Another hot tub wiring question (long) - best 3-phase service?

This rather long question is directed to electricians and engineers
who are familiar with wiring and service installation requirements for
a commercial hot tub business. The question is what is the best type
of service class (voltage - # of phases) for a U.S. installation with
6 or more hot tub units.
With the exception of some 120 V. installations, virtually all of the
residential hot tub units in the USA specify single-phase 240 V. four
wire service (usually at 50 or 60A). This consists of two hot wires,
a neutral, and a safety ground wire. The code requirements specify a
hefty Ground Fault Circuit Interupter for each unit.
The problem I am encountering in a commercial installation is as
follows. When combining six or more of these units, it is presumably
desirable to connect to a commercial size three phase service entrance
in such a way that all of these large current-drawing pump motors and
electric heaters present a balanced load to the incoming feeders. The
problem is that even the commercial tubs require 240 volt single phase
4 wire service + the GFCI. Hot tub pump motors that operate on 208V
single or 3-phase are non-standard inventory and difficult to obtain.
The same applies to the factory standard electric heaters which expect
240 volts and take a performance hit if the only voltage available is
208.
Given the standard services available in the USA (Edison split-phase
120/240, 3 phase wye 120/208, 3 phase High Leg Delta 120/240, and
others, etc.) each present difficulties to these requirements.
Standard Edison split-phase 120/240V service would require wiring all
6 hot tubs to one (or two if Delta primary) service phases and create
a highly imbalanced situation as far as maximum current draw. I
suspect that the power company would not be happy with this if the
entire building were loaded onto one phase.
3 phase wye 120/208 service does not match with the required 240V.
that the tubs specify for pumps and heaters.
3 phase High Leg Delta 120/240V service provides for the proper 240
voltage, but the problem is that two sides of the Delta do not have a
center-tapped neutral, thus causing difficulties with the code
requirements for a neutral and the GFCI.
My guess would be that the 3 phase wye 120/208 with three boost
autotransformers to step up the 208 to 240 volts would be the best way
to go, but I'm not an expert and certainly not as familiar with the
code as some of the participants in this newgroup. The goal is a safe
installation that meets and exceeds the NEC requirements at the lowest
possible cost.
I will be doing further investigations and talking to my electrical
contractor and the power company for advice, but I was wondering if
anyone out there had all ready encountered a similar installation and
perhaps could describe how they did it.
Incidently, this seems to be the one example I've found where the
European System (High Leg at 240 V., current carrying neutral, and
safety ground) seems to present an advantage over the US 3 wire split
phase system. If the former were allowed in the USA, we would specify
wye connected transformers with 240 volt secondaries, use Euro style
tubs with their own GFCI's and evenly distribute the load on each
phase. I'm not sure if the US code would permit this, however....
Beachcomber
Reply to
Beachcomber
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snipped , this seems to be the one example I've found where the
The 240 volt service, with high leg may create more problems that the GFI. If you mistakenly get the controls on the high leg,,, poof you have let all the magic out of the unit.
We ran into this a long time ago with 3 phase services to apartments. Water heaters were 240 and the service was 208. We just pulled the covers off the water heaters and raised the temps. They worked just at a reduced output. Same for the ranges. I doubt that anyone even noticed.
As for the utility and the load on a 240v service they do not give a damn. They will just charge the crap out of the owner for demand. The load for these tubs will not even make the utilty lines sneeze. Now if you had 3-4 200 MVA loads they would be talking to ya immediately.
You could put in a buck boost transformer and raise the voltage to 240. Sounds like a lot of money for nothing to me.
Reply to
SQLit
This may help... from NEC 680.44(B) (B) Other Units. A field assembled spa or hot tub rated 3 phase or rated over 250 volts or with a heater load of more than 50 amperes shall not require the supply to be protected by a ground-fault circuit interrupter.
3 phase pumps are certainly available and heaters just have to be sized to reflect the voltage available, as the other poster pointed out. Residential spas are usually the "skid pack" 50a unit you refer to but a commercial spa can easily be field assembled.
