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240 volts vs. 208 volts in a residential building

Discussion in 'Electrical Engineering' started by [email protected], Nov 29, 2006.

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  1. Guest

    Which of the following methods of wiring a large residential building
    would be preferred between these two choices (there certainly are other
    choices, but I'm focusing on these two right now).

    The building is supplied by power at 480Y/277 volts, or at a higher
    voltage stepped down in a secured electrical room to 480Y/277. One or
    more large capacity 480Y/277 volts circuits feed through the vertical
    core of the building to an electrical room on each floor. On each
    floor, a dry-type transformer steps 480 volts down to service voltage
    for each customer.

    1.

    Each floor is powered through a three phase transformer that steps the
    480Y/277 volt subfeed down to 208Y/120 volts. Each tenant is supplied
    with just TWO phases of the three phase service, with the choice of
    phases approximately balanced.

    2.

    Each floor is powered through a single phase transformer that steps
    just two legs of the 480Y/277 volt subfeed from 480 volts to 240/120
    volts. Each tenant is supplied with this normal single split phase
    voltage. The diversity of the floors are approximately balanced.

    For both of these cases, assume the total load is within the capacity
    of available transformers, or that multiple transformers could be used
    to supply each floor when there is a greater load than one transformer
    can supply. Also assume that special building-wide loads such as
    elevators and centralized HVAC can be powered by 480Y/277 directly if
    applicable, or by a voltage system derived from 480Y/277 as needed.

    As a variation of choice #2, where more than one transformer is needed
    for a building with large floors, these transformers can be balanced
    as reasonably possible over the core subfeed phases. Also assume that
    additional subfeed circuits can be separately wired if a single feed
    would be inadequate, up to as many feeds as needed, such as one feed
    separately to each floor.

    I'm not specifying a particular size for this building. Instead, what
    I want to focus on is the practicality of supplying 120/240 volts for
    single phase tenants (and generally residential will be single phase)
    instead of 120/208 volts.

    Another question: would this preference be any different if some or
    all of the tenants were light business use, such as lawyer offices,
    corporate branch sales offices, recruiters, etc, with no unusual
    electrical needs (but would have a small kitchen with normal cooking
    facilities for employee use such as lunch breaks)?
     
  2. Guest

    208 sucks in a residential building unless the landlord owns the
    laundry, provides HVAC and all the heat producing appliances are
    fossil fuel.
    Consumer grade dryers, ranges, water heaters and A/C units are
    designed for 240. You can 208 volt equipment but it costs more and
    the output of 240v heaters at 208 is significantly less.
     
  3. John Gilmer

    John Gilmer Guest

    Personally, I would prefer single phase 120/240 over a "2 out of 3" 120/208
    service.

    There is, maybe, a better selection of 240 volt over 208 "stuff" including
    office equipment.

    OTOH if someone "needed" 120/208 3 phase with the help of a two transformers
    he could "reconstitute" is from the 3 wire 120/208.

    I
     
  4. Beachcomber

    Beachcomber Guest


    I agree that 208 sucks and is to be avoided if possible, but you can
    purchase electric range heater elements and dryer conversion kits for
    208V. It's a pain in the A though if residents don't understand and
    buy or bring the wrong replacement equipment. A 240V dryer running on
    208V, in my experience will run a lot longer to dry the clothes
    properly.

    Of course if you have gas ranges and gas dryers, it doesn't matter.

    The larger (portable) air conditioners are rated for 240V and you may
    have trouble finding one that works OK on 208v. Fixed AC (compressor
    units are usually rated for 240v but sometimes wired on 208V.
    circuits. Then there might be elevator motors, sump pumps, air
    handlers, vent fans and other motors to consider.

    Beachcomber
     
  5. Guest

    On Wed, 29 Nov 2006 20:00:50 -0500 wrote:

    | On 29 Nov 2006 22:48:47 GMT, wrote:
    |
    |>Which of the following methods of wiring a large residential building
    |>would be preferred between these two choices
    |
    | 208 sucks in a residential building unless the landlord owns the
    | laundry, provides HVAC and all the heat producing appliances are
    | fossil fuel.

