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#130598 - 08/11/06 02:42 PM What are all these transformers for?  
kmt  Offline
Junior Member
Joined: Aug 2006
Posts: 3
Garlstorf, Niedersachsen, Germ...
Hi to everyone in here!

I'm a Newbie from Germany and I've been
wondering about all these transformers
I've seen in many code violation pics.

I'm not very used to the PoCo systems in
the U.S., and I'm just curious what the
purpose of all these transformers could be?

Or, asked the other way round, how does
electricity come to you in the "normal"
houses (eg single family house)? Which voltage/current and how many live and neutral wires?

In Germany, electricity normally comes in
3 live wires and 1 neutral wire, 230V between
live wires and neutral, 400V between 2 live
wires. Mostly 80A each live wire. And I'm
just curious how's it done in the U.S.?

Greetz from Hamburg

Tools for Electricians:

#130599 - 08/12/06 03:36 PM Re: What are all these transformers for?  
Alan Belson  Offline
Joined: Mar 2005
Posts: 1,803
Mayenne N. France
kmt. You will have the same power of transformers per consumed x000kw in Germany as the US, UK or France. The distribution of power from the Generating Plant at perhaps 132,000 volts, for economy of transmission, stepping down to 66,000v - 6600v in the suberbs, and finally to 230v per phase at a local level. The ecomomic transmission distance of 230v is only a few kilometers, so there will be a lot of them near you in Hamburg. Perhaps they are hidden in 'buildings' (or behind them), like here in France where traditionally they built a masonary/concrete enclosure to house the transformers? You must also note that many US pics are in big cities with high power consumptions and that the US seems to have opted for pole-mounted [aka 'Pole Pigs' ] transformers serving a few homes/businesses, rather than ground level [pad mounted] units of larger capacity. Pole mounting, and hence visibility, is now the norm here, so perhaps they are cheaper to install?


Wood work but can't!

#130600 - 08/13/06 06:47 AM Re: What are all these transformers for?  
kmt  Offline
Junior Member
Joined: Aug 2006
Posts: 3
Garlstorf, Niedersachsen, Germ...
I see... so these transformers are just
step-down transformers to convert higher
voltage down to 120V. I first thought
that these could be transformers from
delta connection (without neutral) to
Y-connection (with neutral)?!?

But nevertheless my question: how is
electricity delivered to "normal" living
houses in the U.S.? With or without neutral/
how many live wires and which voltages are
standard? Is there at least a "standard"?


#130601 - 08/13/06 12:01 PM Re: What are all these transformers for?  
WFO  Offline
Joined: Sep 2005
Posts: 202
Cat Spring, TX
A "typical" house in the US gets a 3 wire, single phase service at 120/240 volts. It will be two hot legs off a transformer with the center point tapped to ground and bonded to the system neutral (i.e., 120 each leg to ground, 240 leg to leg). least that's the case in beautiful Cat Spring, Texas! [Linked Image]

#130602 - 08/13/06 02:43 PM Re: What are all these transformers for?  
SteveFehr  Offline
Joined: Mar 2005
Posts: 1,213
Chesapeake, VA
I believe that's how it is throughout most of the US, WFO, but there are always areas with wierd setups.

kmt, mostly, US distribution is high voltage 3-wire 3-phase delta, with ground wires strung above more as lightning rods than anything else, and grounded at every tower- the earth itself carries the lion's share of the unbalanced return current. This is stepped down to medium voltage in regional substations, sometimes in multiple steps.

Power run on the poles into residential neighborhoods is typically medium voltage 3-wire 3-phase delta (again with a small ground wire atop, grounded at every pole, or every other pole). Then, a small transformer will tap 2 of the phases. (Or, in my neighborhood, there is only 1 MV wire run with no ground, and it's tapped straight to the earth.) The secondary is a center-tapped coil with the center tap grounded to the earth so that either end is +/- 120V to ground and 240V between them. These 2 wires are run along the street, where several houses will tap off it to their respective service panels, where the ground is again grounded to earth, and neutral bonded to that. Most individual circuits are 15A or 20A 2 wire (120V/neutra) plus ground. Larger loads use 3-wire + ground 240V.

Commercial and industrial sites will often get 3-phase power (stepped down from MV to LV at a transformer right outside the building), sometimes delta, sometimes wye, and at a variety of voltages. The most common I've seen are 208/120V and 480/277V, but there are oddballs, too. The neutral is always derived at the switchboard and in every wye-secondary transformer.

[This message has been edited by SteveFehr (edited 08-14-2006).]

#130603 - 08/14/06 04:36 AM Re: What are all these transformers for?  
SteveFehr  Offline
Joined: Mar 2005
Posts: 1,213
Chesapeake, VA
Here's the pole pig feeding my house. The line on the top is MV, I'm not sure exactly what voltage. The 3 wires below feed 2 blocks worth of houses, which are just 2 houses at the moment, but they're building more- similar 25kVA transformers in my neighborhood feed 5 or 6 houses. The top wire is ground, the bottom two are +/-120V. The thick cable on the bottom is Cable TV coax. There is a street lamp coming off this pole, too.

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#130604 - 08/14/06 04:47 PM Re: What are all these transformers for?  
classicsat  Offline
Joined: Nov 2002
Posts: 456
I don't know exactly above 66KV, but generally, the MV circuits supplying the pole-pings or PMTs are a Wye supply, above that, Delta is commonly used.

Around here I think 44KV (straight from the neighbourhood nuke plant) goes to the substation to convert to 7KV Wye, which is split up amongst the roads or ran to 3 phase customers, which gets converted to 120/240V.

In many neighbourhoods the primary and secondary share the same neutral conductor, hence the apparent lack of a primary return.

FWIW, common residential services have 100 to 200 A per leg, go fron the trasnformer to the meter, thent to the service disconnect, usually in the customers breaker panel, sometimes at the meter.

#130605 - 08/14/06 05:34 PM Re: What are all these transformers for?  
kmt  Offline
Junior Member
Joined: Aug 2006
Posts: 3
Garlstorf, Niedersachsen, Germ...
Many thanx for all these informations. I'm
a little bit concerned about how few power these transformers carry. You said, a 25kVA
transformer for up to 5-6 houses? Looks very
weak to me, especially when I think of
all the air condition units running everywhere.
As I said, here in Germany, 1-family-buildings in residential
neighborhoods get about
63A * 3 phase + neutral * 230V
= 44kVA in total. I understand that the PoCos
in the U.S. are interested to keep the
tension as high as possible and as near to
the custom as possible to avoid loss in the leads (sorry for my bad technical English), but I can't believe that it should be easier
to service these lots of funny (sorry :-) trashcans on a pole compared to one central
transformer station per street or block.
Nevertheless It's very interesting
to see how power supply is done in other
parts of the world.

Greetz from Hamburg

#130606 - 08/14/06 11:44 PM Re: What are all these transformers for?  
Larry Fine  Offline
Joined: Feb 2005
Posts: 693
Richmond, VA
KMT, your English is just fine.

One advantage of more, smaller transformers is that fewer homes lose power when one goes bad.

Larry Fine
Fine Electric Co.

#130607 - 08/15/06 03:35 PM Re: What are all these transformers for?  
Alan Belson  Offline
Joined: Mar 2005
Posts: 1,803
Mayenne N. France
[Linked Image]

Electricité de France transformateur, c/w building.

[This message has been edited by Alan Belson (edited 08-15-2006).]


Just noticed, Zut alors! The crane is the other side of those power lines. [Linked Image]

[This message has been edited by Alan Belson (edited 08-15-2006).]

Wood work but can't!

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