Residential Street Wiring & Pole Transformer Quest's, Please

[Robert11]

Hello:

Was wondering about the service wires running along my street
(live outside of Boston)

a. There are 3, typically, so I guess the main service on the street is 3
Phase.
Right?

b. If so, what voltage do they typically run, phase to phase; I believe
it's
about 13,000 V phase to phase, but am not sure ?

c. What would it be phase to Neutral ?

d. Then, it seems, they tap off of a phase with a single wire going to a
pole mounted transformer
every half block or so that services a "few" houses. For "balance", I
believe, but am not sure, that they
(probably) tap off of a different phase further down the block. Correct ?

This is what confuses me:

The secondary of the transformer seems to consist of three wires which are
eventually brought into the house: this being 220V and a ground.
I imagine that this is just the transformer secondary wiring with a center
tap for the ground.

Right ?

What I can't figure out is If the tap off the high voltage (one of the 13K
phases) to the transformer primary
is a single wire, which it seems to be, where is the second wire for the
primary winding ?

Is it wound as an "auto-transformer" with the return leg of the primary
wired directly to the secondary wiring inside the
transformer, or is it wired internally to the secondary's center tap,
presumably the ground, and uses that ?

If anyone can clarify all of this a bit for me, would be most appreciative.
I looked on the web, but couldn't find any appropriate drawings or
explanations.
Are there any ?

Thanks,
Bob

 

[daestrom]

Hello:

Was wondering about the service wires running along my street
(live outside of Boston)

a. There are 3, typically, so I guess the main service on the street is 3
Phase.
Right?

b. If so, what voltage do they typically run, phase to phase; I believe
it's
about 13,000 V phase to phase, but am not sure ?

c. What would it be phase to Neutral ?

Three wires on insulators usually means 3 phase. 13000 sounds a bit high
for street distribution. 2400 or 4160 is more common. But 13kV *could* be
right. !3.8kV is a common voltage used in electrical equipment.

Vneutral = Vline-line / sqrt(3) For *any* 3 phase system. So 13.8kV /
1.73 = 7976V (if your guess of 13kV is correct).

d. Then, it seems, they tap off of a phase with a single wire going to a
pole mounted transformer
every half block or so that services a "few" houses. For "balance", I
believe, but am not sure, that they
(probably) tap off of a different phase further down the block. Correct ?

Yes, that is a very common setup.

This is what confuses me:

The secondary of the transformer seems to consist of three wires which are
eventually brought into the house: this being 220V and a ground.
I imagine that this is just the transformer secondary wiring with a center
tap for the ground.

Right ?

What I can't figure out is If the tap off the high voltage (one of the 13K
phases) to the transformer primary
is a single wire, which it seems to be, where is the second wire for the
primary winding ?

You have to look closer, and remember the neutral is probably grounded. So
the transformer can has one high-voltage bushing (insulator thingy) sticking
out with a wire on the end of it. The other wire of the primary is *not* on
a high-voltage bushing because the neutral is grounded. This saves the cost
of one high-voltage bushing for each transformer.

Is it wound as an "auto-transformer" with the return leg of the primary
wired directly to the secondary wiring inside the
transformer, or is it wired internally to the secondary's center tap,
presumably the ground, and uses that ?

No, most US distribution 'cans', that I've seen are not auto-transformers,
they have separate high and low voltage windings. Look again at the three
high-voltage wires. You'll probably find an uninsulated wire running with
them for the neutral. The 240/120 is usually run lower down on the pole and
kept separate from the HV stuff.

daestrom

 

[Charles Perry]

Three wires on insulators usually means 3 phase. 13000 sounds a bit high
for street distribution. 2400 or 4160 is more common. But 13kV *could*
be right. !3.8kV is a common voltage used in electrical equipment.

? Most utilities in the US are phasing out 2400 and 4160V. They are
converting to 15kV (12.47, 13.8, etc), 25kV, or 35kV. Yes, 35kV is quite
common as a distribution voltage now. 15kV is by far the most common.

Charles Perry P.E.

 

[daestrom]

? Most utilities in the US are phasing out 2400 and 4160V. They are
converting to 15kV (12.47, 13.8, etc), 25kV, or 35kV. Yes, 35kV is quite
common as a distribution voltage now. 15kV is by far the most common.

ISC (I Stand Corrected). Thanks,

daestrom

 

[Beachcomber]

Also, 35kV (34,200-34,800) is also the highest voltage that the
lineman can use rubber gloves and work on energized circuits in
relatively close proximity.

Beachcomber

 

[Charles Perry]

Charles Perry wrote:

Is the nominal voltage the phase to phase or phase to ground. When the
local PEPCO lads say 13.9 are they talking about to ground or not.

Line to Line. The standard is to identify three phase systems by their line
to line voltage. Thus you hear 12.47kV for a 12.47/7.2kV wye, and 35kV for
a 34.5kV/19.9kV wye, etc. This goes for transmission voltages as well,
such as 138kV, 161kV, 500kV, 765kV.

Charles Perry P.E.

 

[[email protected]]

d. Then, it seems, they tap off of a phase with a single wire going to a
pole mounted transformer
every half block or so that services a "few" houses. For "balance", I
believe, but am not sure, that they
(probably) tap off of a different phase further down the block. Correct ?

For what it's worth:
Con Edison (power utility serving us here) told me that there
is a "rule" as to how many houses per transformer, and the
maximum distance from a transformer to a house. That
makes sense, so it is probably correct, and I expect there
is some similar "rule" for other utilities. I would also guess
that the "rule" applies to certain zoning situations and not
others. Commercial/light industrial would have to be
different than residential, for example.

In my case, the need for that "rule" was clearly demonstrated.
The neighbor's air conditioner would kick on, and *my*
lights would dim. In addition, we had fairly regular power
"blips", often finding the microwave clock blinking 12:00
When I finally got Con Edison to come out and look at
it, their finding was that my house was too far from the
pole pig and it serviced too many houses. They added
a brand new pole and transformer across the street from
my house. Haven't had a power "blip" since then, and
my lights no longer dim. Four different neighbors came
to me to ask what I did, as their lights no longer dim, and
their microwaves no longer blink 12:00 either!

Ed

 

[[email protected]]

FPL is doing an interesting thing in front of my house. There is one common
secondary set feeding about a dozen houses being fed from 3 transformers on a
single primary. It seems pretty stable. I haven't put my Dranitz on it lately
but it used to run inside a +/-3v filter for days and no logs.

Before Con Ed added the transformer, we had 14 houses
on the original pig and they told me that exceeded
their rule. 'Course there are different size transformers,
and I'm clueless as to how they decide what size to use.

I think the parallel transformer setup you describe is
fairly standard. At least that's how it appears to be
wired around here.

Ed