Closed Delta 120/240V 3-phase service

On Thu, 21 Feb 2008 18:52:59 +0000 (UTC) Michael Moroney
|>Actually this is quite common. Even if three phase distribution goes
|>through a residential area, the transformers on the poles for homes |>along the way are usually just one transformer supplying one phase to |>that bunch of homes. | | Different utilities in different areas have different practices, including | the willingness to bring three phase down a street that doesn't need it | yet, or will never need it.
I can't rule that out. And maybe I've seen it, as I have seen many cases of three phase MV on a residential street without noting whether it was going through to somewhere else or if all three phases were used (such as by three separate single phase taps). A few streets in town here have two sets of three phase MV. I'm guessing some have special uses (like a hospital circuit) or are feeding strictly to further distance circuits.
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snipped-for-privacy@ipal.net wrote:

In the situation under discussion were the only significant three phase load is the elevator a three phase auto transformer wired in boost configuration would solve the problem at a reasonable cost.
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Tom Horne

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Tom:
Thanks for the suggestion. Could you elaborate a bit more on how this would be connected please...
Are you saying that you would have standard split phase 120/240V from one transformer... Then in addition to that, you would have 3 autotransformers (or one 3 phase autotransformer) for the 3 phase load?
I've never heard of anything like this. Aren't auto transformers not permitted (from the utility primary to customer secondary) because of safety concerns? Or if it is a customer provided transformer (perhaps a buck-boost XFR), wouldn't you need a utilility transfomer on the primary 3-phase line anyway?
It doesn't seem that you would be saving anything here.
Beachcomber
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| |>In the situation under discussion were the only significant three phase |>load is the elevator a three phase auto transformer wired in boost |>configuration would solve the problem at a reasonable cost. |>-- |>Tom Horne |> | | Tom: | | Thanks for the suggestion. Could you elaborate a bit more on how this | would be connected please... | | Are you saying that you would have standard split phase 120/240V from | one transformer... Then in addition to that, you would have 3 | autotransformers (or one 3 phase autotransformer) for the 3 phase | load?
With just 2 extra small transformers, you can add a couple extra phases to a normal 120/240 split phase and end up with 4 lines, 3 of which are the same as 208 wye, if you use the original incoming three phase distribution to derive it.
Or you can do it with just 1 weird transformer. Using a single phase transformer with a 139/277 volt split phase, you wire it's primary B-C if the original single phase is A-N (or wire it C-N if the original single phase is A-B). Connect the 139/277 center tap to one line end of the 120/240. Voila, you now have a Scott-T emulating 277 delta.
* | | N <-- this is the original 120/240 | | *-----*-----* <-- this is the added 139/277
| I've never heard of anything like this. Aren't auto transformers not | permitted (from the utility primary to customer secondary) because of | safety concerns? Or if it is a customer provided transformer | (perhaps a buck-boost XFR), wouldn't you need a utilility transfomer | on the primary 3-phase line anyway?
Autotransformers are safe as long as they are "up there" on the pole out of reach and properly grounded, or well sealed in a pad mount enclosure.
| It doesn't seem that you would be saving anything here.
Probably not.
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| In the situation under discussion were the only significant three phase | load is the elevator a three phase auto transformer wired in boost | configuration would solve the problem at a reasonable cost.
What if you don't have any need for three phase power at all? Suppose your building has elevators that work fine on single phase power. The building is so big, however, the power company doesn't want to drop only one phase into it; they want to have all three phases coming in, in some approximation of a balanced load. And you need to supply 240 volts, not 208, to each tenant. What would you set up?
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snipped-for-privacy@ipal.net wrote:

What makes you think that its up to you to chose? The utility company is going to tell you what is available. The elevator company is going to tell you what they need, as well. As usual, you're playing another of your long winded, and useless games of 'What if'...
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snipped-for-privacy@earthlink.net says...