    Exactly.


    | Consumer grade dryers, ranges, water heaters and A/C units are
    | designed for 240. You can 208 volt equipment but it costs more and
    | the output of 240v heaters at 208 is significantly less.

    There are 208 volt elements for many things, but not all. That itself
    is clear indication that 208 volts for 240 volt stuff sucks.
     
  6. Guest

    | On Wed, 29 Nov 2006 20:00:50 -0500, wrote:
    |
    |>On 29 Nov 2006 22:48:47 GMT, wrote:
    |>
    |>>Which of the following methods of wiring a large residential building
    |>>would be preferred between these two choices
    |>
    |>208 sucks in a residential building unless the landlord owns the
    |>laundry, provides HVAC and all the heat producing appliances are
    |>fossil fuel.
    |>Consumer grade dryers, ranges, water heaters and A/C units are
    |>designed for 240. You can 208 volt equipment but it costs more and
    |>the output of 240v heaters at 208 is significantly less.
    |
    |
    | I agree that 208 sucks and is to be avoided if possible, but you can
    | purchase electric range heater elements and dryer conversion kits for
    | 208V. It's a pain in the A though if residents don't understand and
    | buy or bring the wrong replacement equipment. A 240V dryer running on
    | 208V, in my experience will run a lot longer to dry the clothes
    | properly.

    Not all ranges have 208 volt elements available. Some do. Most don't
    as far as I've seen. The more expensive ones seems to be the ones that
    do, but you're just tossing out a fine 240 volt element after special
    ordering a 208 volt element.


    | Of course if you have gas ranges and gas dryers, it doesn't matter.
    |
    | The larger (portable) air conditioners are rated for 240V and you may
    | have trouble finding one that works OK on 208v. Fixed AC (compressor
    | units are usually rated for 240v but sometimes wired on 208V.
    | circuits. Then there might be elevator motors, sump pumps, air
    | handlers, vent fans and other motors to consider.

    I have seen some for 208 volts. But not very many. And those were
    commercial units for motels.

    IMHO, three phase should be limited to 480 volts in the USA.
     
  7. Guest

    | Personally, I would prefer single phase 120/240 over a "2 out of 3" 120/208
    | service.
    |
    | There is, maybe, a better selection of 240 volt over 208 "stuff" including
    | office equipment.
    |
    | OTOH if someone "needed" 120/208 3 phase with the help of a two transformers
    | he could "reconstitute" is from the 3 wire 120/208.

    You probably mean:

    A *
    \ / \
    N C
    /
    B

    But it still doesn't solve the 240 volt problem.
     
  8. Ben Miller

    Ben Miller Guest

    Nothing unique about this. It is common. In fact, I saw one today. The
    distribution panels in the metering room on each floor are three-phase, each
    tenant gets 208/120 single phase. I don't know what 240 volt loads are in
    the units. If you use gas for the cooking and laundry, the entire issue of
    208 vs. 240 in the tenant units goes away. The only other load would be A/C,
    and those are normally dual rated for 230/208 volt.
    Distributing three phase is more efficient in terms of the wiring, panel,
    and transformer cost. At the same voltage, you can carry 73% more power with
    33% more copper (25% more copper if you have an egc as well).

    Often done. Not a problem.

    Talk to some building owners about what they are willing to pay for extra
    transformers and wiring just so their tenants can have 240/120 volts instead
    of 208/120, and I think you will see the practical problem. I have clients
    (owners) trying to squeeze every last penny out of the cost of the
    electrical systems in these buildings. They just aren't going to buy into
    the extra $$, when there really isn't any problem. If appliances were
    failing every week, that would be different.
    Nope. Many of those buildings also use 208/120 four wire. It is all about
    the cost of distributing the power through the building. There is no
    compelling reason to add cost or complexity.

