Closed Delta 120/240V 3-phase service

|> So that's the way you would go? What if the utility says they don't want |> to have the loss of an extra set of transformers with MV -> 480 -> 120/240
|> and insist on no more than one transformer per MV -> meter path? |> | | The utility cannot insist on anything if the service you order is | tariffed by the state public utilities commission they must provide it | on demand. The tariff may allow them to surcharge you if the service | does not draw enough current to justify the additional expense. If the | type of service you want is not tariffed then the utility must apply for | a tariff for that new class of service and obtain it prior to connecting | the service. If they don't want to provide it then you would be faced | with the insurmountable cost of applying for the tariff against their | opposition.
I've read many tariffs from various areas. Most have limits on the maximum capacity a utility is required to provide as single phase, and allows the utility to require three phase for higher capacity services. They all seem to indicate that 240 delta services are no longer required to be provided.
In some cases there was some specification of rate compensation when the utility required providing the service at a higher voltage, to account for the loss in the customer transformer. I presume that is when the metering is at the higher voltage. In the example case I described with a large apartment building, presumably metering would be done at the 120/240 end, since there would not be individual transformers for each unit.
| Dairy farmers in several states have obtained tariffs for ungrounded | delta service to supply customer owned transformers in order to keep | utility neutral currents off of the farm and away from their livestock. | The utilities initially fought the tariffs very hard for reasons that | were not clear to me. Once the problem with dairy cows and stray Multi | Grounded Neutral (MGN) currents became well known some utilities began | offering special transformers that do not permit the MGN currents to | flow on the premise service wiring. I've yet to learn how they | accomplish that but if it were my livelihood I'd want the ungrounded | Delta supply that was tariffed in spite of the utilities opposition.
One possible reason they fought those tariffs was because they believed that the ungrounded transformers would be subject to damage caused by a lightning strike involving an arc between primary and secondary when the secondary is not grounded. This could damage the winding insulation.
The special transformers are probably ones that do have an ungrounded secondary, but have a special arc gap device to allow a lightning strike or surge to jump across between primary and secondary, reducing the risk of damage to the winding insulation.
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| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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On Thursday, February 14, 2008 at 1:16:11 PM UTC-5, snipped-for-privacy@ipal.net wr ote:

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I have been in the electrical craft since Nineteen Sixty Six and I have see n a lot of different service voltages supplied to buildings. First I am no t an Electrical Engineer although I have always been the go to guy on theor y questions that the trainees and apprentices come up with. So I'll just g o ahead and admit that I don't understand how you could get a six phase out put out of a three phase input. If it is a delta secondary it can only be corner or center of one phase grounded as the code specifically requires th ose as the only place the bonding jumper can be connected. I have worked i n manufacturing plants were the delta was ungrounded but elaborate measures were taken to insure that any ground fault was detected well before a seco nd such fault on a different phase had any chance to occur. You simply can not connect any delta system to ground on two separate phases or you have a dead fault. I have yet to see any supply system that allows me to take ha lf phase loads from more than one phase.
Someone has already pointed out that if you have six secondary windings in a star configuration that they would have to be supplied from six phase dis tribution or two of each of the secondary windings would have same primary winding exciting them and would therefore be functionally parallel with no phase difference or voltage between them.
It would seen to me that either 208/120 wye connected supply or a 240/120 c enter tapped phase delta with the resultant high leg on one phase would do what is needed. Either would give you Three phased power for elevator moto rs, air conditioning chiller or whatever the bigger Three phased load is.
As to actually insisting that individual units have 240 volt, Three phase, I have yet to see a situation were that is actually needed. Any Three phas e heat pump, air conditioner, or other motor load that will run on 240 volt three phase that I have had to install could be re-tapped to run on 208 Th ree phase or the motors were labelled as 240,208 Three phase that would run on either without modification. Even single phase home appliances will ru n on either in most cases. Why then would one need to have actual 240 volt s in single or Three phase all the way to the individual unit. I have neve r actually seen Three phase power run to the individual units of an apartme nt building.
If you actually must have 240 volts to each unit in the original posters sm all apartment building; although no one has explained why that might be tru e; then you will need either a single phase supply or a separate branch or feeder to each unit for the 240 volts. I have seen existing buildings with Fourteen units with single phase supply but I have not seen any new constr uction wired that way in the last several decades.
If the elevator motor or some other relatively large load had to have 240 v olt Three phase then the designer should compare the cost of installation a nd operation of either two separate services to supply different voltages o f Three phase or instal a boost transformer off of the 208 volt Three phase to derive the needed 240 volt Three phase. Keep in mind that most utiliti es charge additional monthly fees for a second service to the same premise even if it is only in the form of a fee for reading a second meter each mon th.
As for the utility insisting on anything, they must supply any service that is in the regulatory tariffs maintained by the state regulatory body. Sin ce the derived system transformers that would produce the 120/240 volts fro m the 480/277 are not part of the distribution network they are beyond the utilities power to control. Like anything else on the load side of the dem arcation point it is the exclusive province of the Authority Having Jurisdi ction (AHJ) over the enforcement of the locally adopted electrical code. T he only exceptions I have encountered in almost Fifty years is with publicl y owned utilities because some of those do the electrical code enforcement.
-- Tom Horne
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On 17/06/2015 8:59 AM, Tom Horne wrote:
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seen a lot of different service voltages supplied to buildings. First I am not an Electrical Engineer although I have always been the go to guy on theory questions that the trainees and apprentices come up with. So I 'll just go ahead and admit that I don't understand how you could get a s ix phase output out of a three phase input. If it is a delta secondary i t can only be corner or center of one phase grounded as the code specific ally requires those as the only place the bonding jumper can be connected . I have worked in manufacturing plants were the delta was ungrounded but elaborate measures were taken to insure that any ground fault was det ected well before a second such fault on a different phase had any chance to occur. You simply cannot connect any delta system to ground on two s eparate phases or you have a dead fault. I have yet to see any supply sy stem that allows me to take half phase loads from more than one phase.
You can have 3 single phase transformers with center tapped secondaries connected on the primary to the HV side (delta preferred) and secondary center taps connected to ground so that there would be 120V on each pair of adjacent legs but 120/240 between opposite legs. This idea has been used for many years for rectifier supplies Technically speaking it is a 6 phase star (the center tapped single phase setup (edison) can be considered as a 2 phase star (although it is
called this only in countries that don't use it. The idea of doing this fordistributioncan have advantages- at higher distribution voltages as the clearances can be reduced. In some areas this is worth while- chopping big holes through heritage trees is not a good idea. .

in a star configuration that they would have to be supplied from six pha se distribution or two of each of the secondary windings would have same primary winding exciting them and would therefore be functionally paralle l with no phase difference or voltage between them.