Reply to
Greg
If I understand what you are suggesting, this would give you a 240V 3phase Y. The problem here is that the line to neutral voltage will be about 138 volts and not suitable for any 120 volt loads within each unit.
You could use three 208 to 120-240V single phase transformers (not autotransformers) and feed three seperate single phase sub panels. But this is getting expensive.
You could also rewire the units to seperate the 120 volt loads from the 240 volt heater and motors. The 120 volt loads could be connected to the center tapped leg of a 240V delta and wouldn't produce significant imbalance. The problem here is that any UL cert. of these units would be voided, since it applies to the unit as built. This might work if the components are individually certified and the rewired units could be approved as 'field assembled' per another poster's suggestion.
Reply to
Paul Hovnanian P.E.
Thanks for the suggestions. One question though... If I did as you suggest above with three single phase 120-240 transformers, I have concerns about the derived neutrals. Since the primaries would be coming from different phases, it would seem to me that there would be a potential difference between the neutrals of each sub panel if each neutral were connected to the center-tap of the secondaries. In other words, is a neutral truly a neutral if it is not grounded somewhere? If everything were isolated, it would be as though the appliance were operating through a full isolation transformer and would lose the benefits of a safety ground? Does the code allow this?
Hence I thought that by using autotransformers, the original continuity of the (grounded) neutral at the center of the wye would be preserved.
I hope I am expressing this clearly. It seems to be a complex issue.
Beachcomber
Reply to
Beachcomber
| You could put in a buck boost transformer and raise the voltage to 240. | Sounds like a lot of money for nothing to me.
You'd be raising the hot to neutral voltage of the two legs being boosted up to 139 volts (relative to getting 240 volts). An autotransformer will not isolate you from the original neutral which will show the two hots as 120 degrees apart instead of the 180 degrees you want. What is needed is a true isolation transformer. Connect the 208 volt side to two hot legs and the 240/120 volt side will have its neutral grounded properly. Now you'll have two 120 volt hot legs + or - 30 degrees from the hot legs the primary is attached to, and genuine 240/120 volt split phase. Do this for 2 of the 6 hot tubs. Repeat twice on the other 2 phases for the other 4 hot tubs. Of course be sure all the grounding is brought together correctly.
I have seen 240/120 to 208/104 normal isolation transformers, which turned around and used backwards could do the job if sized right. See:
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scroll down to group 2. The ones with connection numbers 10 or 11 could do the job. You can see how those are wired up here:
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scrolling down to connection 10 or connection 11. I have no idea what the pricing is; you'd have to contact them or ask your electrical supplier.
Check on what the actual drop into the building is. In many places it really is 480Y/277 and gets transformed down to 208Y/120 with a transformer in the electrical room. If you are so lucky, you can get 480 to 240/120 single phase transformers a lot cheaper (because there is a market for them) than a 208 to 240/120 one.
Be sure you are using a fixed width font, such as Courier, to view this:
Start with a normal 208Y/120 volt system:
*
Reply to
phil-news-nospam
If you want to sell this in the US you had better use 240/120 volts 1-phase 3 wire or 208/120 volt 3-phase 4 wire. These are the standard voltages and phases. Try anything else and you are asking for trouble.
Reply to
Mr. Smith
Actually, the standard is 240/120 volts 1-phase 4 wire (Hot, Hot, Neutral, Safety Ground) for most residential "skid pak" hot tubs. (Some will run on 120V if that is all that is available). The code is rather stringent about the use of a GFCI for any residential installation. (Siemens makes 50A and 60A GFCI's for this purpose.
Agreed, three phase pumps and three phase hot tub heaters would be better, cheaper, more efficent, quieter, and probably last longer. The problem is that, as far as I can tell, the hot tub industry manufacturers do not stock them as standard equipment. Of course it is possible to build a custom system using 3-phase pumps and 3-phase DHW heaters are certainly available. There are increased costs and warranty issues for the custom construction option, however.
