Future three phase - what is my power company doing?

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jlkunka

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Moseley, VA
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mechanical engineer
I recently got service hooked up in a building that I am renovating. Years ago it had three phase service (it was a sawmill, but all equipment was auctioned off), but I wanted to get up and running quickly with 400A single phase. I discussed my needs with the power company, and said I would be going back to 480Y/277 three phase a few years out, as my business grew. They said they would accommodate this and "rough in" for the three phase.

See links to attached photos for what they did. I have a 100kva single phase transformer (on the left), and two empty transformer boxes. Primary service is 34500/19920 and there is a hot lead run into each of the empty boxes. My question is, what are they going to put in when I go to three phase? Do they replace the single phase transformer with three discrete transformers connected to give me 480Y/277? I need to begin planning for what to install inside to be ready for the three phase.

Installed Transformers Transformer Tag
 
I doubt they would make a three phase bank from 3 single phase pads. I have never heard of such a thing. I am pretty sure it would be a single three phase unit. Do you know how large your future service might be? IIRC pad size changes above 100kva.
 
electrofelon said:
I doubt they would make a three phase bank from 3 single phase pads. I have never heard of such a thing. I am pretty sure it would be a single three phase unit. Do you know how large your future service might be? IIRC pad size changes above 100kva.
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What you say makes sense but look at the photos that are linked below my question. They placed 3 discrete boxes about 10' apart and there is one primary wire coming up into each Box. The transformer on the left is my single phase transformer with low voltage leads coming over to my building. The other 2 boxes just have the primary wire to them. I know there are examples of discrete Pole mounted 3 phase Transformers to service a building, and I figured they had the same for pad mounted Transformers but I'm unable to find any examples of this.

Guessing my three phase service would be 100kva as well. The two dummy pads are concrete and the single phase pad is composite.
 
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I have seen pole mount transformers used as pad mount, they had a large enclosure that they put them in. Their logic behind it was they have to keep a spare in the yard, and since at the time, was the only pad mount 480/277 service they had, they didn’t want that amount of money sitting around, but it was only one pad.
 
jlkunka said:
What you say makes sense but look at the photos that are linked below my question. They placed 3 discrete boxes about 10' apart and there is one primary wire coming up into each Box. The transformer on the left is my single phase transformer with low voltage leads coming over to my building. The other 2 boxes just have the primary wire to them. I know there are examples of discrete Pole mounted 3 phase Transformers to service a building, and I figured they had the same for pad mounted Transformers but I'm unable to find any examples of this.

Guessing my three phase service would be 100kva as well. The two dummy pads are concrete and the single phase pad is composite.
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Ya Ya, I saw the photos. It is odd, dont know what they are thinking. As I said, never heard of a three phase bank made up from 3 single phase pads and cant imagine it being done that way. It is theoretically possible, but note that each single phase unit would need to be 277 to make a 480y/277 bank and that is an odd duck (for a pad). IF I were the POCO, I would just leave the two spare phases coiled up in the first pad, or perhaps the second since it is quite reasonable that the first would remain for single phase service.
 
Where is this at? Rural in all underground area?
I could see where this would be done to bring all three phases in, although I don’t know why they wouldn’t use a three phase vault.
later on they will bring your big three phase XF from the three singles into one.
just get the infrastructure into place.
I'm with felon here also.. it won’t be 277/480 from single phase XFs.
i have built open deltas and 120/208 from singles, but it isn’t recommended because of the way pads are built And their proximity from a fuse.
 
electrofelon said:
Ya Ya, I saw the photos. It is odd, dont know what they are thinking. As I said, never heard of a three phase bank made up from 3 single phase pads and cant imagine it being done that way. It is theoretically possible, but note that each single phase unit would need to be 277 to make a 480y/277 bank and that is an odd duck (for a pad). IF I were the POCO, I would just leave the two spare phases coiled up in the first pad, or perhaps the second since it is quite reasonable that the first would remain for single phase service.
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I've seen a few three phase "banks" made up of single phase padmounts. Majority of the ones I have seen are at least 40 years or so old. All of them I have seen are delta secondary, even 480 delta on at least one I am aware of.
 
