At the transformer, as per the diagram in that excellent link. When you do
the actual service lateral, you pull three primary cables and each one has
an aluminum (usually) center conductor and a copper concentric grounded
conductor. The grounded conductors are joined at the transformer and also
bonded to the transformer case and ground rods.
The concentric on the primary cable along with the transformer grounds
are not always a metallic return to the substation. Primary neutral and
common neutral (common to primary and secondary, run in the secondary
position) are metallic returns to the substation. You find primary
neutrals in the older 4KV where they've used 2400V transformers phase to
neutral (star or wye). We also run a common neutral system in our 21KV
that we use 12000V transformers on.
A side note to the underground cable, is they are not getting the life
expectancy out of it that they thought when they put it in. One of the
problems that we've run into is some soil conditions dissolve the copper
neutral on the older installations where it was direct buried. We've
since gone to a jacketted cable and require everything to be in conduit
now. Another problem they find is when the cable is faulting it's
through hairline cracks through the poly. We've replaced a considerable
amount of this at great expense, and have recently been working with a
company that injects silicone in the end of the strands that fills these
hairline cracks. At 5 dollars a foot, it's about half the price of cable
replacement. This year we have some 40000' coming up for cable
replacement and another 20000' that we are going to do the silicone
You're way above my head, are you talking about upstream of the
customer-owned cable? In the typical case, the concentric grounds are the
only return, right?
That's probably the way to go- I once pulled an old lead-jacketed primary
run through about 400' of conduit from a man-hole cover to an underground
vault. It was a real PITA but pulling the new poly-jacketed stuff in was
easy and saved a lot of trenching and restoration. Still, the majority of
installations in this area are direct-bury. The only advantage is that
sometimes you can Biddle the cable and make a quick repair without pulling
the whole line out.
Another problem they find is when the cable is faulting
They inject silicone in the center strand which fills the gaps in the white
poly insulation between the center strand and the semiconducting layer?
That's a pretty neat trick. Is it only practical for utilities? Do they have
to redo the potheads/elbows?
I can't speak to what direction you are referring to about customer
cable. Around here, it varies so much. We have some customer owned
primary and secondary. But when I'm talking about the grounds not being
a return to the sub, say you have a 3-phase primary (just 3 wires, no
neutral) going out from the sub a couple of miles and now you install an
pad-mount transformer (or string of transformers) being fed from a riser
off of this 3-phase primary. Your grounds on all these transformers are
tied together, like you say. But there is no metallic return to the sub
I can't say if it's practical for other than utilities, you'd have to
weigh the costs. And yes, they have to redo the potheads, and elbows.
Around here we don't use that many potheads, we use 3M termination kits.
A note on DC- As of teo years ago, New York transit was still running
DC rotarary converters that are about 100 years old-AC in, DC out, but
not quite a motor generator like a welder. I believe the last went out
of service recently, but I may be wrong. The hookups use have been
thyristor converters (all electronic) since the 50's.
I read recently that the last of the Edison DC service was finally
Also, "Networks of Power"
(Amazon.com product link shortened)
"If you're a history buff, and appreciate the technology that surrounds us
all, you'll love reading "Networks of Power: Electrification in Western
Society, 1880-1930" by Tom Hughes. Hughes takes us back to the days of
fierce rivalry between Edison and Westinghouse; the early era of electric
power generation and consumption where the battle of DC vs. AC consumer
power was born and decided.
Hughes doesn't stop there. Also included in this well-footnoted edition are
in-depth narratives of the evolution of commercial power systems in England
and Germany through 1930. A well written, readable snapshot in time.
Compelling historical reading for the non-technologist as well as the
student of electrical power commercialization."
(Amazon.com product link shortened)
Do they mention anything in the book, "Networks of Power" about the
Folsom Powerhouse? I was privileged enough in my career to work with a
man that operated it. He operated it from 1948 til they closed it in
1955. I worked with him in the '70's when he was Chief Dispatcher with
the company I work for. I took a tour of the powerhouse with him and I
took my video camera. I have him telling a lot of wonderful stories of
things like the collectors flying off the generators, sparks everywhere
and him diving out the window.
Today when you talk about a network in the electrical utility industry
it usually refers to a system in the downtown area of large cities. This
is different from the transformers in the suburbs, in that, the
secondaries are all in parallel. So if you lose a transformer you don't
drop any customers, you just lose KVA. But you have to run your network
with more reserve than the largest transformer.