Beachcomber
Reply to
Beachcomber
|>You could use three 208 to 120-240V single phase transformers (not |>autotransformers) and feed three seperate single phase sub panels. But |>this is getting expensive. |> | | | Thanks for the suggestions. One question though... If I did as you | suggest above with three single phase 120-240 transformers, I have | concerns about the derived neutrals. Since the primaries would be | coming from different phases, it would seem to me that there would be | a potential difference between the neutrals of each sub panel if each | neutral were connected to the center-tap of the secondaries. In other | words, is a neutral truly a neutral if it is not grounded somewhere? | If everything were isolated, it would be as though the appliance were | operating through a full isolation transformer and would lose the | benefits of a safety ground? Does the code allow this?
There is no issue regarding derived neutrals. You don't derive them; you just connect them together. This is what I believe is suggested:
[be sure to display with a fixed width font, such as Courier]
A-----*------------------------------------------------------* B-----|--------------*----* | C-----|--------------|----|--------------*----* | N--* | | | | | | | | 208 | | 208 | | 208 | | \/\/\/\/\/\/\/ \/\/\/\/\/\/\/ \/\/\/\/\/\/\/ | ================= ================= ================= | /\/\/\/\ /\/\/\/\ /\/\/\/\ /\/\/\/\ /\/\/\/\ /\/\/\/\ | | 120 | 120 | | 120 | 120 | | 120 | 120 | | *--------|--------|-|--------|--------|-|--------|--------|-- X1 | | | *--------|--------|-|--------|--------|-- Y1 | | | | | *--------|--------|-- Z1 *----------*--------|----------*--------|----------*--------|-- N *-------------------|-------------------|-- X2 *-------------------|-- Y2 *-- Z2
| Hence I thought that by using autotransformers, the original | continuity of the (grounded) neutral at the center of the wye would be | preserved.
Normally an autotransformer used for boost (or buck) is done with small increments of 12 to 32 volts of change. But technically it could do a larger change. Perhaps what you meant is this:
A----*------------------------------------------------------------ X1 B----|-------------------*---------------------------------------- Y1 C----|-------------------|-------------------*-------------------- Z1 N----|--------*----------|--------*----------|--------*----------- N | | *-|--------|----------|--------|----------- X2 | | | | | *-|--------|----------- Y2 | | | | | | | | *-- Z2 | 120 | 120 | | 120 | 120 | | 120 | 120 | \/\/\/\/ \/\/\/\/ \/\/\/\/ \/\/\/\/ \/\/\/\/ \/\/\/\/ ================= ================= =================
Instead of trying to extend each of the three legs longer (to 139 volts to get 240 between legs), what you'd do in this case is extend a "new leg" in the opposite direction and derive a 120-0-120 configuration from a 0-120 configuration, times three for all phases (splitting hot tub loads across the three phases equally). Someone who knows more about this than I do will need to make sure the ratings are right, and check to make sure this will meet code, especially in the wet situation. Finding such a transformer would be another matter. Something with identical 120 volt windings on primary and secondary might be usable.
I'd still prefer the isolation from the first case, though the second looks like it could work. Either way you get a 6-star output:
Y1 * * Z1 \ / \ / X1 *----N----* X2 / \ / \ X2 * * Y2
Reply to
phil-news-nospam
I am not sure that is really true if you are not simply using a skid pack. Pumps is pumps when it comes to warranty issues. A reputable company will back either one. A 3p unit is generally going to be a better unit than a consumer grade 1p pool pump but they are not that unusual in commercial setups and I thought that was what you were building. You will have less problems with a 3p pump motor since you don't have the "start" hardware.
Reply to
Greg
On 12 Feb 2004 08:51:03 GMT, snipped-for-privacy@ipal.net wrote:>
Phil: Thanks for a most impressive diagram. Now I'm wondering if I can just order my service this way from the power company. (In other words... they would configure the secondaries according to your diagram, using a primary voltage of whatever the incoming lines are set for...