20 center tapped phase delta with the resultant high leg on one phase wou ld do what is needed. Either would give you Three phased power for eleva tor motors, air conditioning chiller or whatever the bigger Three phased load is.

se, I have yet to see a situation were that is actually needed. Any Thre e phase heat pump, air conditioner, or other motor load that will run on 240 volt three phase that I have had to install could be re-tapped to run on 208 Three phase or the motors were labelled as 240,208 Three phase th at would run on either without modification. Even single phase home appl iances will run on either in most cases. Why then would one need to have actual 240 volts in single or Three phase all the way to the individual unit. I have never actually seen Three phase power run to the individual units of an apartment building.

s small apartment building; although no one has explained why that might be true; then you will need either a single phase supply or a separate br anch or feeder to each unit for the 240 volts. I have seen existing buil dings with Fourteen units with single phase supply but I have not seen an y new construction wired that way in the last several decades. I live in an area where air conditioning loads are light and the supply to the transformers on the street is 7200V line to neutral- only one feed from the nearby 3 phase line is taken down this street- hence 120/240 single phase supply to each home. There are rural lines that are single phase. 13-15 KV line to neutral,
ground return (actually a grounded neutral wire is also used) is common. You are right in that generally new construction will have 120/208 in the units- as the load density is sufficient to warrant it.

40 volt Three phase then the designer should compare the cost of installa tion and operation of either two separate services to supply different vo ltages of Three phase or instal a boost transformer off of the 208 volt T hree phase to derive the needed 240 volt Three phase. Keep in mind that most utilities charge additional monthly fees for a second service to the same premise even if it is only in the form of a fee for reading a secon d meter each month. In the case of a larger load- I see no need (technical or economic( for a special 240 3 phase supply

that is in the regulatory tariffs maintained by the state regulatory body . Since the derived system transformers that would produce the 120/240 volts from the 480/277 are not part of the distribution network they are beyond the utilities power to control. Like anything else on the load s ide of the demarcation point it is the exclusive province of the Authorit y Having Jurisdiction (AHJ) over the enforcement of the locally adopted e lectrical code. The only exceptions I have encountered in almost Fifty y ears is with publicly owned utilities because some of those do the electr ical code enforcement.

--
Don Kelly

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<snip>
I did see, many years ago, something close to this. We called it a 'phase-shifting' transformer. IIRC, three of the outputs went to a delta-delta isolation bank and the other three went to a second delta-delta. Each delta-delta isolation bank fed a three-phase rectifier and the two rectifier outputs were combined. Used it to provide a lot of rectified DC.
The phase-shifting transformer's two outputs where shifted 15 degrees and with the isolation transformers greatly reduced the odd harmonics (third, fifth, and seventh) which was a big issue since it was a shipboard installation and the supply was far from 'infinite bus'.
Don't remember exact connections anymore though, that particular 'BeeBee' has fallen out and been replaced by other trivia :-)
daestrom
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snipped-for-privacy@ipal.net writes:

I always wondered if either any electric company offered such a service, or if there was any such thing as a breaker panel that would support it. I strongly suspect the answer to both is an absolute No Way, esp. if you think about how the heck you'd design the panel. You'd have to invent nonstandard breakers that would do lots of skipping to be useful. Of course such a service would really be useful only in an apartment building where there would be multiple standard panels, mostly residential split phase (1/3 A-D 1/3 B-E 1/3 C-F) and a 3 phase panel for elevator service, so no need for an actual panel for this 7 wire service.
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Are we not forgetting about the primary windings for such a scheme?
For three phase, the textbooks say that wye-wye is not used because of undesirable harmonics in the neutral. Would that not apple also to this six phase scheme?
How would the primaries be connected then?
Also, doesn't six phase distribution mean six phase transmission? Six phase transmission for any distance is going to be horrorendously expensive just to keep a few apartment buildings at 120/240 V.
Beachcomber
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| |>> B C |>> \ / |>> \ / |>>A----N----D |>> / \ |>> / \ |>> F E |> |>>A-N and N-D are both wound on the first core. B-N and N-E are both wound |>>on the second core. C-N and N-F are both wound on the third core. A third |>>of the 120/240 loads would be served from A-N-D. A third of the 120/240 |>>loads would be served from B-N-E. A third of the 120/240 loads would be |>>served from C-N-F. Half of the 208Y/120 loads would be served from A,C,E |>>and N. Half of the 208Y/120 loads would be served from B,D,F and N. |> |>I always wondered if either any electric company offered such a service, |>or if there was any such thing as a breaker panel that would support it. | | Are we not forgetting about the primary windings for such a scheme? | | For three phase, the textbooks say that wye-wye is not used because | of undesirable harmonics in the neutral. Would that not apple also to | this six phase scheme?
Nothing about this scheme prevents a delta primary.
Of course, if you want to backfeed power for credit into the utility, you might need a wye-wye anyway.
| How would the primaries be connected then?
Delta.
| Also, doesn't six phase distribution mean six phase transmission? Six | phase transmission for any distance is going to be horrorendously | expensive just to keep a few apartment buildings at 120/240 V.
This would be a traditional E-core transformer. The primaries would be a winding on each bar of the E-core at 480 volts (for example) and wired in the traditional delta configuration. The secondaries would be a pair of 120 volt windings. You can wire the 120 volt windings in parallel for a traditional 480D -> 208Y/120 transformer. You can wire those same 120 volt windings in series for the somewhat unusual 480D -> 416Y/240 (military has generators for 416Y/240 and might have to do just that in the case they get a 480 volt power source instead). That series wiring could be used with the center tap of each secondary connected to neutral instead of one end, to make the 6 phase arrangement.
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| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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snipped-for-privacy@notreal.none (Beachcomber) writes:

Wye or delta, whatever is standard for a 3 phase transformer bank on the distribution system servicing this.

I don't know about that. Around here, nearly every 3 pole pig transformer bank is wired wye-wye. I don't know what's on the inside of pad transformers or the single can 3 phase pole pigs.
Also, this scheme is as much 6 phase as your home service is 2 phase. A standard 3 wire+N 3 phase primary system is all that's needed for the 3 transformers.

No.
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says...

...and how does this "six phase" differ from a center-tapped three- phase?

