3 phase service type ID by powerline tap configuration?

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Clodbuster

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Have a question that's been bugging me for a while. Hopefully somebody can help enlighten me. I've googled extensively but haven't found any answers there yet, and I'm no expert in three phase & POCO stuff.

I live on a farm that has service from the POCO serving both single phase (homes and buildings) and three phase (grain elevator motors). I do not know whether the three phase is delta or wye connection as I've never had reason to check. But it's original per the 1960s installation date in a pretty rural area. There are two fused and one unfused taps at the powelines which run underground to two pad-mount transformers in the yard, then 240/120V on from there.

A year ago the road was improved, which required the POCO move it's lines, at which time they upgraded them from 3 conductors to 4, as shown in the pic. They then hooked my power back up without any changes to my 3-wire service configuration, although the elevator motors ran backwards and we had to swap leads in the panel to correct. And one of their conductors stayed untapped.

Power Lines.jpg

The questions I have are regarding delta vs. wye:

1) Can I tell what configuration I have by looking at the powerline taps or is that determined at the transformer?

2) Am I limited somehow on configurations by the fact that there's only 3 conductors between the powerlines and yard?

3) What is the significance of the powerline change from 3 conductors to 4?

Thanks :blink:
 
You likely have an open delta
secondary, with a wye primary. They probably upgraded the feeder to a true 3 phase feeder, by dropping in the 3rd conducted. 3 phase is possible from 2 transforners, and you likely have 2 pad mounts wired up to serve single phase and 3 phase loads.
Out of curiosity , do you know if you have a high leg? As in one phase to ground voltage of 208v?
You are limited to an open delta or open wye bank, due to the fact that there are only 2 phases in the conduit, a 3rd line has to be added for true 3 phase.

86ca882e20e943d624f2d14fdf52aec3.jpg


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Last edited:
Appreciate the input on this truck41trouble. I'm going to look over the diagrams you sent and try to figure it out. I was hoping someone would send a pic.

truck41trouble said:
you likely have 2 pad mounts wired up to serve single phase and 3 phase loads.
Click to expand...

That is exactly what I have - Two pad mounts side by side, served underground by the POCO taps and then with separate feeds going from there to the single and three phase loads.

truck41trouble said:
Out of curiosity, do you know if you have a high leg? As in one phase to ground voltage of 208v?
Click to expand...

Regarding the high leg, where at? Do you just mean at the main lugs one voltage will be higher lug-lug?
 
Yes, "b" phase after the meter. If you have a high leg it will read 208 volts line to ground/ neutral. Knowing if you have a high leg can confirm the type of connection you have.

Also, do you have one meter, then distribute your load. Or do you have a 3phase and a single phase meter?

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If you have three phase service - your only choice here is open delta primary on your service transformer if you only have two ungrounded and one grounded conductor on the primary supply. Single phase supply is available from the same secondary.

See open delta schematics in previous posts. All other common configurations will require third phase of the primary circuit to be present.
 
170212-1410 EST

Clodbuster:

I am not a power company person or even a power major.

That your transformers were not changed, that there are only two transformers means along with the other information you provided that on the transformer secondary side you have an open delta with a wild leg format. This means one transformer has a secondary center tapped winding and provides you with your single phase service. That center tap is your single phase neutral and is grounded (earthed). The second transformer whether it has a secondary center tap or not is not important, and would be unconnected. The second transformer supplies the open delta wild leg.

Your secondary connections and how they are used is unrelated to whether the primary is a delta or wye source.

In my neighborhood we have a three wire delta primary source, at least from my perspective. Whether it is delta or wye at the substation is of no importance to me. The advantage to me to have this kind of source is that there is no primary neutral that links every home in the neighborhood. I only share my secondary neutral with one other home, and that is next door. Thus, I am isolated, neutral wise, from about 30 other homes, except thru our water line.

.
 
truck41trouble said:
Yes, "b" phase after the meter. If you have a high leg it will read 208 volts line to ground/ neutral. Knowing if you have a high leg can confirm the type of connection you have.

Also, do you have one meter, then distribute your load. Or do you have a 3phase and a single phase meter?

Sent from my VS987 using Tapatalk
Click to expand...

Checked voltages this afternoon and it is 240V leg-leg for each leg, and then 120V to ground on two of the legs. Final leg-ground voltage on the red leg is 240V.

They set it up with two meters here. One for the rental house (single phase only) and one for everything else, both residential single phase and three phase at the elevator.
 
240 leg to leg, 120 leg to ground for two legs and 240 leg to ground for the third leg is just not possible with three phase.
208 is possible, and that seems to be a large measurement error to get 240.
 
GoldDigger said:
240 leg to leg, 120 leg to ground for two legs and 240 leg to ground for the third leg is just not possible with three phase.
208 is possible, and that seems to be a large measurement error to get 240.
Click to expand...

You are correct sir. I double checked and the wild leg is indeed 208V (or 213V to be exact), not 240V as I stated before.

