How are the primaries of the transformers you see wired on the medium voltage side wired, line to line or line to neutral?

Mike Whitt was telling me over on the home inspector board that they are always line to line. I never see them that way in SW Fla. They are wired from one MV phase to the grounded conductor. Is it just a Florida thing?

I'm not an electrician, but I think most residential distribution systems today are wye, with a multi-grounded neutral common to the primary and secondary. The primaries are connected line-to-neutral.

Most single-phase aerial distribution transformers have a single primary bushing, with the other end of the primary winding internally bonded to the transformer case and the secondary neutral. But I occasionally see a two-bushing transformer, where both ends of the primary are brought out to bushings. This type of transformer can be used on either delta systems (connected line-to-line) or wye systems (where one of the primary terminals is externally connected to the neutral).

I think residential delta distribution was much more common in the past.

Thanks Albert, that was what I remembered when I was in the DC area but that was long ago. I looked at about 100 transformers today while I was driving around and I never saw one wired delta primary.

The other strange thing on my street is they have 3 transformers wired in parallel on that 3 wire bus with the 120/240, serving 11 homes. It is only one short stretch, the rest are the regular "maypole" with a transformer serving 2, 3 or 4 houses straight from that pole.
I suspect the bus was put in when there were only a couple scattered houses down the street (30-40 years ago) fed from one transformer and they just added transformers as the lots filled in. This is where the first few houses were built.

Are you sure the transformers are really paralleled? Most often (around here at least), you'll see small strain insulators inserted in the two hot wires of the secondary bus to break it into independent sections, each fed by a single transformer. Or there may be two secondary insulator racks on a pole, but the transformer is only connected to one of them. A secondary section may physically span several poles, or it may only serve the houses connected to one "maypole", but in either case it's electrically separate (except for the neutral).

Secondaries *can* be paralleled or "banked", but I think it's pretty rare in aerial distribution. I believe Detroit and someplace in southern California are some of the few places that do it.

In contrast, urban areas with high load densities often do have such "network" systems, usually underground. These feature breaker-like devices called "network protectors" to isolate failed transformers from the interconnected system.

If you see three transformers of same size on the same pole, it's most likely for a three-phase service. Are there now, or was there previously, and commercial/industrial buildings on your street?

I missed the point that the transformers are on the same pole. That being the case, I'm sure you're correct about the three-phase service.

The transformers are spread down the 900' that this parallel bus is run a few hundred feet apart. You can see that it is one continuous run in that picture I posted. I have more pictures. The whole 3 street run, about 4000 feet comes from one primary leg with 18 transformers ranging from 25 to 50 KVA and feeds about 40 houses. The only place that has this bus scheme is the part near my house. I looked at the 1970 aerial photos and this is exactly where the only 5 houses on the street were. I bet it was all fed with one transformer in 1970. Those houses had old split bus panels, 60 or 100a then, at least one still does.

Actually, looking again at your picture, I think I see insulators dividing the secondary into two sections. Look directly below the transformer at the two bottom (hot) wires - note the two bulges in each wire, sort of a "dog bone" shape. Those look to me like strain insulators, so the transformer would only feed the wires to the left of the pole.

It varies from one POCO to another. Some like to use one high voltage line and neutral/ground, others use a pair of high voltage lines, 2 of the 3 phases.

Also there is variation of how the secondary 120/240V lines are strung along poles. When they use 3 separate wires. Some places will have the neutral as the top of the 3 wires, other places will have the neutral as the middle wire. An arguement for the top wire being the neutral would be that it's the wire that would get touched by falling high voltage distribution wires. Thus shorting directly to ground and less high voltage getting into customers' homes.

Albert, I will go out there and look at those "insulators" tomorrow. That would make sense if that is what they are doing. I thought it was strange. I still think this is a relic from when there were only 5 houses in 2 blocks but I agree they may have split it up. If I see my FPL neighbor I will ask him. This is also his "beat"

For sure the top strand of the 3 is the neutral. I have a picture where it starts and there is only the neutral.

This whole cludge has fascinated me since I first noticed it and I just had an excuse to look closer at it.