Three phase delta?

No need for six-phase transmission to get six phases at the "apartment building". Like you, I don't see the point though.
Dunno, I'm not a power jock...
--
Keith

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| says... |> |> >> B C |> >> \ / |> >> \ / |> >>A----N----D |> >> / \ |> >> / \ |> >> F E |> > |> >>A-N and N-D are both wound on the first core. B-N and N-E are both wound |> >>on the second core. C-N and N-F are both wound on the third core. A third |> >>of the 120/240 loads would be served from A-N-D. A third of the 120/240 |> >>loads would be served from B-N-E. A third of the 120/240 loads would be |> >>served from C-N-F. Half of the 208Y/120 loads would be served from A,C,E |> >>and N. Half of the 208Y/120 loads would be served from B,D,F and N. |> > |> >I always wondered if either any electric company offered such a service, |> >or if there was any such thing as a breaker panel that would support it. |> |> Are we not forgetting about the primary windings for such a scheme? | | ...and how does this "six phase" differ from a center-tapped three- | phase?
When most or all of the loads are single phase, the "6 star" can keep the loading balanced over the 3 incoming phases, while also providing genuine 240 volts to those single phase loads. 240DCT or 240DVCT puts most of the loading on one phase angle (which might be 2 incoming phase lines). 208Y/120 balances the loads, but there's no 240. 240Y/139 is a bit rich on the L-N circuits. 220Y/127 is sometimes a compromise.
|> Also, doesn't six phase distribution mean six phase transmission? Six |> phase transmission for any distance is going to be horrorendously |> expensive just to keep a few apartment buildings at 120/240 V. | | No need for six-phase transmission to get six phases at the | "apartment building". Like you, I don't see the point though.
Which would you be willing to give up? Having 240 volts (and instead you settle for a wimpy 208 volts) or having the phases balanced (this is more of a utility concern)?
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says...

Maybe I'm dense, but I see *no* difference between the two schemes. ELectrically, I don't see how you could tell one from the other.

Before I can tell you my preference in donuts, I need to see the difference between six and half a dozen. ;-)
--
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| says...
|> | says... |> |> |> |> >> B C |> |> >> \ / |> |> >> \ / |> |> >>A----N----D |> |> >> / \ |> |> >> / \ |> |> >> F E |> |> > |> |> >>A-N and N-D are both wound on the first core. B-N and N-E are both wound |> |> >>on the second core. C-N and N-F are both wound on the third core. A third |> |> >>of the 120/240 loads would be served from A-N-D. A third of the 120/240 |> |> >>loads would be served from B-N-E. A third of the 120/240 loads would be |> |> >>served from C-N-F. Half of the 208Y/120 loads would be served from A,C,E |> |> >>and N. Half of the 208Y/120 loads would be served from B,D,F and N. |> |> > |> |> >I always wondered if either any electric company offered such a service, |> |> >or if there was any such thing as a breaker panel that would support it. |> |> |> |> Are we not forgetting about the primary windings for such a scheme? |> | |> | ...and how does this "six phase" differ from a center-tapped three- |> | phase? |> |> When most or all of the loads are single phase, the "6 star" can keep the |> loading balanced over the 3 incoming phases, while also providing genuine |> 240 volts to those single phase loads. 240DCT or 240DVCT puts most of |> the loading on one phase angle (which might be 2 incoming phase lines). |> 208Y/120 balances the loads, but there's no 240. 240Y/139 is a bit rich |> on the L-N circuits. 220Y/127 is sometimes a compromise. | | Maybe I'm dense, but I see *no* difference between the two schemes. | ELectrically, I don't see how you could tell one from the other.
A saw a bunch of different schemes in this thread. Maybe you can list the specific schemes you think are alike but treated as different.
|> |> Also, doesn't six phase distribution mean six phase transmission? Six |> |> phase transmission for any distance is going to be horrorendously |> |> expensive just to keep a few apartment buildings at 120/240 V. |> | |> | No need for six-phase transmission to get six phases at the |> | "apartment building". Like you, I don't see the point though. |> |> Which would you be willing to give up? Having 240 volts (and instead you |> settle for a wimpy 208 volts) or having the phases balanced (this is more |> of a utility concern)? | | Before I can tell you my preference in donuts, I need to see the | difference between six and half a dozen. ;-)
Can you describe an electrical system configuration which is capable of:
1. Supplying 120 and 240 volts (not 208 volts) in single phase to all single phase loads.
2. Divides up the single phase load in three equal groups in order to maintain a balance on each of the three phases of the utility supply.
... that is not the "6 star" I described?
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snipped-for-privacy@ipal.net says...

Specifically, the six-phase "star" and three-phase center-tapped wye. With any two phases any number of others is a few transformers away.

Look harder at a center-tapped wye. There is only 60degrees between the "negative" of phase-A and Phase-B.

How does it divide anything? An imbalance can still be placed on any one (or two).