I've not seen a diagram like this before. Would this be considered a standard class of service? The only thing that causes me to have concern about your circuit is that I've lived in condo buildings that only provided the common wye 208/120 V. (single phase service to each unit - 3-phase incoming service). I'm now wondering why I could not get the more desirable 240/120 V. service (as per your diagram).
Are you saying there is no issue with the three center tapped neutrals all bonded together like that? It would seem that even with no load, you would have current flowing in the neutral even though it might all sum to zero.
Beachcomber
Reply to
Beachcomber
| On 12 Feb 2004 08:51:03 GMT, snipped-for-privacy@ipal.net wrote:> |>[be sure to display with a fixed width font, such as Courier] |> |>A-----*------------------------------------------------------* |>B-----|--------------*----* | |>C-----|--------------|----|--------------*----* | |>N--* | | | | | | |> | | 208 | | 208 | | 208 | |> | \/\/\/\/\/\/\/ \/\/\/\/\/\/\/ \/\/\/\/\/\/\/ |> | ================= ================= ================= |> | /\/\/\/\ /\/\/\/\ /\/\/\/\ /\/\/\/\ /\/\/\/\ /\/\/\/\ |> | | 120 | 120 | | 120 | 120 | | 120 | 120 | |> | *--------|--------|-|--------|--------|-|--------|--------|-- X1 |> | | | *--------|--------|-|--------|--------|-- Y1 |> | | | | | *--------|--------|-- Z1 |> *----------*--------|----------*--------|----------*--------|-- N |> *-------------------|-------------------|-- X2 |> *-------------------|-- Y2 |> *-- Z2 |> | Phil: | Thanks for a most impressive diagram. Now I'm wondering if I can | just order my service this way from the power company. (In other | words... they would configure the secondaries according to your | diagram, using a primary voltage of whatever the incoming lines are | set for... | | I've not seen a diagram like this before. Would this be considered a | standard class of service? The only thing that causes me to have | concern about your circuit is that I've lived in condo buildings that | only provided the common wye 208/120 V. (single phase service to each | unit - 3-phase incoming service). I'm now wondering why I could not | get the more desirable 240/120 V. service (as per your diagram).
The power company would likely not deliver 7 wire service to you. They don't have meters that can work with 7 wires. And if they did this with 3 separate 1 phase meters, they likely can't set that up under a single service account. And then there are electrical code and tariff issues.
To meter it properly, it will have to be brought in as conventional 3 phase. You'll have to put in three transformers, or if you can find such a thing (I've never seen one, and I've seen a lot of transformers listed by manufacturers and used/surplus sellers), a 6-pole 7-wire 3x240/120 3 phase transformer (let me know if you find such a beast).
I also recall seeing some limitations on number of wires on a service drop.
3 phase can be run from 3 separate transformers. The power companies do it all the time. Just look around at the poles in commercial areas and you'll see many with 3 transformers (pole pigs) together. Look closer (from the ground) and see how it is wired. You should see one of the wires going to all three on the secondary terminals, and three (for delta service) or four (for wye service, the most common) wires going into the drop (overhead or underground).
So what you'll likely end up with is normal 3 phase power at some voltage. How you wire this up will depend on which voltage you get.
If you get service at 208Y/120, you can wire the hot tubs up as shown above, with transformers rated for the usage you will have (including any future expansion) for those hot tubs. Then lights and outlets in the rest of the place can just tap off of the 208Y/120 directly.
Alternatively, you can run the entire building/office/whatever from those transformers. Each of the 240/120 volt outputs would need their own breaker panel whether just for the hot tubs or for the whole area.
If your service comes in at 480Y/277 (or 600Y/347 in Canada), you have to have either a 3 phase transformer, or three 1 phase transformers, just to get usable 120 volts circuits, anyway. In these cases, you might as well just do the above circuit for the whole place and have 240/120 (x3) everywhere.