I was very clear with my POCO rep on what service I wanted in the future, 460Y/277 and NOT 208Y/120. I will have motors up to 100HP and I don't want to spend all my money in heavy gauge wire and conduit. He told ME the records on the old service was 208Y/120, but when I opened the (abandoned) transformer it shows it is a 750kva 480Y/277 (see here), so the documentation for this property is/was+ incorrect. He also said this was his first "complicated job", so did he just launch everyone down a strange path? He also had them replace the pole, where the original seemed fine to me.. (new pole seen here).
Once again, looking at the future setup, we have ONE high voltage wire coming into each pad, so how would this be a delta setup? Wouldn't it be a Y with three 277v transformers all grounded individually? When they pull new wires into the building, where do they get a neutral?
 
jlkunka said:
I was very clear with my POCO rep on what service I wanted in the future, 460Y/277 and NOT 208Y/120. I will have motors up to 100HP and I don't want to spend all my money in heavy gauge wire and conduit. He told ME the records on the old service was 208Y/120, but when I opened the (abandoned) transformer it shows it is a 750kva 480Y/277 (see here), so the documentation for this property is/was+ incorrect. He also said this was his first "complicated job", so did he just launch everyone down a strange path? He also had them replace the pole, where the original seemed fine to me.. (new pole seen here).
Once again, looking at the future setup, we have ONE high voltage wire coming into each pad, so how would this be a delta setup? Wouldn't it be a Y with three 277v transformers all grounded individually? When they pull new wires into the building, where do they get a neutral?
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That high voltage wire is concentric neutral cable. If it’s full neutral then it could possibly be something like a 1/0 conductor with a number two copper concentric neutral around it.
 
So I broke down and called my POCO rep for an explanation. They wanted to get the high voltage down off the pole and underground. When I go three phase, since all three phases run into (or past) the box on the right, it will be replaced with a three phase transformer. they'll dig up the other cables and bring them up into that new unit as well. ((see here) I told him that the abandoned transformer was 480Y/277 and he said he wasn't surprised their documentation was incorrect, they wouldn't have reused it anyway. Since the work order is closed, I suspect they plan on storing it on my property forever, or at least until they place a new three phase transformer.
 
jlkunka said:
So I broke down and called my POCO rep for an explanation. They wanted to get the high voltage down off the pole and underground. When I go three phase, since all three phases run into (or past) the box on the right, it will be replaced with a three phase transformer. they'll dig up the other cables and bring them up into that new unit as well. ((see here) I told him that the abandoned transformer was 480Y/277 and he said he wasn't surprised their documentation was incorrect, they wouldn't have reused it anyway. Since the work order is closed, I suspect they plan on storing it on my property forever, or at least until they place a new three phase transformer.
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If that is the plan seems the best plan would be to bring three primary cables up in same place and just use one of them for now, why mess around with the two extra enclosures?
jlkunka said:
I was very clear with my POCO rep on what service I wanted in the future, 460Y/277 and NOT 208Y/120. I will have motors up to 100HP and I don't want to spend all my money in heavy gauge wire and conduit. He told ME the records on the old service was 208Y/120, but when I opened the (abandoned) transformer it shows it is a 750kva 480Y/277 (see here), so the documentation for this property is/was+ incorrect. He also said this was his first "complicated job", so did he just launch everyone down a strange path? He also had them replace the pole, where the original seemed fine to me.. (new pole seen here).
Once again, looking at the future setup, we have ONE high voltage wire coming into each pad, so how would this be a delta setup? Wouldn't it be a Y with three 277v transformers all grounded individually? When they pull new wires into the building, where do they get a neutral?
Click to expand...

Delta would be the secondary, there will be jumpers between each transformer on the secondary to connect the corners of the delta. Only a delta with a high leg has a neutral, it would go to whichever unit has the center tap utilized.
 
kwired said:
If that is the plan seems the best plan would be to bring three primary cables up in same place and just use one of them for now, why mess around with the two extra enclosures?
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kind of what I’m wondering..

makes no sense to put it in three separate enclosures and then dig it up and reinstall it later in a pad mount..
whatever...
 
The POCO messed up. No reason to have separate enclosures to "get it down off the pole". They got the voltage wrong and I'd bet they were planning to make a 240/120 delta service later.

Yes you can do that with pads and I think we still have a couple. No reason to do that anymore except in an emergency. Probably what happened before and temporary became permanent.
 
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