I still don't see the difference between that and a center-tapped wye. Again, I'm not a power jock, so may be missing something subtile.
--
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| snipped-for-privacy@ipal.net says...
|> | says...
|> |> | says... |> |> |> |> |> |> >> B C |> |> |> >> \ / |> |> |> >> \ / |> |> |> >>A----N----D |> |> |> >> / \ |> |> |> >> / \ |> |> |> >> F E |> |> |> > |> |> |> >>A-N and N-D are both wound on the first core. B-N and N-E are both wound |> |> |> >>on the second core. C-N and N-F are both wound on the third core. A third |> |> |> >>of the 120/240 loads would be served from A-N-D. A third of the 120/240 |> |> |> >>loads would be served from B-N-E. A third of the 120/240 loads would be |> |> |> >>served from C-N-F. Half of the 208Y/120 loads would be served from A,C,E |> |> |> >>and N. Half of the 208Y/120 loads would be served from B,D,F and N. |> |> |> > |> |> |> >I always wondered if either any electric company offered such a service, |> |> |> >or if there was any such thing as a breaker panel that would support it. |> |> |> |> |> |> Are we not forgetting about the primary windings for such a scheme? |> |> | |> |> | ...and how does this "six phase" differ from a center-tapped three- |> |> | phase? |> |> |> |> When most or all of the loads are single phase, the "6 star" can keep the |> |> loading balanced over the 3 incoming phases, while also providing genuine |> |> 240 volts to those single phase loads. 240DCT or 240DVCT puts most of |> |> the loading on one phase angle (which might be 2 incoming phase lines). |> |> 208Y/120 balances the loads, but there's no 240. 240Y/139 is a bit rich |> |> on the L-N circuits. 220Y/127 is sometimes a compromise. |> | |> | Maybe I'm dense, but I see *no* difference between the two schemes. |> | ELectrically, I don't see how you could tell one from the other. |> |> A saw a bunch of different schemes in this thread. Maybe you can list |> the specific schemes you think are alike but treated as different. | | Specifically, the six-phase "star" and three-phase center-tapped | wye. With any two phases any number of others is a few transformers | away.
I don't know which you mean by "three-phase center-tapped wye". Maybe you can pick it out from this police lineup:
* * * * * * \ \ / * * * | \ / \ \ \ / / \ / | | \ / \ N---* *---N---* / \ / | *---N---* N * / / \ / \ / | | / / / \ *---N---* *---N---* *---N---* | / * * * * *
1 2 3 4 5 6 7
If you don't see the guilty party, maybe you can draw a picture :-)
Note that drawings are NOT to scale (relative to voltage).
|> |> |> Also, doesn't six phase distribution mean six phase transmission? Six |> |> |> phase transmission for any distance is going to be horrorendously |> |> |> expensive just to keep a few apartment buildings at 120/240 V. |> |> | |> |> | No need for six-phase transmission to get six phases at the |> |> | "apartment building". Like you, I don't see the point though. |> |> |> |> Which would you be willing to give up? Having 240 volts (and instead you |> |> settle for a wimpy 208 volts) or having the phases balanced (this is more |> |> of a utility concern)? |> | |> | Before I can tell you my preference in donuts, I need to see the |> | difference between six and half a dozen. ;-) |> |> Can you describe an electrical system configuration which is capable of: |> |> 1. Supplying 120 and 240 volts (not 208 volts) in single phase to all |> single phase loads. | | Look harder at a center-tapped wye. There is only 60degrees between | the "negative" of phase-A and Phase-B.
I don't know which I need to look at.
|> 2. Divides up the single phase load in three equal groups in order to |> maintain a balance on each of the three phases of the utility supply. | | How does it divide anything? An imbalance can still be placed on | any one (or two).
Of course an imbalance can always happen, and likely will. But it is a statistical thing. If a building has 54 apartments, connecting 18 of them to phase A, 18 to phase B, and 18 to phase C, would be "balanced" enough for utility purposes. If the supply were coming in as 120/240DCT, then all of the apartments would be on just one of the phases and the phase loading would be as lopsided as if the entire building were supplied with single phase power. If the utility insists on balancing the phases and rejects single phase service for this reason, they will reject 120/240DCT. If the supply is 208Y/120, that would satisfy the utility (18 apartments supplied with phases A and B, 18 suppleid with B and C, and 18 supplied with C and A). But it would not satisfied the need to have 240 volts.
|> ... that is not the "6 star" I described? | | I still don't see the difference between that and a center-tapped | wye. Again, I'm not a power jock, so may be missing something | subtile.
Maybe you saw something I didn't see. Check the above police lineup.
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| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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snipped-for-privacy@ipal.net says...

#2 be the culprit. Your "split-phase" three-phase from another post.

Close enough for government work.
<snip>

#2
Sure, but your "six-phase" is no different than the normal three- phase "wye". At least I don't see it.

Nope. You saw it, just one of us isn't "getting it" (I could easily be missing something - 65h work weeks do that after a while).
--
Keith