If you are going with 3 transformers as described above for the whole building/office/area, then you might try to get your service dropped in at 240 volt DELTA (if not the higher voltages listed above). The reason for that is because single phase transformers with an input of 240 volts or 480 volts (usually they come as dual primary 480/240) are cheaper due to mass production.
If you get 240 DELTA, you won't have a neutral coming in, but you'll be creating one on the secondaries (you _must_ run everything from the transformers with this kind of power service). Of course that neutral must be well grounded from the power room where the transformers will be located.
You might also be able to get the oddball power service of 416Y/240 which might be known as 400Y/230, etc. It's uncommon in the US, but not unheard of. The reason it might be possible is because a regular 240/120 volt secondary pole pig can be easily wired up to do it in a WYE configuration by ignoring the center tap. If you do get this kind of service, the wiring diagram I provided will have to be changed to following to be able to use the cheaper transformers:
A-----* B-----|-------------------* C-----|-------------------|-------------------* N--*--|--------------*----|--------------*----|--------------* | | 240 | | 240 | | 240 | | \/\/\/\/\/\/\/ \/\/\/\/\/\/\/ \/\/\/\/\/\/\/ | ================= ================= ================= | /\/\/\/\ /\/\/\/\ /\/\/\/\ /\/\/\/\ /\/\/\/\ /\/\/\/\ | | 120 | 120 | | 120 | 120 | | 120 | 120 | | *--------|--------|-|--------|--------|-|--------|--------|-- X1 | | | *--------|--------|-|--------|--------|-- Y1 | | | | | *--------|--------|-- Z1 *----------*--------|----------*--------|----------*--------|-- N *-------------------|-------------------|-- X2 *-------------------|-- Y2 *-- Z2
If you do get 416Y/240 you can also use the original diagram with 416 volt input transformers, but those are rare and more expensive. There is no reason to get 416Y/240 unless it saves you money (it's uncommon and probably not available just because no one uses it anymore).
Check your power company web site for tariffs to see what you might be able to get.
And of course, a licensed electrician will have to install this and make sure it meets all applicable code, and it will have to be inspected in any scenario. All I'm doing is exploring your options (something I would be doing if I were installing all this). I don't like running 240 equipment on 208 volts ... I've seen it burn out to many A/C motors.
| Are you saying there is no issue with the three center tapped neutrals | all bonded together like that? It would seem that even with no load, | you would have current flowing in the neutral even though it might all | sum to zero.
You would have no current in the neutrals unless the transformers were faulty. If you have current flowing _out_ of the transformer on one wire you have to have current flowing _in_ on another. That's how ground fault breakers are able to work without cutting into the ground wire. What they do is measure the current on all the hot wires and neutral (not ground) wire together (i.e. run them all through a common current transformer). If there is a ground fault and current is flowing out the ground (green) wire somewhere, then the vector sum of currents in the hots+neutral will show such current flowing (and trip the breaker at the appropriate level). If there is no ground leakage, that sum of hots+neutral would be zero.
If you do have potential between transformer secondaries, you have a big problem.
Just be sure you put GFCI breakers on all the circuits going anywhere near the hot tubs, bathrooms, kitchens, concrete floor areas, etc. The codes should spell out all these requirements and your electrician should know them well (I'm not an electrician and I'm assuming you are not, either).
Reply to
phil-news-nospam
Good diagrams. This is what I was thinking. Not as a service, but something to be implemented on the customer's premises.
Not competely correct. We are assuming that there are some 120 volt single phase loads within each hot tub. If all the internal loads were 240 volt, then this discussion would be pointless. It would be perfectly OK to connect 240 volt 2 wire loads across each leg of a 240 volt delta service. Its the internal single phase loads that preclude this possibility since their voltage to the delta's neutral could go to 208 volts if they were on the wrong side of the wild leg.
