phases so to produce
two phases at 90 degrees. What would want to be
fed with _that_?

I've also wondered about a small industrial
building. It has 3 individual heavy wires from
the pole to the building, the pole has 3 "cans".
I found this odd, there should be 4 wires. Looking
closer the secondaries are in a delta configuration
and none are grounded as far as I could see.
The same building has a second feed which appears
to be a standard Y setup, 3 hots and a neutral.
So the first must be some special equipment.

Also what are the advantages of Y and delta
configurations for power wiring? Y provides a
natural neutral for one. For secondary wiring
(the 11,000 volts or whatever that run down your
street) it appears most of it is Y connected but
older is delta. Why? From what I can see the
very high voltage power lines are always delta.
Why? Also, if a power line is described as
carrying 345kV, is that phase to phase or
345kV phase to ground?

2. ### Charles PerryGuest

That is a 240V power bank. It provides a three phase 240V delta service as
well as 120V from the center tapped transformer for "lighting". That
transformer is usually larger than the other two and is used to serve all
single phase load in the facility. Nothing odd about it. It is a quite
common service for commercial facilities.
Open delta. Provides three phase service with only two transformers. Works
well when the three phase load is well balanced and there is little, or no,
single phase load. Again, not unusual at all.
Scott T was developed to supply three phase loads from a two phase
distribution system. This still happens from time to time. It is rather
type of service since it was cheaper than running all three phases the
multiple miles to the top of the mountain.
Delta service is not for "special equipment". Motor drives run on three
phases, no need for a neutral in most cases.
You sure have a lot of questions. The comparison of the
months. One advantage of delta is a savings in the amount of wire that must
be run.

The "11,000 volts that runs down your street" is not secondary, it is
primary and is referred to as medium voltage. It is not 11kV, but usually
12.47, 13.2, 13.8, 25, or 34.5kV. Some 4kV also exists. Most of the US
uses a 4 wire multigrounded wye setup. There are many reasons; two of which
are that most load is single phase and the other being that it is easier to
detect single line to ground faults (these are the most common faults).

345kV refers to the line to line voltage.

Charles Perry P.E.

3. ### Cameron DorroughGuest

You certainly *do* have a lot of questions..
It depends which country you are talking about (silly Americans think they
run the whole world! ;-)

Down under, the most common sight around the back blocks certainly is 11kV
Delta (no neutral or ground). As you get further out of town you find 22kV,
33kV, 66kV and finally 132kV. The easiest way to check the voltage in use
is to count the number of disks on the insulators.

Pole-mounted transformers (D/Y) convert the high-voltage primary to
low-voltage (415V) secondary with a neutral and an earth at each pole. The
main reason for doing this is to save on cable costs.

Oh, and in the country you might find a 22kV SWER line, but that's another
story.

Cameron
Or phase to phase if you prefer ;-)

4. ### User 1.nospamGuest

One was 3 240V
We call this 240 - 4 wire, the wild leg has no reference to ground and can
be most any voltage. I usually see 140-160 volts to ground, tho it varies.
Actually, there is not a lot of 208 equipment, many, many 230 volt motors
are in use at 208...which is often about 215, thanks to the boys at the
power company. They usually work fine and the voltage is within the
tolerance of the motor. 208 is becoming more common in motors due to
hi-efficiency design, but 208 shines in small offices and factories where
both lighting and power can be balanced from one source. It is also often
desired where owners/managers do not feel their maintenance personnel should
deal with higher voltages.

7. ### Nukie PooGuest

Please tell me what SWER is. Thanx

8. ### Cameron DorroughGuest

Single Wire Earth Return. It's a system for supplying single phase power
over very long distances using only one wire.

Cameron

9. ### Dave DahleGuest

You forgot one older service voltage - 2400V delta Not a factor though -
these systems are probably as old as if not older than the 4kV wye systems
mentioned by Charles...

Our municipal utility finally made the push last year to upgrade the last
remaining 2400V lines to 13.8 kV (apparently culminating a 20+ years of
transition)... yet the IOU that *also* operates here in town still has miles
and miles of 4kV in use.

Dave

10. ### Dave DahleGuest

Whattya smoking?

The largest of the 3 (or 2 in the open delta setup) transformers is the
singlephase (or "lighting") side, and it is the center leg of this
transformer that is grounded. Going between any 2 phases, you will see 240V,
but using the grounded leg as a reference, the 2 phases on the lighting side
will measure 120V, while the wild leg WILL show 208V to GROUND.

Whattya smoking?