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| snipped-for-privacy@ipal.net says...
|> |> | says...
|> |> |> | says... |> |> |> |> |> |> |> |> >> B C |> |> |> |> >> \ / |> |> |> |> >> \ / |> |> |> |> >>A----N----D |> |> |> |> >> / \ |> |> |> |> >> / \ |> |> |> |> >> F E |> |> |> |> > |> |> |> |> >>A-N and N-D are both wound on the first core. B-N and N-E are both wound |> |> |> |> >>on the second core. C-N and N-F are both wound on the third core. A third |> |> |> |> >>of the 120/240 loads would be served from A-N-D. A third of the 120/240 |> |> |> |> >>loads would be served from B-N-E. A third of the 120/240 loads would be |> |> |> |> >>served from C-N-F. Half of the 208Y/120 loads would be served from A,C,E |> |> |> |> >>and N. Half of the 208Y/120 loads would be served from B,D,F and N. |> |> |> |> > |> |> |> |> >I always wondered if either any electric company offered such a service, |> |> |> |> >or if there was any such thing as a breaker panel that would support it. |> |> |> |> |> |> |> |> Are we not forgetting about the primary windings for such a scheme? |> |> |> | |> |> |> | ...and how does this "six phase" differ from a center-tapped three- |> |> |> | phase? |> |> |> |> |> |> When most or all of the loads are single phase, the "6 star" can keep the |> |> |> loading balanced over the 3 incoming phases, while also providing genuine |> |> |> 240 volts to those single phase loads. 240DCT or 240DVCT puts most of |> |> |> the loading on one phase angle (which might be 2 incoming phase lines). |> |> |> 208Y/120 balances the loads, but there's no 240. 240Y/139 is a bit rich |> |> |> on the L-N circuits. 220Y/127 is sometimes a compromise. |> |> | |> |> | Maybe I'm dense, but I see *no* difference between the two schemes. |> |> | ELectrically, I don't see how you could tell one from the other. |> |> |> |> A saw a bunch of different schemes in this thread. Maybe you can list |> |> the specific schemes you think are alike but treated as different. |> | |> | Specifically, the six-phase "star" and three-phase center-tapped |> | wye. With any two phases any number of others is a few transformers |> | away. |> |> I don't know which you mean by "three-phase center-tapped wye". Maybe you |> can pick it out from this police lineup: |> |> * * * * * * |> \ \ / * * * | \ / \ |> \ \ / / \ / | | \ / \ |> N---* *---N---* / \ / | *---N---* N * |> / / \ / \ / | | / |> / / \ *---N---* *---N---* *---N---* | / |> * * * * * |> |> 1 2 3 4 5 6 7 |> |> If you don't see the guilty party, maybe you can draw a picture :-) | | #2 be the culprit. Your "split-phase" three-phase from another | post.
That is the same as the six-phase "star" or "6 star". So if you can't see any difference, that's because there isn't any. I would not have called this a "wye" of any sort since it doesn't look like a "Y". But it could be TWO Y's interleaved.
To make this derived system, you need either:
1. 3 single phase transformer cores 2. 1 three phase tranformer E-core
Then you need to wrap these cores with 3 primary windings and 3 secondary windings. Usually the secondaries go on the inside and the primaries go on the outside, so the secondary at lower voltage and higher current has less winding resistance.
Each of the 3 secondary windings needs to be either:
1. a pair of 120 volt windings which you can wire in series 2. a single 240 volt winding with a center tap right in the middle
All of the center points of these windings are wired/bonded together and grounded. Then each of the three phases will have two poles 180 degrees apart. Some people will call this six phases.
|> Note that drawings are NOT to scale (relative to voltage). | | Close enough for government work. | | <snip> | |> |> Can you describe an electrical system configuration which is capable of: |> |> |> |> 1. Supplying 120 and 240 volts (not 208 volts) in single phase to all |> |> single phase loads. |> | |> | Look harder at a center-tapped wye. There is only 60degrees between |> | the "negative" of phase-A and Phase-B. |> |> I don't know which I need to look at. | | #2
Let me relabel the terminals like so:
C' B \ / \ / A----N----A' / \ / \ B' C
Between A and A' you have 240 volts. Between B and B' you have 240 volts. Between C and C' you have 240 volts. Between A and B you have 208 volts. Between B and C you have 208 volts. Between C and A you have 208 volts. Between C' and A' you have 208 volts. Between A' and B' you have 208 volts. Between B' and C' you have 208 volts. Between A and N you have 120 volts. Between B and N you have 120 volts. Between C and N you have 120 volts. Between A' and N you have 120 volts. Between B' and N you have 120 volts. Between C' and N you have 120 volts. Between A and C' you have 120 volts. Between C' and B you have 120 volts. Between B and A' you have 120 volts. Between A' and C you have 120 volts. Between C and B' you have 120 volts. Between B' and A you have 120 volts.
Which connection pair do you need to ask about?
|> |> 2. Divides up the single phase load in three equal groups in order to |> |> maintain a balance on each of the three phases of the utility supply. |> | |> | How does it divide anything? An imbalance can still be placed on |> | any one (or two). |> |> Of course an imbalance can always happen, and likely will. But it is a |> statistical thing. If a building has 54 apartments, connecting 18 of them |> to phase A, 18 to phase B, and 18 to phase C, would be "balanced" enough |> for utility purposes. If the supply were coming in as 120/240DCT, then |> all of the apartments would be on just one of the phases and the phase |> loading would be as lopsided as if the entire building were supplied with |> single phase power. If the utility insists on balancing the phases and |> rejects single phase service for this reason, they will reject 120/240DCT. |> If the supply is 208Y/120, that would satisfy the utility (18 apartments |> supplied with phases A and B, 18 suppleid with B and C, and 18 supplied |> with C and A). But it would not satisfied the need to have 240 volts. | | Sure, but your "six-phase" is no different than the normal three- | phase "wye". At least I don't see it.
The normal three phase WYE is just:
B / / A----N \ \ C
Between A and B you have 208 volts. Between B and C you have 208 volts. Between C and A you have 208 volts. Between A and N you have 120 volts. Between B and N you have 120 volts. Between C and N you have 120 volts.
There aren't any other ways to connect, and no way to get 240 volts.
|> |> ... that is not the "6 star" I described? |> | |> | I still don't see the difference between that and a center-tapped |> | wye. Again, I'm not a power jock, so may be missing something |> | subtile. |> |> Maybe you saw something I didn't see. Check the above police lineup. | | Nope. You saw it, just one of us isn't "getting it" (I could easily | be missing something - 65h work weeks do that after a while).
OK, hope the above helps.
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snipped-for-privacy@ipal.net says...
<attempt to snip to size>

No, not "interleaved" but split. Invert each phase, as in a center- tapped transformer (I.e. split) and you're there.

Kinda like they do now on the pole.

Kinda...
Ok, I'm confused. This is different how?

Exactly (is there a difference?).

Right; split-phase "wye".

I don't. You seem to see a difference between this "star" and an ordinary 3-phase "wye" that delivers residential 240V split-phase. I don't see anything new or particularly interesting here, but am trying.

That's funny, because that's exactly how I get the 240V for my house off the 3-phases on the pole.