The original question referred to what current should be on the pole ground rod and how much gets reflected back on the drop to the house. More gee whiz info. The pole grounds are cruising at about a half an amp down at my end of the street until you get up near the feed 2000' away, one is 1.8 and the first pole with a transformer has almost 3a.
more gee whiz info
With my service disconnect open I have about the same as the pole out front, on my service neutral at the service point, a bit less than half an amp so it is coming from the FPL side, not my house. The pole stayed the same with my SD on or off.

In my area, it's all wye, and looks just like Greg's photo. MV on top, then the 3x 240V residential lines below (I forget which is the neutral out my way.) No protective grounded conductor on top, and plenty of blinking lights every time there's a lightning storm. This way, they only have to run one MV wire conductor out, and can share the grounded neutral for both the MV and LV lines.

They also seem to love to undersize them. Lessee... per NEC, my house requires a 200A service and is a dedicated fridge circuit from bumping up to a 400A service, and my two neighbors are similar... so of course the POCO puts a 25kVA pole pig up that's only rated for 104A to feed us all.

5 new houses just down the street, each with electric heat and requiring 200A service? CW (and NEC) says 240kVA. POCO puts a 50kVA on the pole. And we wonder why the power fails so much on really hot or really cold days...

They seem to have 25s for 2 houses, 37s for 3 and 50 for 4.
Just looking out the window, there are none of those "insulators" between the two transformers I can see from here. I will get up the street to look at the rest in a little while.

Albert was right. Every segment is isolated one way or the other and those are insulators in front of that transformer. I love it when I learn something. Thanks.

Ahh that all looks almost identical to what we have here in my city.. For single phase stuff the trannies are hooked to a neutral common to both the primary and secondary, and phase voltage is anything as low as 2400V all the way up to 16,000V depending on where you live in the area.
Greg: 4 houses to a 50 KVA transformer?? In an older area of the city my parents live in, ( built in 1967) there are 13 houses on a 50!! Mine is probably no better ( Cant tell they are all back yard drops) but at least the PoCo recently upgraded all the primary conductor and plunked in 75KVA cans.

A.D

Yup they are pretty consistent 2 on 25, 3 on 37 and 4 on 50.

I suppose it might be because we are all electric. Everything we have spins the meter. I did a load calc on my house and it comes out close to 200a with the original method and 150 or so with the optional. My house is actually on the small side compared to anything built in the last quarter century.

It's interesting how much smaller the utility's secondary wires are in contrast to the houses' service entrance conductors, which are governed by the NEC.

Part of that is the free air aspect of a service drop. I agree I still don't think it explains all the liberties they take. They do have a lot of experience so they are probably right

It comes to economics. A WO is issued to reconnect a service upgrade. Unless it specically says replace the tri-plex, they won't. If someone made the effort in the field and it was not on the WO, someone will be "on the carpet" for it. New construction they typically carry a couple of sizes of wire and no one on the POCO side will take the time to crunch any numbers. It is the sparkies installation that usually throws the peception off. We have a 200 amp service with either 2/0, 3/0, or a 4/0 cable hanging out for the POCO to connect to. How often does residential service draws enough power to require such a size? 99.999% of the time, never. In theory the service will never reach it's demand level let alone maxing the service out.

Look at a electric stove circuit for example. A 12kW stove has only a 8kw ciruit and when was the last time the breaker tripped on it?

The POCO's takes advantage of the demand load on the service. Partly to save cost on wire and to minimize strain on the anchors holding the tri-plex at both end.

At my house in Palm Bay, there are 8 houses on one pole pig. I can't remember what us stamped on it, but it is only 25 or 35 KW. All of these houses are 150 amp service min. with the newer houses at 200A. I am surprised that the tranny doesn't glow in the dark sometimes.

I don't think I have ever seen a house served with anything but 2ga triplex since they started replacing the 3 strands with the triplex 30 years ago. I know when I did my service upgrade ("bootlegged 100" to 200) I asked the engineering department at FPL about my drop and all they asked was if it was twisted.

My place is fed by an underground service that climbs up a power pole, 2ga in conduit (for a 100A service). This pole is shared by a neighboring house, which also has an underground service using 4ga. The POCO has an overhead connection that looks fairly heavy crossing the street to get to the line that in turn goes to a 3rd pole that has the transformer. It looks like the power company gave us the heavy line to our pole because it serves 2 houses. Which makes for less voltage drop when we turn on heavy loads in our house. If we had an overhead service, that wire would likely be much thinner.

