3-phase overhead with three conductors?

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Old 12-18-07, 08:06 AM
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3-phase overhead with three conductors?

I was looking at my overhead drops this morning for other reasons and noticed that the small 3-phase services that we have are wired with two insulated conductors and one uninsulated conductor. The insulated conductors are marked: one with blue tape and one with red.

I've never paid much attention to these before but wouldn't the "black" phase have 208 or whatever voltage to ground? They go into weatherheads fairly high up on the building, but I figured they would have insulated all the ungrounded conductors. These sure look like they are going back to a 3-phase transformer, and there is no other visible feed for our 3-phase service. The spans between poles run in parallel to single-phase service from a different transformer.
 
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Old 12-18-07, 09:28 AM
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Can you record an accurate voltage-reading where the Service Conductors terminate on the Main breaker?

If the voltage across the two insulated ( Un-Grounded ) conductors is 208 , and the voltage between any insulated conductor and the "Neutral" ( bare or white Grounded Conductor) is 120 , then the "source" is a 3-phase, 4-wire, 208/120 volt system.

The maximum voltage-to-Ground is 120 volts for both 3-phase and single-phase systems.
 
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Old 12-18-07, 09:49 AM
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open delta and Y-delta are a bit different
 
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Old 12-18-07, 10:06 AM
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Thanks, 120 volts, I got that. But I don't want to get fixated on the voltage. I am wondering about the uninsulated conductor. Is this typical? Am I missing something?
 
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Old 12-18-07, 10:13 AM
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240 volts across the "live" conductors = single-phase system

208 volts across the "live" conductors =3-phase system.

The un-insulated conductor is the "Neutral" of the system which is required to be connected to a Grounding Electrode ( metallic water-line , rod )
 
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Old 12-18-07, 11:45 AM
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I doubt that the uninsulated conductor is a phase. You might only be looking at the two (additional) phases.

And, I don't know that i've ever seen a pole mounted 3 phase transformer. But, I believe there is a way to get 3 phases with 2 transformers.
 
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Old 12-18-07, 02:56 PM
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=PATTBAA;1279076]240 volts across the "live" conductors = single-phase system

208 volts across the "live" conductors =3-phase system.
are you sure about this PATTBAA??
 
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Old 12-18-07, 03:23 PM
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Originally Posted by telecom guy View Post
I doubt that the uninsulated conductor is a phase. You might only be looking at the two (additional) phases.

And, I don't know that i've ever seen a pole mounted 3 phase transformer. But, I believe there is a way to get 3 phases with 2 transformers.
Have a look here, tell me what you think.

The small weatherhead in the first photo is the one that is being fed by the transformer in the second and third photos.




 
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Old 12-18-07, 03:35 PM
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that POCO single can it can be wired either Delta or wye connection.


but looking at the photo for service drops sound like you got one set run on corner grounded delta system this system is very tricky to trobleshooting.

And for any average DIY this part is way out of their scope.

useally a qulaifed electrician will deal with this system.

the way with corner ground voltage read this

L1 -L2 = 240 volts , L2-L3 = 240v, L3-L1 = 240 v but here is the kicker

L1-N 120 v , L2-N 120 volts L3-N = 0 [ yeah zero volts ]


reedit it shouldbe L1-G , L2-G, both will read 120 volts but L3-G will read zero and also on L1-L2=240 L2-L3=240 L3-L1= 0 volts because this corner grounded system is intened to be grounded it very easy to get mix up with single phase system if not carefull.


and you will notice on the photo on the right side a smaller service entrance drop that is singe phase on that side the rest of it on left side all three riser is 3 system

the other way you can tell is check the meter face plate it will marked 3 3w = delta [ can be CG or NCG ]

if marked 34W that is 4 wire delta which you dont have that on the photo

if marked 13W that is single phase

but some case it will feed off from network so L-L will be 208 volts

If need more question please do post it here

Merci, Marc
 

Last edited by french277V; 12-18-07 at 05:15 PM. Reason: add more info and reedit and corrected the mistake Thanks Nap !!
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Old 12-18-07, 04:59 PM
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french, you're slipping. With a corner grounded delta, as in your description, L3 is your neutral (or more accurately; grounded conductor) Would you like to tell me how you get 240 volts L1 to L3 but 120 volts L1 to ground?

the way to get 120 and 240 from a delta is if it is center tapped on one winding. That is where you end up with a 208 hot to neut/ground, 120 to ground for the other 2 legs and 240 leg to leg.

Argmematey; in the photos of the x-former, I see no bare conductor. Is there one I just can't see?

telecomm guy; I have seen 3 pigs on a pole before. Actually quite common around here. and yes, you can have an "open delta" transformer set up.

her is a link that is pretty good with showing x-former connections.
 
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Old 12-18-07, 05:11 PM
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Nap:

Thanks for catching this one i was only half wake after working long day today i feel like this >>

anyway i will try to edit it and see if i can get this corrected before it get out of hand.

i should typed " ground " instead of Netrual

that only way i can get the referince point.