Nope. I'm still wondering why you see your "star" as any different than what we see on 90% of the poles in the country.
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| snipped-for-privacy@ipal.net says...
| | <attempt to snip to size> | |> |> | Specifically, the six-phase "star" and three-phase center-tapped |> |> | wye. With any two phases any number of others is a few transformers |> |> | away. |> |> |> |> I don't know which you mean by "three-phase center-tapped wye". Maybe you |> |> can pick it out from this police lineup: |> |> |> |> * * * * * * |> |> \ \ / * * * | \ / \ |> |> \ \ / / \ / | | \ / \ |> |> N---* *---N---* / \ / | *---N---* N * |> |> / / \ / \ / | | / |> |> / / \ *---N---* *---N---* *---N---* | / |> |> * * * * * |> |> |> |> 1 2 3 4 5 6 7 |> |> |> |> If you don't see the guilty party, maybe you can draw a picture :-) |> | |> | #2 be the culprit. Your "split-phase" three-phase from another |> | post. |> |> That is the same as the six-phase "star" or "6 star". So if you can't |> see any difference, that's because there isn't any. I would not have |> called this a "wye" of any sort since it doesn't look like a "Y". But |> it could be TWO Y's interleaved. | | No, not "interleaved" but split. Invert each phase, as in a center- | tapped transformer (I.e. split) and you're there.
You mean like having 2 three phase 208Y/120 transformers, one fed 180 degrees out of phase from the other, and combining their outputs with neutrals tied together? That's what I meant by interleaved. If that's not what you are saying, then I don't understand what it is you are saying.
|> To make this derived system, you need either: |> |> 1. 3 single phase transformer cores | | Kinda like they do now on the pole.
Yes. Except instead of using 120-volt-only transformers (there are such things ... just 2 lugs on the secondary), they need to use the 120/240 volt transformers (plenty of those around) and understand the concept to know how to wire it up.
Metering that could be an issue. In the case of the apartment building, individual tenants would be metered at 120/240 so it's not an issue. But if this needs to be metered in its "6 star" configuration, it can be done with 3 CTs, each carrying the 180-degree-opposite conductor pairs running in opposing directions just as a single CT would be used for single phase power.
|> 2. 1 three phase tranformer E-core | | Kinda...
It could be done. The manufacturing just has to provide a pair of 120 volt windings, or a series 120/240 tapped winding, and sufficient terminal board space to deal with it.
|> Then you need to wrap these cores with 3 primary windings and 3 secondary |> windings. Usually the secondaries go on the inside and the primaries go |> on the outside, so the secondary at lower voltage and higher current has |> less winding resistance. | | Ok, I'm confused. This is different how?
The difference is that a traditional E-core 208Y/120 transformer would have only ONE 120 volt winding per core bar (times the 3 core bars). Two such windings at 120 volt each, or a 120/240 volt series winding (connected in series inside the winding instead of at the terminal board) would be needed for the "6 star" configuration.
|> Each of the 3 secondary windings needs to be either: |> |> 1. a pair of 120 volt windings which you can wire in series |> 2. a single 240 volt winding with a center tap right in the middle | | Exactly (is there a difference?).
Yes, there is a difference. In #1 you have 4 wires coming from the secondary winding to the terminal board. In #2 you have 3 such wires because the winding is series connected, possibly a continuous single wire, inside the winding itself, and just tapped at a mid-point.
It's a difference in construction. Some people might be more familiar with one over the other.
|> All of the center points of these windings are wired/bonded together and |> grounded. Then each of the three phases will have two poles 180 degrees |> apart. Some people will call this six phases. | | Right; split-phase "wye".
Keep the "wye" in quotes, then; it's not really wye. I would never call it a wye at all. It's a 6 pointed radial star.
|> |> Note that drawings are NOT to scale (relative to voltage). |> | |> | Close enough for government work. |> | |> | <snip> |> | |> |> |> Can you describe an electrical system configuration which is capable of: |> |> |> |> |> |> 1. Supplying 120 and 240 volts (not 208 volts) in single phase to all |> |> |> single phase loads. |> |> | |> |> | Look harder at a center-tapped wye. There is only 60degrees between |> |> | the "negative" of phase-A and Phase-B. |> |> |> |> I don't know which I need to look at. |> | |> | #2 |> |> Let me relabel the terminals like so: |> |> C' B |> \ / |> \ / |> A----N----A' |> / \ |> / \ |> B' C |> |> Between A and A' you have 240 volts. |> Between B and B' you have 240 volts. |> Between C and C' you have 240 volts. |> Between A and B you have 208 volts. |> Between B and C you have 208 volts. |> Between C and A you have 208 volts. |> Between C' and A' you have 208 volts. |> Between A' and B' you have 208 volts. |> Between B' and C' you have 208 volts. |> Between A and N you have 120 volts. |> Between B and N you have 120 volts. |> Between C and N you have 120 volts. |> Between A' and N you have 120 volts. |> Between B' and N you have 120 volts. |> Between C' and N you have 120 volts. |> Between A and C' you have 120 volts. |> Between C' and B you have 120 volts. |> Between B and A' you have 120 volts. |> Between A' and C you have 120 volts. |> Between C and B' you have 120 volts. |> Between B' and A you have 120 volts. |> |> Which connection pair do you need to ask about? | | I don't. You seem to see a difference between this "star" and an | ordinary 3-phase "wye" that delivers residential 240V split-phase. | I don't see anything new or particularly interesting here, but am | trying.
A true WYE is not split phase. That makes a contradiction of terms. But if you know of a manufacturer that makes such a transformer AND calls it "split phase wye" or some such thing, please do point to their catalog reference. I've looked at a lot of transformer catalog info online and have never seen such a thing in a single transformer.
|> |> |> 2. Divides up the single phase load in three equal groups in order to |> |> |> maintain a balance on each of the three phases of the utility supply. |> |> | |> |> | How does it divide anything? An imbalance can still be placed on |> |> | any one (or two). |> |> |> |> Of course an imbalance can always happen, and likely will. But it is a |> |> statistical thing. If a building has 54 apartments, connecting 18 of them |> |> to phase A, 18 to phase B, and 18 to phase C, would be "balanced" enough |> |> for utility purposes. If the supply were coming in as 120/240DCT, then |> |> all of the apartments would be on just one of the phases and the phase |> |> loading would be as lopsided as if the entire building were supplied with |> |> single phase power. If the utility insists on balancing the phases and |> |> rejects single phase service for this reason, they will reject 120/240DCT. |> |> If the supply is 208Y/120, that would satisfy the utility (18 apartments |> |> supplied with phases A and B, 18 suppleid with B and C, and 18 supplied |> |> with C and A). But it would not satisfied the need to have 240 volts. |> | |> | Sure, but your "six-phase" is no different than the normal three- |> | phase "wye". At least I don't see it. |> |> The normal three phase WYE is just: |> |> B |> / |> / |> A----N |> \ |> \ |> C |> |> Between A and B you have 208 volts. |> Between B and C you have 208 volts. |> Between C and A you have 208 volts. |> Between A and N you have 120 volts. |> Between B and N you have 120 volts. |> Between C and N you have 120 volts. |> |> There aren't any other ways to connect, and no way to get 240 volts. | | That's funny, because that's exactly how I get the 240V for my house | off the 3-phases on the pole.
Which connections give you 240 volts? If you are getting 240 volts from A-B or from B-C or from C-A, and if the phase angles really are 120 degrees as a true three phase WYE would be, then you are going to get 139 volts at A-N, B-N, and C-N. I don't think that is what you want.
FYI, I did find one utility offering 240Y/139 service for some portions of their service area, as a replacement for 240D.
Maybe you are getting ONE phase of 120/240 via ONE split phase transformer tapped to ONE phase (connected L-N) or TWO phases (connected L-L) of the primary distribution lines. But just because there is three phase on the distribution does NOT mean you are getting it. You are most likely getting one of: 208Y/120 three phase (my grandfather actually did get this at his home), or 120/240 single phase, or that old 240DCT/120 setup.
|> |> |> |> ... that is not the "6 star" I described? |> |> | |> |> | I still don't see the difference between that and a center-tapped |> |> | wye. Again, I'm not a power jock, so may be missing something |> |> | subtile. |> |> |> |> Maybe you saw something I didn't see. Check the above police lineup. |> | |> | Nope. You saw it, just one of us isn't "getting it" (I could easily |> | be missing something - 65h work weeks do that after a while). |> |> OK, hope the above helps. | | Nope. I'm still wondering why you see your "star" as any different | than what we see on 90% of the poles in the country.
208Y/120 is _very_ different. 208Y/120 has THREE line wires coming out at 120 degree phase angle equal intervals. The "6 star" (maybe we can call it 240*/208/120) has SIX line wires coming out at 60 degree phase angle equal intervals.
If I provide you with exactly 3 transformers which are wired up with one winding for the primary voltage and one winding for the secondary at 120 volts, it can be wired up by connecting the primaries in whatever they need for the type of service (delta or wye) and connecting the secondaries in a wye configuration. There, you have 208Y/120 just like 90% of the three phase poles in the country (I'll just accept your stats of 90% as I do not know the actual figures).
I have never seen, and never heard of, any "6 star" or 240*/208/120 setup anywhere. I have never seen any utility tariff (I've looked through a few dozen over the past few years) that offers such a service.
I HAVE seen a couple three phase setups where a 120/240 volt pole pig was used, and only HALF of it was wired up to get 120 volts. I HAVE seen one manufacturer detail that they do make cans with the 120 volt windings in parallel internally, and still have 3 lugs with one of them not connected. So these are not necessarily a case of wasting half the capacity. The 3rd lug may simply be there are part of the process of manufacturing only one set of empty cans instead of two different sets.
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| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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snipped-for-privacy@ipal.net says...

