Question about rural power distribution

[bw77]

This is in rural upstate NY.

Power pole carries 3 wires on top, and 3 wires below.
2 of the lower wires come into the transformer. And a ground comes
up to the transformer. 3 wires enter the house.

House has 120 and 240 circuits.

My question is about the 2 wires coming down to the transformer.
Are these 2 legs of a single phase? If so, what is the 3rd wire that
is not coming to the transformer.

Or are the lower 3 wires 3-phase? But if so I don't understand how
that would work.

Thanks.

 

[nh_yota]

Top and bottom rows are both 3 phase but at different voltages. Transformer is hooked between two of three phases.

 

[wyliesdiesels]

as has been said both are 3-phase.

what is it that u dont get?

 

[md21722]

I think he's wondering why the transformer has 2 wires.

In many rural areas there is only one 7200 volt wire running down the road and only one wire to the transformer...

 

[matt_i]

This is one reason the PoCo doesn't want to just "give away" 3phase power. They have to furnish and mount 2 more transformers on the pole at their cost...

 

[wyliesdiesels]

I think he's wondering why the transformer has 2 wires.

In many rural areas there is only one 7200 volt wire running down the road and only one wire to the transformer...

Thats called SWER

https://en.m.wikipedia.org/wiki/Single-wire_earth_return

 

[ishiboo]

Thats called SWER

https://en.m.wikipedia.org/wiki/Single-wire_earth_return

The OP's is not a SWER setup though. (I know you know that, just clarifying for the OP) The neutral is center-tapped from the transformer.

To answer the OP's question... voltage is the difference in potential between two points. A 120v outlet has one wire at 0V, and another at 120V. Put two together at a 180 degree shift and you have how single-phase is distributed in the US - whatever one leg is at, the other is at the exact opposite. The transformer brings the distribution voltage down to +/- 120V per leg, with a 0V neutral in the middle. So even though 2 wires go into the transformer, 3 come out. Then it gets to your main panel, disconnect, etc. and becomes 4 wires.

 

[86turbodsl]

This is in rural upstate NY.

Power pole carries 3 wires on top, and 3 wires below.
2 of the lower wires come into the transformer. And a ground comes
up to the transformer. 3 wires enter the house.

House has 120 and 240 circuits.

My question is about the 2 wires coming down to the transformer.
Are these 2 legs of a single phase? If so, what is the 3rd wire that
is not coming to the transformer.

Or are the lower 3 wires 3-phase? But if so I don't understand how
that would work.

Thanks.

There are 2 runs of primary there, likely different circuits and they will diverge somewhere down the road. Are you close to the substation? Since there's 3 wires each, and i don't see a ground wire below any of them, i'd say that's 2 separate phases on each crossbar, with a central ground. If you see 3 wires on a crossbar with a single below a couple feet, that's true 3phase. You could likely get 240V 3phase grounded leg at your location with having 2 phases there.

 

[bw77]

Are you close to the substation?

It's about 4 miles from the substation.

 

[bw77]

as has been said both are 3-phase.

what is it that u dont get?

I have always thought of 3-phase power as being used to power 3-phase
motors.

At home, in the suburbs, we have a single hot wire coming down to a
transformer, a ground coming up, center tapped, 2 hots and a neutral
coming into the house. So the 2 hot legs are 180 degrees apart,
providing 240v.

Visiting relatives in the country, I see they have 2 lines coming into
the transformer and I was not sure what it was. If they are 2 lines
of 3-phase does that mean that the 2 hots are 120 degrees apart?

 

[theoldwizard1]

Visiting relatives in the country, I see they have 2 lines coming into the transformer and I was not sure what it was. If they are 2 lines of 3-phase does that mean that the 2 hots are 120 degrees apart?

It depends on the type of 3 phase power, Delta or Wye.

On 208V 3 phase, any one of the 3 hots and the neutral will give 120V.

 

[wyliesdiesels]

The OP's is not a SWER setup though. (I know you know that, just clarifying for the OP) The neutral is center-tapped from the transformer.