A single phase, 240 volt 3 wire circuit can be ground-fault protected by summing both hot legs plus the neutral. > If you do have potential between transformer secondaries, you have a big > problem. > > Just be sure you put GFCI breakers on all the circuits going anywhere near > the hot tubs, bathrooms, kitchens, concrete floor areas, etc. The codes > should spell out all these requirements and your electrician should know > them well (I'm not an electrician and I'm assuming you are not, either). > > -- > ----------------------------------------------------------------------------- > | Phil Howard KA9WGN |
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| > | (first name) at ipal.net |
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| > -----------------------------------------------------------------------------
Reply to
Paul Hovnanian P.E.
In the trade it is called 240/120 volts 1-phase 3 wire and 208/120 volts 3-Phase 4 wire. The grounding conductor is not considered part of the system description. Just for your information.
Reply to
Mr. Smith
|> | Are you saying there is no issue with the three center tapped neutrals |> | all bonded together like that? It would seem that even with no load, |> | you would have current flowing in the neutral even though it might all |> | sum to zero. |> |> You would have no current in the neutrals unless the transformers were |> faulty. | | Not competely correct. We are assuming that there are some 120 volt | single phase loads within each hot tub. If all the internal loads were | 240 volt, then this discussion would be pointless. It would be perfectly | OK to connect 240 volt 2 wire loads across each leg of a 240 volt delta | service. Its the internal single phase loads that preclude this | possibility since their voltage to the delta's neutral could go to 208 | volts if they were on the wrong side of the wild leg.
Oops, meant to say no current between transformers on the neutral, unless he hooked up some cross phase or 3 phase load (which needs to be done via a 3 phase breaker panel, not a 1 phase one, which he would have 3 of for these 3 1 phase setups).
|> If you have current flowing _out_ of the transformer on one wire |> you have to have current flowing _in_ on another. That's how ground fault |> breakers are able to work without cutting into the ground wire. What they |> do is measure the current on all the hot wires and neutral (not ground) |> wire together (i.e. run them all through a common current transformer). |> If there is a ground fault and current is flowing out the ground (green) |> wire somewhere, then the vector sum of currents in the hots+neutral will |> show such current flowing (and trip the breaker at the appropriate level). |> If there is no ground leakage, that sum of hots+neutral would be zero. | | A single phase, 240 volt 3 wire circuit can be ground-fault protected by | summing both hot legs plus the neutral.
Or all three hot legs plus neutral if running three phase.
Reply to
phil-news-nospam
Correct. In fact, the neutrals of the three derived single phase systems would be independent. The only connection that they would have would be through the grounding system. Electrically, there is a connection between the systems neutrals, but it should carry no load currents.
Reply to
Paul Hovnanian P.E.
An option that saves one transformer is to get 240V delta service with the center tap of one transformer grounded (I forget the exact name of this service) Get two 240-240CT transformers and connect the primary of one transformer between the high leg "B" and "A" of the service, and the other between the high leg "B" and "C". For the third you don't need a transformer, just use the "A" "N" and "C" of the service. Note that "B" is the high leg, 208V (NOT 120V!) to neutral. "N" is the center tap of the transformer that connects "A" to "C" and is grounded.
A modified version of Phil Howard's ASCII drawing follows:
Now that I think of it, you can cheat even more and use only one transformer! Again, get a 240V center tapped grounded delta service, and use a "Scott-T" transformer configuration. This essentially generates 2 phase service from 3 phase service, and yes it is balanced among the 3 phases as long as the load is balanced among the two sources. The transformer is 208V-240VCT and has to be 50% beefier than the transformers in the previous examples. Connect as follows:
Connect 3 directly to the service phases "A" "N" "C" (Z1-N-Z2 on the drawing), and the other 3 to X1-N-X2 on the drawing. Again, "B" is the high leg.
Note the transformer powers 3, not 2 tubs. It needs to be large enough. Terminals X1 and Z1 are 90 degrees out of phase, not 120 degrees as in 3 phase service.
If you do do this, you may have to do a lot of explaining to get the electrical inspector to understand it.