Greg's image shows the Multi Grounded Common Neutral (Refered to as "MGCN") "version" for 1Ø Medium Voltage Distribution.

I see this type of Primary feeds in Nevada (Las Vegas), and in rural areas of Oregon.

Most of California - at least Southern California, have L-L Primary Feeders for 1Ø Distribution.

Some areas with 12470Y/7200 3Ø 4 Wire Primaries will connect L-N, but this is not a Common Neutral type system - as the Grounded Conductor is not common to both the Primary and Secondary feeders of a given Transformer.

Here are a few Schematics from the Tech. Reference section, covering Residential Distribution Systems:


Fig. 1.1: Multi-Grounded Comon Neutral (MGCN)


Fig. 1.2: 3 Phase 4 Wire Distribution - typical in my area on older 4KV Circuits, and some newer 12470Y/7200 4 Wire Circuits


Fig. 1.3: 3 Phase 3 Wire "L-L" Distribution - typical in my area.

More images may be seen at Residential Distribution Systems page.
Go to the "Menu" at the Tech Reference area, and select from there

Scott

BTW: I just found out the links at the "Menu" need to be re-directed, so I perform the necessary tweeks ASAP.

Fixed Tech Reference "Menu" hyperlinks!!!

Check it out at:

Menu For Technical Reference Section

Scott

I have seen a similar scheme on some older poles around here, although I'm pretty sure they are not isolated. If multiple transformers are running on the same primary, and their secondaries run to a common bus, the load should be evenly distributed, shouldn't it?

Newer installations are all triplex, and the newest services I've seen are completely insulated with plastic (no more ceramic) hangars. They look pretty weird, as the phases and grounding conductor can't be spaced more than 12" from each other, all using the same hangar.

The transformer that supplies my house is a 25KVA, and currently has 9 service drops running from it. Most of them are probably 100A, but that's still quite a stretch.

11 amps per house? You all must be Amish

"11 amps per house? You all must be Amish"

LOL

My guess is that your light bulbs last a long time due to low voltage.

My UPS reports 120.6 VAC on the line-in. But, I haven't replaced a bulb since I moved in September.

Last Summer, Con Ed of NYC had some spectacular cascading failures in areas where they had multiple transformers feeding a common bus. Seems some of the transformers failed, and the transformers still alive had more loading, and the bus wires started literally burning up in places. A lot of underground wiring had to be replaced, and some customers had to be fed with generators Con Ed set up locally on the street. It was a mess, and the city government was rather unhappy about it.

In the U.K. the xfmr primaries are always wired phase-to-phase, since our MV/HV never distributes a neutral.

3-phase xfmrs and 240/415 Y local distribution is the norm for anywhere where there are more than a few houses, but where single-phase spurs are run for remote rural homes, the HV is always run as two phases:



Xfmr above is 11kV primary, 240V secondary.

I was in Tampa for a few days up north of town in the lake area. They have single primaries on the poles and wire L/N there too. Must be an FPL thing

Very interesting!

Square-ish aerial transformers seem to be common in many countries outside the US. Virtually all of ours are cylindrical, at least since the 1940s.

In the 1960s GE introduced the "SealPak" transformer which had a shape somewhat like the one in your picture, but I don't think they caught on; at least I've never seen one except in ads.

Do aerial transformers in the UK typically have primary fusing or cutouts? In the photo, it appears that the primary leads are connected to the lines by hot-line clamps, which would serve as disconnects.

They've certainly been the norm here for a very long time. Here is another single-phase xfmr, this time feeding 3-wire 240/480V local distribution lines in a rural area where there are a few dozen homes scattered along a backlane (primary is 11kV again):



The typical 3-phase xfmr looks very similar, although obviously with 3 HV bushings and 4 LV terminals.

We've started seeing one or two new types in the last few years though, like this cylindrical unit which is much more North-American-looking:



Or this one, which at first sight seems odd for having the HV bushings vertical instead of at an angle like almost all of our older xfmrs:





There's not usually any fusing on overhead xfmr connections here. Cut-outs and sectionalizers can be found in some places.

I've seen a few rectangular air-cooled pole-mounted transformers in my area (VA, USA). I'll take a photo if I get a chance.