Appreciate the input from everyone on this. In looking at the panel and the diagrams it all makes sense now. I can definitely see why the POCO did this, as rural as we are here. Saved themselves some wire. And for seasonal motor loads the loss of efficiency isn't too bad for us.

Also, in educating myself about this I discovered that the previous electrician wired the 240V/50A welder outlet using the wild leg, so one line-ground on the outlet is 213V. :eek:
 
Clodbuster said:
Also, in educating myself about this I discovered that the previous electrician wired the 240V/50A welder outlet using the wild leg, so one line-ground on the outlet is 213V. :eek:
Click to expand...

That is often not a problem, but check with the welder manufacturer.
If the insulation in the welder is not good for more than 150V, for example, you might have a problem. Not likely though....

mobile
 
170213-2443 EST

Clodbuster:

How did the power company save any wire? Three wires minimum are required for three phase (a delta source). A wye source with neutral would use four wires. However, I could use two hots and neutral from a wye source to create a three phase output after transformers.

.
 
gar said:
170213-2443 EST

However, I could use two hots and neutral from a wye source to create a three phase output after transformers.

.
Click to expand...

Yes, although I admit I have never understood how that works. I understand open delta, and have seen the diagrams, but it still doesnt click. There must be some hocus pocus going on ;)

Regarding the POCO running two phases and a neutral, why would they do that? Only thing I can think of is such a line was originally two wire, and they ran a third later to expand the capacity of the line. If the original line was a phase and neutral, they could run another phase and pick up some of the loads with that, or if it was originally 2 phases, running the neutral would allow them to connect existing transformers phase to neutral and up the voltage by 1.73.
 
Clodbuster said:
...Also, in educating myself about this I discovered that the previous electrician wired the 240V/50A welder outlet using the wild leg, so one line-ground on the outlet is 213V. :eek:
Click to expand...

There is not a problem with a branch circuit using the high leg for a 240V load but I bet it's tied to a 120/240V breaker and not a straight 240V breaker which means the max voltage to ground the breaker is rated for is 120V.
 
The underground medium voltage cable the POCO used has a concentric neutral. (Center ungrounded conductor, insulation, then the neutral wraps around that sort of like the neutral in SEU cable). Sometimes that concentric neutral is the outer portion of the cable, though today there usually is another covering over the neutral.

POCO saves 1 more run of the cable, the neutral is already in each cable, but is not insulated for medium voltage so is not going to be an option to cheat somehow and use one of those neutrals as an ungrounded conductor.

To utilize open delta primary on a bank you need three supply conductors, it can be all three phases or it can be two phases and a neutral.

Here we sometimes find pole top open delta bank in the middle of what looks like a single phase primary line. What it really is, is one phase and neutral coming from one direction and another phase and neutral coming from the opposite direction.
 
gar said:
170213-2443 EST

Clodbuster:

How did the power company save any wire? Three wires minimum are required for three phase (a delta source). A wye source with neutral would use four wires. However, I could use two hots and neutral from a wye source to create a three phase output after transformers.

.
Click to expand...
Not only the power company, but the user saves some money/ wire too. The cost to bring the 3rd phase in to his riser pole can be very expensive, as well as the cost of another ug primary run.

As long as the 3 phase load isnt crazy, this setup works well. I still build out an open delta bank on occasion, it just depends on the customers needs and load.

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truck41trouble said:
Not only the power company, but the user saves some money/ wire too. The cost to bring the 3rd phase in to his riser pole can be very expensive, as well as the cost of another ug primary run.

As long as the 3 phase load isnt crazy, this setup works well. I still build out an open delta bank on occasion, it just depends on the customers needs and load.

Sent from my VS987 using Tapatalk
Click to expand...
Or how many miles the third phase conductor needs to be run, especially for somewhat limited load service.
 
electrofelon said:
Yes, although I admit I have never understood how that works. I understand open delta, and have seen the diagrams, but it still doesnt click. There must be some hocus pocus going on ;)

Regarding the POCO running two phases and a neutral, why would they do that? Only thing I can think of is such a line was originally two wire, and they ran a third later to expand the capacity of the line. If the original line was a phase and neutral, they could run another phase and pick up some of the loads with that, or if it was originally 2 phases, running the neutral would allow them to connect existing transformers phase to neutral and up the voltage by 1.73.
Click to expand...
Sometimes that isn't a "true" neutral. By true I mean, not returning to the substation or the isolation bank.

If 2 phases of delta are already present, and the end use warrants an open delta bank, we usually run a "fake" neutral about 3-4 poles away in each direction, with ground rods at every pole.

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truck41trouble said:
Sometimes that isn't a "true" neutral. By true I mean, not returning to the substation or the isolation bank.

If 2 phases of delta are already present, and the end use warrants an open delta bank, we usually run a "fake" neutral about 3-4 poles away in each direction, with ground rods at every pole.

Sent from my VS987 using Tapatalk
Click to expand...
if you actually have a grounded neutral at the source then you are simply using earth as a path back to the neutral. If you don't have a neutral at the source it shouldn't work, period.
 
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