To answer the OP's question... voltage is the difference in potential between two points. A 120v outlet has one wire at 0V, and another at 120V. Put two together at a 180 degree shift and you have how single-phase is distributed in the US - whatever one leg is at, the other is at the exact opposite. The transformer brings the distribution voltage down to +/- 120V per leg, with a 0V neutral in the middle. So even though 2 wires go into the transformer, 3 come out. Then it gets to your main panel, disconnect, etc. and becomes 4 wires.

SWER has nothing to do with whether there is a center tapped neutral or not.

SWER would be on the primary side and I have seen distribution with SWER in California. One primary bushing connects to overhead wire, other primary bushing connects to ground rod at base of pole AND the center tapped neutral secondary bushing.

 

[mburrus]

what you have here is likely 33kv up top, running from town to town feeding small substations... below is likely 4-13kv, in a delta configuration. because it is delta connected, there is no ground, and thus 2 phases have to be brought to the transformer to complete the circuit. there also appear to be no fuses on the high side, indicating that this is a "completely self protected" transformer. the neutral for the house service is generated at the transformer, and likely bonded to ground at the house, and possibly the pole, if there is a pole ground.

 

[wyliesdiesels]

There are 2 runs of primary there, likely different circuits and they will diverge somewhere down the road. Are you close to the substation? Since there's 3 wires each, and i don't see a ground wire below any of them, i'd say that's 2 separate phases on each crossbar, with a central ground. If you see 3 wires on a crossbar with a single below a couple feet, that's true 3phase. You could likely get 240V 3phase grounded leg at your location with having 2 phases there.

Out here PoCos put all 4 wires on the same crossbar.

Why would the OP want corner grounded Delta 240v as opposed to 120v/240v high leg Delta 4-wire?? 4-wire Delta is more versitle.

The OP would have to install his own split phase transformer if he wanted 120v.

I have always thought of 3-phase power as being used to power 3-phase motors.

At home, in the suburbs, we have a single hot wire coming down to a
transformer, a ground coming up, center tapped, 2 hots and a neutral
coming into the house. So the 2 hot legs are 180 degrees apart,
providing 240v.

Visiting relatives in the country, I see they have 2 lines coming into
the transformer and I was not sure what it was. If they are 2 lines
of 3-phase does that mean that the 2 hots are 120 degrees apart?

Any 2 legs of a 3phase supply will give u line voltage

So what u have is an SWER primary distribution. The main advantage is savings on wire costs since theres only 1 hot wire.

Yes the 3 legs of a 3-phase supply are 120* agrees apart.

It depends on the type of 3 phase power, Delta or Wye.

On 208V 3 phase, any one of the 3 hots and the neutral will give 120V.

Hes NOT talking about voltage. Hes talking about phase angles.

 

[altec876]

It's a delta primary system and "y" secondary to the house. Most likely the transformer has a small lever on the side which is a internal switch. The older cans had these a lot especially with co-ops less extra parts cheaper to install.

 

[wyliesdiesels]

It's a delta primary system and "y" secondary to the house. Most likely the transformer has a small lever on the side which is a internal switch. The older cans had these a lot especially with co-ops less extra parts cheaper to install.

Its only a single phase can. Theres only 2 primary bushings. How can it be a y secondary?

In town here i know of 1 house that is fed wye secondary 208/120v 3-wire but it comes from a 3-phase 3 can bank.

 

[TractorJeff]

Those do not look like 13kv capable insulators on the lower beam. (Upper Insulator height looks tall enough for 14.4kv, IMO) Lower Beam look more like 4160 volt insulators. The clearance between the two live wire insulators is double the clearance to the closest insulator in photo. This wire would probably maybe be the ground?
Its been 30 years since I studied Power Distribution in NYS so cost and standardization has surely changed.

 

[boomer12831]

34.5 kv on top - sub transmission, 4800 delta in the middle - primary 3 phase, 120/240 secondary on the bottom single phase