If you get any other service other than 240V high leg delta you can still do a "Scott-T" configuration with two transformers - if you can find the right ones. One has a center tap _primary_ and goes between two phases, and has a 240VCT secondary. (This one is skipped above as it would be 240VCT-240VCT, both CTs grounded) The second has a primary 0.866 times the first, and it connects from the third phase to the center tap of the primary of the first transformer, also with a 240VCT secondary.
Reply to
Michael Moroney
| snipped-for-privacy@ipal.net wrote: |> |> |> |> |> | Are you saying there is no issue with the three center tapped neutrals |> |> | all bonded together like that? It would seem that even with no load, |> |> | you would have current flowing in the neutral even though it might all |> |> | sum to zero. |> |> |> |> You would have no current in the neutrals unless the transformers were |> |> faulty. |> | |> | Not competely correct. We are assuming that there are some 120 volt |> | single phase loads within each hot tub. If all the internal loads were |> | 240 volt, then this discussion would be pointless. It would be perfectly |> | OK to connect 240 volt 2 wire loads across each leg of a 240 volt delta |> | service. Its the internal single phase loads that preclude this |> | possibility since their voltage to the delta's neutral could go to 208 |> | volts if they were on the wrong side of the wild leg. |> |> Oops, meant to say no current between transformers on the neutral, |> unless he hooked up some cross phase or 3 phase load (which needs to be |> done via a 3 phase breaker panel, not a 1 phase one, which he would |> have 3 of for these 3 1 phase setups). | | Correct. In fact, the neutrals of the three derived single phase systems | would be independent. The only connection that they would have would be | through the grounding system. Electrically, there is a connection | between the systems neutrals, but it should carry no load currents.
OTOH, if you were to connect a load to a hot leg of one transformer, and a hot leg of a different transformer, while the neutrals are all tied together, you'll get either 208 volts or 120 volts, depending on which you pick, and that load's current will flow across those neutral interconnects. Do that in balance three ways and you cancel it back out. It's all just a 6-star.
But the OP's perception that it would be a "short" of some kind isn't true. One could tie the end terminals of the windings as neutral and produce a 416Y/240 configuration.
Reply to
phil-news-nospam
Open delta.
The above will work. It really shouldn't matter whether the utility provides the 240V 3ph-4w service from a full or open delta. Just as long as the service neutral is the center tap of the A-C phase (as you stated).
Reply to
Paul Hovnanian P.E.
| An option that saves one transformer is to get 240V delta service with | the center tap of one transformer grounded (I forget the exact name of | this service) Get two 240-240CT transformers and connect the primary | of one transformer between the high leg "B" and "A" of the service, | and the other between the high leg "B" and "C". For the third you don't | need a transformer, just use the "A" "N" and "C" of the service. | Note that "B" is the high leg, 208V (NOT 120V!) to neutral. | "N" is the center tap of the transformer that connects "A" to "C" and | is grounded.
That would be overlaying an "open delta" on an existing delta so as to isolate the phases to allow tying the neutrals together. That could work.
| Now that I think of it, you can cheat even more and use only | one transformer! Again, get a 240V center tapped grounded delta | service, and use a "Scott-T" transformer configuration. This essentially | generates 2 phase service from 3 phase service, and yes it is balanced | among the 3 phases as long as the load is balanced among the two | sources. The transformer is 208V-240VCT and has to be 50% beefier | than the transformers in the previous examples. Connect as follows:
I believe the current angle will be plus and minus 30 degrees reactive on 2 of the phases with this. A Scott-T is meant to supply a 3 phase load, not a 1 phase load. The power company might not like this due to the low power factor (0.5).
| If you get any other service other than 240V high leg delta you can still | do a "Scott-T" configuration with two transformers - if you can find the | right ones. One has a center tap _primary_ and goes between two phases, | and has a 240VCT secondary. (This one is skipped above as it would be | 240VCT-240VCT, both CTs grounded) The second has a primary 0.866 times | the first, and it connects from the third phase to the center tap of the | primary of the first transformer, also with a 240VCT secondary.
This would likewise have the reactive phase problem due to half the loads being of a different current angle than any of the phases.
Reply to
phil-news-nospam

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