What if Phil bought his own bigass transformer?
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krw wrote:

Progress Energy, who bought Florida Power, wouldn't let you provide any transformers. They did the calculations, and set their own transformers. Three phase availability was determined by your location, but generally any building above a certain level was three phase. Even small convenience stores and gas stations are three phase around here. The only home I've seen with an elevator had three phase power, along with an Onan diesel three phase generator with an automatic transfer switch.
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On Sun, 17 Feb 2008 03:39:08 -0500 Michael A. Terrell
| krw wrote: |>
|> > |> > What makes you think that its up to you to chose? The utility |> > company is going to tell you what is available. The elevator company is |> > going to tell you what they need, as well. As usual, you're playing |> > another of your long winded, and useless games of 'What if'... |> |> What if Phil bought his own bigass transformer? | | | Progress Energy, who bought Florida Power, wouldn't let you provide | any transformers. They did the calculations, and set their own | transformers. Three phase availability was determined by your location, | but generally any building above a certain level was three phase. Even | small convenience stores and gas stations are three phase around here. | The only home I've seen with an elevator had three phase power, along | with an Onan diesel three phase generator with an automatic transfer | switch.
Maybe Florida has no provision to require the utility to provide standard voltages (like 240 volts). Some other states do. And such a requirement is NOT a problem to meet (although if you want to change an existing setup I can expect you have to pay the full costs).
Do you know if that home has 208Y/120 or 240DCT/120 ?
This system is overkill for a single home:
* * \ / *--*--* / \ * *
But this system could do the job adequately:
* / *--*--* \ *
Which to choose depends on whether the distribution supply needs to be kept in balance or not. Utilities require (and this is a reasonable requirement) that larger services be three phase so they can keep the lines in the area in balance.
I've not seen generators that readily do either of these diagrammed examples, so I suspect that house is either 208Y/120 or 240DCT/120. And the 240DCT/120 can be done at least three different ways.
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snipped-for-privacy@ipal.net says...

...and the house to the north gets:
* * \ / *--* \ *
...and the house to the south gets:
* / *--* / \ * *
And pretty soon you have something on the pole that looks kinda like:
* * \ / *--*--* / \ * *

Three phase also sorta allows them to deliver more power for a given conductor size.

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|> Do you know if that home has 208Y/120 or 240DCT/120 ? |> |> This system is overkill for a single home: |> |> * * |> \ / |> *--*--* |> / \ |> * * |> |> But this system could do the job adequately: |> |> * |> / |> *--*--* |> \ |> * | | ...and the house to the north gets: | | * * | \ / | *--* | \ | * | | ...and the house to the south gets: | | | * | / | *--* | / \ | * * | | And pretty soon you have something on the pole that looks kinda | like: | | * * | \ / | *--*--* | / \ | * *
It would certainly be possible to deliver three such configurations to three different houses from the same pole with three transformers of the 120/240 split phase design. And you say this has been done? If so, then I think that's cool. But that is not what is generally done in 99% or more of three phase installations.
|> Which to choose depends on whether the distribution supply needs to be kept |> in balance or not. Utilities require (and this is a reasonable requirement) |> that larger services be three phase so they can keep the lines in the area |> in balance. | | Three phase also sorta allows them to deliver more power for a given | conductor size.
When comparing single phase L-L voltage to a delta with the same voltage that is true by 15.47%. If you take 240 delta, and compare it to 240 single phase 2 wire (regardless of where the grounding point is on either system), the 240 delta has more power in the same conductor size.
But if you look at both systems relative to the center of the system, the single phase is 2x 120 volts whereas the three phase is 3x 139 volts. You get the extra power based on the relative system voltage effectively being higher. Compare 120/240 volt single phase to 240Y/139 three phase. Put in 6 conductors, 3 per pole for 120/240 volt single phase and 2 per pole for 240Y/139 three phase. The latter gets its gain by virtue of the higher voltage.
Compare 208Y/120 to 120/240 and there is no such gain. It's all in the voltage comparison where delta allows it to look like it has a lower voltage than it really does.
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snipped-for-privacy@ipal.net wrote:

208Y/120 They also had a couple 12" wells with three phase pumps, and a diesel powered backup pump.
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On Sun, 17 Feb 2008 19:21:25 -0500 Michael A. Terrell
| snipped-for-privacy@ipal.net wrote: |> |> On Sun, 17 Feb 2008 03:39:08 -0500 Michael A. Terrell
|> | krw wrote: |> |>
|> |> > |> |> > What makes you think that its up to you to chose? The utility |> |> > company is going to tell you what is available. The elevator company is |> |> > going to tell you what they need, as well. As usual, you're playing |> |> > another of your long winded, and useless games of 'What if'... |> |> |> |> What if Phil bought his own bigass transformer? |> | |> | |> | Progress Energy, who bought Florida Power, wouldn't let you provide |> | any transformers. They did the calculations, and set their own |> | transformers. Three phase availability was determined by your location, |> | but generally any building above a certain level was three phase. Even |> | small convenience stores and gas stations are three phase around here. |> | The only home I've seen with an elevator had three phase power, along |> | with an Onan diesel three phase generator with an automatic transfer |> | switch. |> |> Maybe Florida has no provision to require the utility to provide standard |> voltages (like 240 volts). Some other states do. And such a requirement |> is NOT a problem to meet (although if you want to change an existing setup |> I can expect you have to pay the full costs). |> |> Do you know if that home has 208Y/120 or 240DCT/120 ? |> | | | 208Y/120 They also had a couple 12" wells with three phase pumps, | and a diesel powered backup pump.
OK, so they didn't have any 240 volt supply.
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On Sat, 16 Feb 2008 14:16:04 -0500 Michael A. Terrell
| snipped-for-privacy@ipal.net wrote: |>
|> |> | In the situation under discussion were the only significant three phase |> | load is the elevator a three phase auto transformer wired in boost |> | configuration would solve the problem at a reasonable cost. |> |> What if you don't have any need for three phase power at all? Suppose your |> building has elevators that work fine on single phase power. The building |> is so big, however, the power company doesn't want to drop only one phase |> into it; they want to have all three phases coming in, in some approximation |> of a balanced load. And you need to supply 240 volts, not 208, to each |> tenant. What would you set | | | What makes you think that its up to you to chose? The utility | company is going to tell you what is available. The elevator company is | going to tell you what they need, as well. As usual, you're playing | another of your long winded, and useless games of 'What if'...
Maybe you should read some of the tariffs. Most jurisdictions do have some requirements that certain services must be made available of so requested. The only "what if" here is "what if someone knew what they were allowed to really ask for".
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I think that logically, the process goes something like this.
1. The architect or building contractor responsible for the electrical system needs to research the utility tariffs and policies and determine what services are available.
2. Further research may be necessary to see what services are available at the specific location of the building under construction.
This may be different then #1.
3. Given the available choices, it is up to the person designing the electrical system within the building the most economical and useful service, given the specific needs of the building (elevators, HVAC, pumps, machine shop tools, etc.) That is, should it be 120/208, 120/240, volt single phase or three phase, open wye, or closed-delta, etc.
#3 is the most interesting and the most difficult, apparently, from the comments offered on this thread. It doesn't seem that there are any carved-in-stone rules for what is best, only tradeoffs and different opinions.
Have I got this right?
Beachcomber
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| |>| What makes you think that its up to you to chose? The utility |>| company is going to tell you what is available. The elevator company is |>| going to tell you what they need, as well. As usual, you're playing |>| another of your long winded, and useless games of 'What if'... |> |> | I think that logically, the process goes something like this. | | 1. The architect or building contractor responsible for the | electrical system needs to research the utility tariffs and policies | and determine what services are available. | | 2. Further research may be necessary to see what services are | available at the specific location of the building under construction. | | | This may be different then #1. | | 3. Given the available choices, it is up to the person designing the | electrical system within the building the most economical and useful | service, given the specific needs of the building (elevators, HVAC, | pumps, machine shop tools, etc.) That is, should it be 120/208, | 120/240, volt single phase or three phase, open wye, or closed-delta, | etc. | | #3 is the most interesting and the most difficult, apparently, from | the comments offered on this thread. It doesn't seem that there are | any carved-in-stone rules for what is best, only tradeoffs and | different opinions. | | Have I got this right?
Basically, yes. One point is that to choose 120/240 when the building size (in terms of load) is above the point where three phase is required means a greater cost. Unlike single homes, such developments, even if residential in nature, are treated like commercial, at least up to the point where the tenant metering takes place. So the developer has to pay most or all of the electrical service installation costs. Since traditional 208Y/120 is lower in cost (one stock transformer, one entrance disconnect), it may well be the one chosen. It can cost more to then get special appliances for each unit to operate at 208 volts. Or, as appears to be the case quite often, the developer just passes the cost on to the owner(s) or tenants in the form of poorly chosen appliances, or leaving it up to condo buyers to find the 208 volt appliances, or suffer with 240 volt appliances. The fact that so many cases are decided in bad ways does not mean better choices are not available; they just cost a little more.
I do know some apartment buildings are large enough to have transformers on each floor. I was in one building many years ago that had 8 units on each of 12 floors. There was a transformer on each floor, and metering for each tenant was in a small room on the same floor. The meter room did not have the transformer. That would certainly be essential of the feed was at distribution voltage (I don't know what it was in that building as I did not actually inspect the rooms). I do know it had 208 volts to each unit, and specially bought 208 volt appliances. But they could have just done this with a single phase transformer on each floor and saved on the cost of appliances but getting normal 240 volt ones. In this case, it would not have been any more expensive, and it would have been at least as well balanced across the distribution phases as the existing setup. Any needs for three phase power could have been satisfied with a separate three phase transformer at the top or bottom of the building. It would have to be separately metered, anyway.
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snipped-for-privacy@ipal.net wrote:

Obviously, you don't and you've been flogging another dead horse. You can take any simple subject and turn it into mind numbing drivel. You've flogged this one for a week and made zero progress. Even if you got answers, they would only be valid for one location, not the entire planet earth.
You bitch about no availability of 208 volt appliances. How many do you see that are marked 240 VAC only? I see a lot that are marked 208-240 VAC. So your water heater is a few volts low. It won't make much difference in the output. If you buy at the right places, 208 volt stuff isn't hard to find, even in the deep south. If you are in an apartment, it is up to the landlord to provide the major appliances, and repair or replace them when they quit. So, if the building is big enough, they are supplied three phase. no matter how much you want to yak about what ifs, no one really gives a damn, except the local power company, and the owner of the building.
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Power is a function of the voltage squared. So a 3kW heater at 240v becomes 2.25kW at 208v. It may not be significant if it's used for storage heating but as an instantaneous heater it would matter.
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charles wrote:

Gee thanks for that lesson. I've know and used it since the '50s. If you need that extra bit of heat, you can buy the proper heating element. Not at the over the counter type 'DIY' dumps, but at a real plumbing supply house.
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On Sun, 17 Feb 2008 13:10:44 -0500 Michael A. Terrell
| charles wrote: |> |> Power is a function of the voltage squared. So a 3kW heater at 240v |> becomes 2.25kW at 208v. It may not be significant if it's used for storage |> heating but as an instantaneous heater it would matter. | | | Gee thanks for that lesson. I've know and used it since the '50s. | If you need that extra bit of heat, you can buy the proper heating | element. Not at the over the counter type 'DIY' dumps, but at a real | plumbing supply house.
Which means it is more expensive. And in many cases they have to special order it. I've already experienced the special order cost case with a 208 volts single phase motor (it was twice as much as a 240 volt motor of the same HP and mounting).
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