Actually we have quite a few square and rectangular pole mounted single phase transformers around my area as well, but they are older. I would guess some are from the 50's and 60's era when the cases were painted black or green but I have seen a few painted grey which seemed the norm around the 70's, unless of course they were "rebuilts".
All 3-phase transformers remain rectangular, and I will try to get pix as well in the next few days as well.

A.D

Same around here, the original transformers here from the 50s were/all are rectangular. So are the three phase transformers, but Peco doesn't hang many of those around here anymore, instead using three separate transformers.

Ian A.

I don't think I've ever seen a US pole mounted three phase transformer before in real life. (Post those pics guys )
I remember seeing a photo in an old book of a pole mounted transformer: it had four secondary bushings and four wires extending down to feed the secondary rack. There where both four wire and three wire service drops extending from the secondary rack. I thought immediately that it was a three phase pole mounted transformer. The weird thing was that I could only see two primary bushings (the side mounted kind) and two primary wires extending down from the primary crossarm and cutouts. How could that work? I have observed that modern single phase (and single primary bushing) transformers that are connected phase to neutral have the primary grounded neutral conductor tied to the center tap of the secondary. (essentially the primary and secondary neutrals are the same wire) But I didn't think that the three phase transformer in the old photo would be wired this way. (It looked to be connected the same way as a single phase transformer would be to a delta primary)

Any Ideas?

(digging deep in my memory, I believe the book was on appliance repair, early sixties vintage, and it had a few pages on how electricity is delivered to homes)

The only time I saw a pole-mounted 3ph was in New Hampshire. There, they also seem to put much larger transformers on poles than I would think to (200KVA+) and have been using the insulated primary system with the plastic hangers for a while now. In fact, I think the wire and hangers were invented and manufactured in New Hampshire. IIRC, it had three primary bushings and four secondary lugs.

Never seen a pole-mounted square transformer in use, but there is a PUD on the coast here in Oregon that uses what appear to be clear-coated stainless steel transformers (on the topic of nonstandard pole pigs). None of them seem to be rusting, and all are shiny which suggests that they didn't just have the paint peel off. They look interesting, since pretty much everywhere else has the same old gray ones.

Interesting the differences between the UK and NZ.
We use in NZ the two phase supply or the SWER (single wire earth return) system. (washing line)

In the second case the 11 or 6.6 kV single wire is taken off an 11/11 or 11/6.6 kV isolating TX to avoid having the feeder trip on earth fault. The secondary voltage is 230/460 or 240/480 Volts.

But on the first photo of this thread, it looks like a SWER system set up.

Sounds like it might have been an open delta with one primary leg grounded. Either either that, or the other bushings were just hidden from view.

That could just be open delta wired wye primary. Actually pretty normal here using 2 transformers. One of the transformers is center tapped on the secondary and off you go.
I can get a picture of that at the "shop and rob" at the end of my street.

Yah, I've seen that 2 transformer connection very often. But those transformers are usually connected to a 4 wire wye system with a grounded neutral.

I found a picture of a transformer like I was asking about:


http://www.electricalphotos.com/showphoto.php/photo/425/ppuser/28

This picture is very similar to the one I seen in that other book. Could it be a three phase unit or not?

Okay, I happened to catch one in the background of a pic I took today (yes of traffic lights).



Closeup of single transformer:


Ian A.

Re: 3 phase rectangular pole mounted transformers..
Ya usually the PoCo will use 3 separate cans for a 3 phase service drop to a large building but it is NOT uncommon to see a single 3phase transformer of 150 KVA or less on a pole. Usually I have seen them where a building is an older one and had a separate 3 phase service strictly for an elevator or other motor driven equipment.

Don't worry fellas, I am working on those pix still. I know where there is a nice brand-new installation of one of these transformers, right outside my friends 7'th floor balcony.. I will have them in the next few days!

A.D

I have never seen 2 transformers wired wye secondary. It is usually red leg delta around here.

Sorry I should have been more clear. I meant to say the Primary is wye connected.(each transformer is connected phase to neutral). Yes the secondary is open delta with a high leg.

10-4 That sounds regular here. I will try to remember my camera tomorrow.

This is the label on a 50KVA like the one at the top of this thread.