Merci, Marc
 
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Old 12-19-07, 03:59 AM
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Originally Posted by nap View Post

Argmematey; in the photos of the x-former, I see no bare conductor. Is there one I just can't see?
In the second photo, showing the secondary wiring, the center jumper from the side of the can looks like it is h-tapped to the bare conductor on the "inboard" secondary distribution. All three H-taps are together, but two are insulated and the third, on the bare conductor, is not.
 
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Old 12-19-07, 04:19 AM
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Originally Posted by french277V View Post


... and you will notice on the photo on the right side a smaller service entrance drop that is singe phase on that side the rest of it on left side all three riser is 3 system

the other way you can tell is check the meter face plate it will marked 3 3w = delta [ can be CG or NCG ]

if marked 34W that is 4 wire delta which you dont have that on the photo

if marked 13W that is single phase
1. The smaller service is the one fed by the pictured can. I know that the disconnect and meter are marked 3, but I asked the people who switched out the meters about this and they clearly didn't know the answer.

I will check on the inside conduit to see how things are routed inside the building, but is "open delta" meaning two phases from one transformer, and the third phase from the other?

2. The larger service is 1, not 3. If I look at the far left side of the weatherhead photo, I can see that the bare conductor is tied to the insulator on the wall, and the bare H-tap connects the bare conductor to the house wiring that is marked with white electrical tape.
 
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Old 12-19-07, 05:56 AM
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I see that it certainly is a 3 phase pole mounted transformer, pretty rare to see down here. Usually, they mount 2 or 3 single phase to get 3 phase. In any case, your picture clearly shows a 3 phase delta system, with one leg apparently grounded.

Also, what is that "stuff" wrapped to the neutral pole to pole conductor(s)? Down here, I don't see anything wire wrapped like that except for telephone and CATV.
 
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Old 12-19-07, 07:45 AM
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Originally Posted by telecom guy View Post

Also, what is that "stuff" wrapped to the neutral pole to pole conductor(s)? Down here, I don't see anything wire wrapped like that except for telephone and CATV.
From the looks of it the neutral is used as the messenger and the wire suspending the hots from the neutral is called lashing.
 
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Old 12-19-07, 01:57 PM
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french, you didn't make the correction I was speaking of. On a corner grounded delta, there is only one voltage, typically 240 or 480. That is all you will ever read. There is no 120 voltage available from corner grounded delta. (unless it was a 120 volt secondary, something I have never seen from a POCO. it would have to be in house)

argmematey, did you go to that link I gave earlier?. It shows an open delta config.
 
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Old 12-19-07, 05:24 PM
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Originally Posted by nap View Post
french, you didn't make the correction I was speaking of. On a corner grounded delta, there is only one voltage, typically 240 or 480. That is all you will ever read. There is no 120 voltage available from corner grounded delta. (unless it was a 120 volt secondary, something I have never seen from a POCO. it would have to be in house)

argmematey, did you go to that link I gave earlier?. It shows an open delta config.
I did. Do I understand this right:

A-B 240
B-C 240
C-A 240

And this is the part I find confusing:
A-Ground 0 VAC (Since that's the bare conductor)
B-Ground 240 VAC?
C-Ground 240 VAC?

I figure if A is bare it must be at 0 relative to ground.
So then both B and C must be at 240 relative to A.

Is that right?

Also I don't really get how the rotation works since I don't see the 120-degree relationship between A and the other two. It would be nice to see a can cross-section to see how this is actually done. But that will probably have to wait until my next lifetime, when I will be an EE.
 
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Old 12-19-07, 08:40 PM
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Nap:
that methold i used is my common pratice to find corner grounded delta system.

[ we dont have many corner grounded delta system around here now getting more rarer now. and all the new service must have WYE system reguradless of what the system voltage now.]

for exsting comuster we still have the delta system on here and when we upgrading it most of the time POCO will not use any transformer larger than the main breaker or load demand is there that about it with delta system IE 200A on single phase load and 40A on three phase load the POCO will expand the delta size to the limit of the exsting main breaker but if go from 200 amp to 400 amp the POCO useally will say just swap from delta to wye connection.

Merci, Marc
 
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Old 12-23-07, 01:09 PM
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Consider that the structure may be suppiled by two seperate Services, as permitted by 230.2 (D) which reads----

"Additional Services (are) permitted for different voltages, frequencies, phases, or uses ---"

There may be a 400 amp, 3-phase Service ( 240 volts?) for a power system, and a single-phase , 3-wire, 120/240 volt system for receptacles , liting, etc.
 
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Old 04-13-09, 05:12 PM
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What is POCO???

is poco an abbreviation for something?
 
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Old 04-13-09, 05:18 PM
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Yes

POwer COmpany

that would be the folks that sell you your electricity.
 
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