Dave

11. ### John GilmerGuest

Except that there usually are only TWO transformers used to provide the
service. One BIG transformer for the 120/240 "lighting" and one "dogleg"
to provide the third phase node.

It is very uncommon for secondaries to be connected in delta. A delta load
is usually driven by a Y transformer bank.
With two transformers, it is VERY common for the service for larger
restaurants. The distribution utilities seem to be pushing the 120/208
service over the 120/240/240/240 service described. They want the
"balance" but the customers end up with inferior power for their heavy loads
(208 v 240 volts).

12. ### Nukie PooGuest

Are you serious? That sounds like an old telegraph setup. That doesn't sound
too safe with the "return" current running through the ground like that. I
remember reading in an old IEEE transaction about stray currents near the
Brooklyn bridge causing severe corrosion problems (there's a power plant
nearby on the Long Island side in Ravenswood). About 20yrs ago, I had a
customer that complained about getting zinged when his foot was immersed in
his swimming pool whilst the other was still on the ground. To make along
story short, I wound up temporarily disconnecting his service drops to prove
that his problem was caused by stray current and a utility issue (he had a
private well so it wasn't coming in on any water main). SWER sounds like an
invitation for such problems.

14. ### User 1.nospamGuest

It is very uncommon for secondaries to be connected in delta. A delta
It is very common in the coalfields of West Virginia and Kentucky to have
480 delta, often with a "corner ground", but not always. Then they run the
ground thru a resistor and monitor the current, if it exceeds a
predetermined level, an alarm sounds and maintenance takes over.

Some mines even take 4160 into the mine where a unit substation reduces it
to utilization voltage.

15. ### User 1.nospamGuest

The high voltage stuff where (around here, northeast US) the wires hang
Substation to subtation = Transmission lines.....usually 120KV and up....to
750KV.
Substation to transformer poles = Distribution lines....12,470 volts up to
Don't, you will never get close enough to measure the voltage before you are
You may be looking at a capacitor bank, used for power factor correction on
long lines. Common in rural areas. These do not provide power to any user.

16. ### Charles PerryGuest

Those are not stepdown transformers, they are voltage regulators. They
adjust the voltage on the conductors.

Most lines with 2 poles per structure are either subtransmission or
transmission. Most lines with a single pole are distribution but could be
subtransmission or transmission.

Go here for a decent explanation:

http://science.howstuffworks.com/power.htm

Charles Perry P.E.

17. ### Cameron DorroughGuest

Interesting stuff.. stray currents can be a hassle and IIRC most old
telegraph setups were two-wire DC (at least the ones here were)..

Cameron

18. ### Cameron DorroughGuest

Quite correct. BTW, it was not uncommon to see mist rising from pools of
rainwater around the base of the transformer pole early on a winter's
morning.. It was a great place to warm the feet when a long way from the
house.

Cameron

19. ### daestromGuest

I have seen some distribution networks 'upgraded' by reconnecting the
transformers from delta primary to wye primary, then changing the
transformers feeding into the distribution from delta secondary to wye
secondary. This simply raises the voltage by sqrt(3) on the distribution
lines. Often insulators/fuses/etc... are rated to take the higher voltage
anyway. Or they can be changed out to higher voltage rating equipment
slowly over a period of time before reconnecting the transformers. If this
can be done, then the same conductors can now carry more power without
overheating.

daestrom

20. ### Peter FisherGuest

The Scott Transformer system was also used on UK Distribution Networks, but
you have to be aware of the history. Back in the days of old, electricicl
energy was supplied from the DC power station in the town during the day,
and at night, when the load was negligible, by a large battery. The battery
was centre earthed, so running along the high street was a three wire cable.
(Positive, Earth, Negative). And the towns residents were happy

As the electricity networks developed, Three-phase equipment was installed
at the power station, and connections were made to an embryonic 33kV Grid.

The customers were connected to the AC system, using the old cables as two
phases + neutral. The towns residents weren't too keen about having their
High Street dug up just to install a different cable. Also these old DC
cables were usually quite large cross sectional area, and in fairly good
condition, being installed in an age when good engineering was over
engineering. In order to achieve a balanced three-phase load on the high
voltage system, the Scott Transformer was installed.

Having said the above, I'm not aware of any remaining in service. I believe
that in the Croydon area, south of London we used to have such a network,
but I think it was decommissioned back in the 1970's, before I joined the
company. I do know of quiet residential areas where the old three core
cables are still in use, but they have been cut up into short lengths, and
used for cross road services and such like.

Peter Fisher - EDFenergy Networks Branch - CDM co-ordinator