<more>

Either that or CT secondaries. Same difference.

IF there is any advantage (doubt it), they'll figure out how to wire it up. It's rather obvious.

Yes, that's why I see nothing unique about your "idea". It's done today, except there is no need for the inversion.

There is likely a reason they don't. Though in your case of an apartment building they do, though likely with individual transformers. It's done on 90% of the poles in the country.

No different than what happens on 90% of the poles in the country, except those are in three cans rather than one.

A distinction without a difference. WHo cares how many screws are on the terminal board?

Sure. If there is no reason to make a widget there is a good chance that most are unfamiliar with the widget.

"Wye" is usually in quotes, even in a normal implementation.

Certainly it is, on 90% of the poles in the country.

The CT on any leg.

It's a "wye" not a delta. Three transformers with CT secondaries from A-N, B-N, and C-N. You just made you "star" configuration.

I didn't say an individual home was getting three phase, but it's there on the pole. ...including your "star".
<snip>

Look at 100 random poles. You'll see 90 of them wired as 3-phase "wye" (really doesn't matter if they are delta). The secondaries are CTed, so you have the six points of your "star". Nothing new here.

You've seen it, just haven't called it that. There is no point to it, so it isn't named.

Irrelevant manufacturing detail.
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| snipped-for-privacy@ipal.net says...
|> | |> | <attempt to snip to size> | <more> |> |> That is the same as the six-phase "star" or "6 star". So if you can't |> |> see any difference, that's because there isn't any. I would not have |> |> called this a "wye" of any sort since it doesn't look like a "Y". But |> |> it could be TWO Y's interleaved. |> | |> | No, not "interleaved" but split. Invert each phase, as in a center- |> | tapped transformer (I.e. split) and you're there. |> |> You mean like having 2 three phase 208Y/120 transformers, one fed 180 degrees |> out of phase from the other, and combining their outputs with neutrals tied |> together? That's what I meant by interleaved. If that's not what you are |> saying, then I don't understand what it is you are saying. | | Either that or CT secondaries. Same difference. | |> |> To make this derived system, you need either: |> |> |> |> 1. 3 single phase transformer cores |> | |> | Kinda like they do now on the pole. |> |> Yes. Except instead of using 120-volt-only transformers (there are such |> things ... just 2 lugs on the secondary), they need to use the 120/240 volt |> transformers (plenty of those around) and understand the concept to know |> how to wire it up. | | IF there is any advantage (doubt it), they'll figure out how to wire | it up. It's rather obvious.
There is an advantage to having genuine 240 volts. Not all circumstances make it easy to use separate single phase transformers. The ones that do would already have multiple transformers. Otherwise it increases cost. And there are no "6 star" transformers made (yet).
|> Metering that could be an issue. In the case of the apartment building, |> individual tenants would be metered at 120/240 so it's not an issue. But |> if this needs to be metered in its "6 star" configuration, it can be done |> with 3 CTs, each carrying the 180-degree-opposite conductor pairs running |> in opposing directions just as a single CT would be used for single phase |> power. | | Yes, that's why I see nothing unique about your "idea". It's done | today, except there is no need for the inversion.
Maybe there is a confusion here. CT could mean "center tap" or it could mean "current transformer". In the above, I mean "current transformer" for the purpose of metering.
|> |> 2. 1 three phase tranformer E-core |> | |> | Kinda... |> |> It could be done. The manufacturing just has to provide a pair of 120 |> volt windings, or a series 120/240 tapped winding, and sufficient terminal |> board space to deal with it. | | There is likely a reason they don't. Though in your case of an | apartment building they do, though likely with individual | transformers. It's done on 90% of the poles in the country.
What is done in on 90% of the poles? The "6 star" suggest is certainly not.
|> |> Then you need to wrap these cores with 3 primary windings and 3 secondary |> |> windings. Usually the secondaries go on the inside and the primaries go |> |> on the outside, so the secondary at lower voltage and higher current has |> |> less winding resistance. |> | |> | Ok, I'm confused. This is different how? |> |> The difference is that a traditional E-core 208Y/120 transformer would have |> only ONE 120 volt winding per core bar (times the 3 core bars). Two such |> windings at 120 volt each, or a 120/240 volt series winding (connected in |> series inside the winding instead of at the terminal board) would be needed |> for the "6 star" configuration. | | No different than what happens on 90% of the poles in the country, | except those are in three cans rather than one.
The "6 star" is not done anywhere I have ever heard of. You must be referring to a common WYE, which is not the same thing (WYE is a "3 star" not a "6 star").
|> |> Each of the 3 secondary windings needs to be either: |> |> |> |> 1. a pair of 120 volt windings which you can wire in series |> |> 2. a single 240 volt winding with a center tap right in the middle |> | |> | Exactly (is there a difference?). |> |> Yes, there is a difference. In #1 you have 4 wires coming from the secondary |> winding to the terminal board. In #2 you have 3 such wires because the |> winding is series connected, possibly a continuous single wire, inside the |> winding itself, and just tapped at a mid-point. | | A distinction without a difference. WHo cares how many screws are | on the terminal board?
You need more screws for the extra wires. How many depends on if you want to double up on a single lug or not.
|> It's a difference in construction. Some people might be more familiar with |> one over the other. | | Sure. If there is no reason to make a widget there is a good chance | that most are unfamiliar with the widget. | |> |> All of the center points of these windings are wired/bonded together and |> |> grounded. Then each of the three phases will have two poles 180 degrees |> |> apart. Some people will call this six phases. |> | |> | Right; split-phase "wye". |> |> Keep the "wye" in quotes, then; it's not really wye. I would never call it |> a wye at all. It's a 6 pointed radial star. | | "Wye" is usually in quotes, even in a normal implementation.
Really? I see it more NOT in quotes. The term WYE is a longish form of "Y" which is a depiction of a 3 pointed star configuration, not 6. The "*" more resembles 6 under most fonts (some have 8, but that's another issue).
