# Residential Street Wiring & Pole Transformer Quest's, Please

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
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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).

Yes, that is a very common setup.

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.

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
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daestrom wrote:

Many localities use the same conductor for the distribution voltage neutral and the service voltage neutral. A careful look with binoculars at many pole pigs will reveal that the primary and secondary neutrals are strapped to each other and connected to the secondary neutral. Since most of the US uses a Multi Grounded Neutral (MGN) using the same conductor for both neutrals reduces cost and improves the grounding of the entire system. whenever I have a chance to measure neutral current on the neutral of a residential service with the transformer disconnected from the grid I always find some measurable current using the homes neutral. What I have never been able to determine with the time and equipment in hand is whether the current is from other homes that have neutral problems or from some of the primary neutral current returning to the sub stations transformers through the earth. -- Tom Horne
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? 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.
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ISC (I Stand Corrected). Thanks,
daestrom
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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. -- Tom H
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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
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<snip>
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.
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Robert11 wrote:

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

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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.
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Greg wrote:

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
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If you are talking about what I think you are, that is right.

For what it's worth, I'm also in the general Boston area, and I've seen labels on transformer cans stating they were for 7970/13800 volts.

Phase-phase voltage divided by sqrt(3). For 13,800V, it's about 7970 volts.

Usually grounded internally.
Look carefully in different areas. You'll eventually see a transformer with two high voltage bushings with one lead connected to a hot wire and the other to the neutral (usually below the transformer, and usually with the neutral of the secondary connected to it). You'll also see transformers with two HV bushings with both connected to (different) hots. The first isn't usually done because transformers with two HV bushings are more expensive than those with only one.
--
-Mike

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On Wed, 9 Feb 2005 03:40:19 +0000 (UTC) Michael Moroney
|>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 ? | | Usually grounded internally. | | Look carefully in different areas. You'll eventually see a transformer | with two high voltage bushings with one lead connected to a hot wire and | the other to the neutral (usually below the transformer, and usually with | the neutral of the secondary connected to it). You'll also see | transformers with two HV bushings with both connected to (different) hots. | The first isn't usually done because transformers with two HV bushings are | more expensive than those with only one.
Generally a separately derived system is one where a whole new neutral is established, as well as all phase conductors. The utility practice of connecting primary and secondary neutral together means it is acting more like an autotransformer, and is not a true separately derived system. Draw up the schematic, and you can see it's really a buck/boost (both at the same time) transformer with the ground attached at the convergence point (not your usual autotransformer).
FYI, if the ground wire on the pole ever fails, then the only ground on the transformer is the service drop. If the primary lines ever have a neutral go open (something that can be caused by lightning), then you have the classic "open neutral", but at much higher voltage. And it will be your service drop that is sinking that voltage to ground.
--
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| Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
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