(Amazon.com product link shortened)
It's been a few years since I read it, but I don't remember that. A lot of
the book dealed with rationalization (standardization and the
rationalization movement) of the power industry, the balance between
competition and the need for a unified grid. On the technical side, much of
what we take for granted, such as the concept of a feed with two hots and a
neutral, (vs. one hot and one neutral) was once a bright new idea. With your
background, I think you would enjoy at least some of the book.
And anyone in Southern California that wants to see a working 2400
VAC 60Hz 6-phase to 600 VDC Rotary Converter station only has to go to
the Orange Empire Railway Museum on an operating weekend.
http://www.oerm.org There's a nice little GE station there, with a
completely restored and re-wound rotary converter (ask about how that
restoration came about, it's interesting). And it's all self-starting
and self-shutdown, done with a big drum controller - just hit the
button and watch it come up.
Looks like something Rube Goldberg designed, complete with a
ball-and-worm armature shaft wiggler to keep the brushes wearing
evenly, and big live-front contactors with open arc chutes...
They now have a solid-state converter for everyday use, but they can
still fire up the rotary - for demonstrations, if they're working on
the other power plant, or if they're going to be running a lot of
rolling stock at once.
--<< Bruce >>--
Bruce L. Bergman, Woodland Hills (Los Angeles) CA - Desktop
Electrician for Westend Electric - CA726700
His suggestion for ASCII art made me cringe too, but it actually came
out perfect on my computer.
Bit I also had some problem "following" along.
On Fri, 09 Jan 2004 19:43:45 GMT, "Bob Davis"
Clarke, you must have never heard of Delta Power.
In that system, Two of the three phase are 120 to ground.
The third phase is 208 to ground.
between any two phases is 240 volts. Really.
With Delta Power, you have 120 volts from two of the three
phases, and you have 240 volts three phase.
NO YOU WON'T.
In 208V 3-phase service, the phase-to-phase voltage is 208V, and the
phase-to-neutral voltage is 120V.
Delta or wye, it doesn't matter. You won't get 240V anywhere in a 208V 3-phase
Doug Miller (alphageek at milmac dot com)
How come we choose from just two people to run for president and 50 for Miss America?
It was probably set up as a warehouse originally. This was often done
because all that was needed was power for lights. The 208 wye service
has the added advantage that your load is more balanced since you have
three 120 circuits to use not just two. Plus you can run just about
any 3-phase power machinery.
The only drawback to the 208 wye service is that any 240 2-phase stuff
that you use won't get the power for which it was designed. But then,
for just about anything that you find in a shop, that's not a problem
anyhow. For instance, if you've got a 240 2-phase air compressor it
will run happily on 2 legs of the 208 service. The power output might
be a bit less, but you'll never notice it - plus the manufacturer
overstated it in the first place anyhow.
Oh, and one more thing: Where Doug was saying "Wrong" and "NO.." up
there. You now know that the correct answers are "Right" and "YES.."
He is talking about "red leg" delta. That is 240 between phases though. What
they do is put 2 regular pole pigs up (delta primary), tap the first one like
the normal 120/240 and connect the end of the secondaries together, leaving the
3d side of the delta open (also called "delta Vee" or "open leg delta").
It is just a cheap way for the utility to give someone 120/240 and also 3P 240.
The L/N will be 120/120 and 208 on the "red leg", actually orange conductors in
the code. You see this in light industrial areas. It works fine as long as the
3p loads are balanced across the phases.
I have had problems in computer rooms connected this way since the load will be
a combination of 3p and 1p loads inside the computer. We can usually fix it by
rolling the phases around so all the single phase loads are on the center
tapped transformer and the open leg only sees 3p loads.
The utility generally upsizes the center tapped pig. You can identify this on
the street by looking for 2 transformers together on a pole and a 3P drop going
to the building. If you see 3 transformers it will be 3P wye (208) about 99% of
the time. I suppose you could spec 3P delta if you really wanted 240v 3P but it
might cost more and you won't have any 120v without the center tap and red leg
service. You are also left with how you ground this service. Corner grounded
kicks open another can of worms but you could use single phase service
equipment. Usually these are impedance grounded or even floating for certain
We have 440v and 220v 3 phase in our building. When we need 220v
single phase we just take any two lines from the 220v 3 phase. When
using plugs, we use the 3 phase plugs, but only use two of the taps
going to the load. Works on motors, welders, everything, everytime.
I never knew there was a problem until I read this thread. (this
ain't theory, we been doing this for years). Oh yes, when we need
110v we use one leg from the 220v 3 phase.
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.