The 50 KVA up close

Here is an American open delta setup with a 3-wire (no MGN) primary. Grounding is done by local rod at the pole. This particular setup is in the high desert area in Sothern California Edison territory where I noticed that most, if not all of the local distribution, was done with no MGN and the transformers connected L-L. This would be considered unusual by US standards which is mostly MGN distribution.

Thanks for the pics guys.



I would agree. 95% of the lines around me are 4 wire wye primary with grounded neutral. Do you know what primary voltage they use? In my area, I have noticed that some medium to small towns that are not served by the local electric co-op(s) are served by either their own municipal utility, or by the larger utility and have a small substation are more likely to be supplied with the 3 wire delta. (usually 2.4 kv) I think for several reasons: first, most of these towns have a steady population and the system is adequate. There aren't a lot of businesses that demand more than what a system like this handle. (many of these systems are quite old) There really is no need to replace it. Most of the distribution transformers that they use can be ran from many different voltages and systems. For example: take a two bushing transformer that can run on 2400 Delta, 4160Y2400 wye, 7200 delta or 12470Y7200 volt wye, plus some of the transformers have another tap for a 7.9 KV, the phase to ground voltage for a 13.8 kv wye system like in my town. 3 voltage taps, 5 different systems, one transformer.

A couple pics I shot in towns that have a 3 wire delta:
one older:


One newer installation:

This is a 3p wye

... and a 3p open delta center tapped (red leg)

Do power companies follow a standard method or rule, as to which transformer in a 3p open delta center tapped (wild leg) is to be on the leading phase of the two (120 degrees ahead) and which is always on the lagging phase (120 degrees behind)? Something like "The wild leg transformer shall be on the lagging phase of the 2 high voltage primary supply lines, as compared to the line the main transformer (the one with the grounded center-tap) is fed by". Or is it essentially random?

It is usually "B" but I am not sure it is really a rule.

It seams different utility co vary in there distribution norms . In the Centerpoint (utility co)of course 120/240 single phase is the most common, next is 120/240 open delta, with 120/240 delta and 480/277 Y next. Rare is 120/208 y. Some motel drops but most 120/208 is from customer owned transformers with 480 drops. Of the 100+ commercial customers in Centerpoint territory none have 120/208. Now in Entergy (utility co.)territory (north of Houston) again 120/240 single phase is most common, followed by 120/208 y then 120/240 open delta, then 120/240 delta & 480/277Y. My shop is in Entergy territory and is served by 120/208 y although I have only 120/208 single phase serving my suite.

It's amazing how much variation there is among utilities in their construction standards, as contrasted with work on the customer side of the meter.

For example, in my area (Dominion Virginia Power territory), there seems to be no effort to identify phases on distribution lines or to keep them in the same positions on the pole. It's common to see primary conductor positions interchanged where the line turns a corner. Linemen refer to wires only by their relative locations on the crossarm at a given point: "road phase", "center phase" and "field phase". Even on service drops from aerial three-phase transformer banks, I don't see any phsae identification. It appears that phase rotation is only established when connection is made to the cuttomer's service wires.

In contrast, PEPCO, the neighboring utility to the north (serving DC and its Maryland suburbs), seems to identify phase positions consistently on its primary lines. On certain poles I see wooden placards with nailed-on metal letters "A", "B" and "C", which I assume are phase labels. I haven't looked closely at PEPCO's construction so I don't know how the identification is carried through the primary and secondary lines.

Around here the PoCo used red, white or blue " labels" on some crossarms or at the bottom of the pole to identify what phase is what, especially if there is more than one circuit...
Even the single phase pad-mount transformers have either a red, white or blue sticker on them to identify what phase they are supplied from.

A.D

Where phases cross at angle poles was apparantly a necessary 'roll' of the wire to maintain separation. And you are right, very rarely are phases marked on overhead. Anytime a corner/junction pole is changed out, a major part of the work is making sure everything is put back exactly the same, so that customer rotation is not changed, and so that everything phases out for backfeeding purposes.

As far as marking phases on service drops, it really doesn't matter to us except that on newer meters the high leg, if there is one, needs to feed through the meter on the far right (the customer can move it to the middle on the next wire landing). Although, if you see quad (or triplex) service wire (underground or overhead), the wires are marked Phase A, B, and C from the factory.

Obviously, if work is performed on a three phase service, rotation will be maintained.