|> |> Which connection pair do you need to ask about? |> | |> | I don't. You seem to see a difference between this "star" and an |> | ordinary 3-phase "wye" that delivers residential 240V split-phase. |> | I don't see anything new or particularly interesting here, but am |> | trying. |> |> A true WYE is not split phase. That makes a contradiction of terms. |> But if you know of a manufacturer that makes such a transformer AND |> calls it "split phase wye" or some such thing, please do point to |> their catalog reference. I've looked at a lot of transformer catalog |> info online and have never seen such a thing in a single transformer. | | Certainly it is, on 90% of the poles in the country.
Since well more than 10% of poles have a traditional 3 pole wye, there cannot be 90% with a 6 pole three phase star arrangement.
|> |> Between A and B you have 208 volts. |> |> Between B and C you have 208 volts. |> |> Between C and A you have 208 volts. |> |> Between A and N you have 120 volts. |> |> Between B and N you have 120 volts. |> |> Between C and N you have 120 volts. |> |> |> |> There aren't any other ways to connect, and no way to get 240 volts. |> | |> | That's funny, because that's exactly how I get the 240V for my house |> | off the 3-phases on the pole. |> |> Which connections give you 240 volts? | | The CT on any leg.
Be specific. First of all you know you need TWO connection points for a voltage. Simply saying "CT" (which I assume to mean "center tap" instead of "current transformer") means ONE connection. You get no voltage from one connection.
So how do you get 240 volts from a 208Y/120 wired transformer bank? Answer: you don't.
|> If you are getting 240 volts from |> A-B or from B-C or from C-A, and if the phase angles really are 120 degrees |> as a true three phase WYE would be, then you are going to get 139 volts |> at A-N, B-N, and C-N. I don't think that is what you want. | | It's a "wye" not a delta. Three transformers with CT secondaries | from A-N, B-N, and C-N. You just made you "star" configuration.
What are you labeling A-N, B-N, and C-N? The primaries or secondaries? Your statement "CT secondaries from A-N, B-N, and C-N" makes no sense. If there were 3 split-phase secondaries involved, there would be a lot more connection points than just these three. Even if all the center taps were connected together, you would still have 6 connection points to label. Starting at A, that runs to F.
|> FYI, I did find one utility offering 240Y/139 service for some portions |> of their service area, as a replacement for 240D. |> |> Maybe you are getting ONE phase of 120/240 via ONE split phase transformer |> tapped to ONE phase (connected L-N) or TWO phases (connected L-L) of the |> primary distribution lines. But just because there is three phase on the |> distribution does NOT mean you are getting it. You are most likely getting |> one of: 208Y/120 three phase (my grandfather actually did get this at his |> home), or 120/240 single phase, or that old 240DCT/120 setup. | | I didn't say an individual home was getting three phase, but it's | there on the pole. ...including your "star".
It's not there if what's on the pole is 208Y/120, which most poles supplying three phase power have. The only voltages available are 208 volts and 120 volts. Every possible pairing of 2 connections on these very common setups gives either 208 volts or 120 volts. I listed them previously. I left none out. None have 240 volts.
| <snip> | |> |> |> |> OK, hope the above helps. |> | |> | Nope. I'm still wondering why you see your "star" as any different |> | than what we see on 90% of the poles in the country. |> |> 208Y/120 is _very_ different. 208Y/120 has THREE line wires coming out at |> 120 degree phase angle equal intervals. The "6 star" (maybe we can call |> it 240*/208/120) has SIX line wires coming out at 60 degree phase angle |> equal intervals. | |> If I provide you with exactly 3 transformers which are wired up with one |> winding for the primary voltage and one winding for the secondary at 120 |> volts, it can be wired up by connecting the primaries in whatever they |> need for the type of service (delta or wye) and connecting the secondaries |> in a wye configuration. There, you have 208Y/120 just like 90% of the |> three phase poles in the country (I'll just accept your stats of 90% as I |> do not know the actual figures). | | Look at 100 random poles. You'll see 90 of them wired as 3-phase | "wye" (really doesn't matter if they are delta). The secondaries | are CTed, so you have the six points of your "star". Nothing new | here.
You really think that?
Sure, a 120/240 volt split phase transformer used normally for single phase service could be used for 208Y/120 by connecting only ONE SIDE. And it might be done in a few places. But it would be very rare since it is a waste of half the transformer capacity, and thus a higher cost than needed. They do make transformers that can deliver their entire capacity on just 2 lugs at 120 volts. There is no center tap. Or if there is, the center of 120 is 60 so it would not be used.
The majority of 208Y/120 services derived using single phase pole pigs is done with 3 cans that have 120 volt ONLY secondaries.
|> I have never seen, and never heard of, any "6 star" or 240*/208/120 setup |> anywhere. I have never seen any utility tariff (I've looked through a few |> dozen over the past few years) that offers such a service. | | You've seen it, just haven't called it that. There is no point to | it, so it isn't named.
So you really do have 208Y/120 mixed up with "6 star" or whatever someone else might refer to it as.
|> I HAVE seen a couple three phase setups where a 120/240 volt pole pig was |> used, and only HALF of it was wired up to get 120 volts. I HAVE seen one |> manufacturer detail that they do make cans with the 120 volt windings in |> parallel internally, and still have 3 lugs with one of them not connected. |> So these are not necessarily a case of wasting half the capacity. The 3rd |> lug may simply be there are part of the process of manufacturing only one |> set of empty cans instead of two different sets. | | Irrelevant manufacturing detail.
It's very relevant. It decides if the transformer has only 120 volts or if it has 120/240 volts. It decides in the case of 2 separate 120 volt windings whether they can be paralleled or not (and they need to be in parallel for the 208Y/120 service). The only transformer design that allows an external choice of configuring 120 volt parallel or 120/240 volt series center tapped is one with 4 lugs. I have seen such a transformer. But virtually all the rest of 3 lugs or 2 lugs.
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| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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