    Ben Miller
     
  9. John Gilmer

    John Gilmer Guest

     
  10. Guest

    | |> On Wed, 29 Nov 2006 21:16:45 -0500 John Gilmer <>
    | wrote:
    |>
    |> | Personally, I would prefer single phase 120/240 over a "2 out of 3"
    | 120/208
    |> | service.
    |> |
    |> | There is, maybe, a better selection of 240 volt over 208 "stuff"
    | including
    |> | office equipment.
    |> |
    |> | OTOH if someone "needed" 120/208 3 phase with the help of a two
    | transformers
    |> | he could "reconstitute" is from the 3 wire 120/208.
    |>
    |> You probably mean:
    |>
    |> A *
    |> \ / \
    |> N C
    |> /
    |> B
    |>
    |> But it still doesn't solve the 240 volt problem.
    |
    | Quite correct. I just wanted to give the ONLY reason to give the units
    | 120/208 over 120/240.
    |
    | I'm definitely on the 120/240 side.
    |
    | Old Tom Edison got it right the first time!
    |
    | BTW: That is a VERY good ASCII representation of the way of generatig the
    | 3rd phase.

    Well, Tom had part of it right. I'm still fence sitting on the DC thing.
    AC is certainly more practical right now because everything is designed
    around it ... just as so many things are designed around 240 volts instead
    of 208 volts.

    I wonder what the power company would think if I put big single phase loads
    on that derived C-N connection :)
     
  11. Guest

    | |> Which of the following methods of wiring a large residential building
    |> would be preferred between these two choices (there certainly are other
    |> choices, but I'm focusing on these two right now).
    |>
    |> The building is supplied by power at 480Y/277 volts, or at a higher
    |> voltage stepped down in a secured electrical room to 480Y/277. One or
    |> more large capacity 480Y/277 volts circuits feed through the vertical
    |> core of the building to an electrical room on each floor. On each
    |> floor, a dry-type transformer steps 480 volts down to service voltage
    |> for each customer.
    |>
    |> 1.
    |>
    |> Each floor is powered through a three phase transformer that steps the
    |> 480Y/277 volt subfeed down to 208Y/120 volts. Each tenant is supplied
    |> with just TWO phases of the three phase service, with the choice of
    |> phases approximately balanced.
    |
    | Nothing unique about this. It is common. In fact, I saw one today. The
    | distribution panels in the metering room on each floor are three-phase, each
    | tenant gets 208/120 single phase. I don't know what 240 volt loads are in
    | the units. If you use gas for the cooking and laundry, the entire issue of
    | 208 vs. 240 in the tenant units goes away. The only other load would be A/C,
    | and those are normally dual rated for 230/208 volt.

    It is in fact A/C that I had my first real life experience with that
    taught me why 208 is bad when things are designed for 240. After
    several burnouts of the A/C blower motor, the service people finally
    had to special order a 208 volt motor. The motor will run on a lower
    than normal voltage, but during peak heat, our computer room drove it
    to nearly continuous operation. The ambient heat plus extra current
    heat apparently rose it above the heat some part of the motor just
    could not withstand for the time periods involved.


    |> Each floor is powered through a single phase transformer that steps
    |> just two legs of the 480Y/277 volt subfeed from 480 volts to 240/120
    |> volts. Each tenant is supplied with this normal single split phase
    |> voltage. The diversity of the floors are approximately balanced.
    |>
    |
    | Distributing three phase is more efficient in terms of the wiring, panel,
    | and transformer cost. At the same voltage, you can carry 73% more power with
    | 33% more copper (25% more copper if you have an egc as well).

    I don't see where you get those exact figures. I know three phase can
    be more efficient to distribute power, but those numbers don't seem
    right. By saying "33% more copper" that sounds like you are going from
    a 3-wire Edison style single phase to a 4-wire Wye derived three phase
    with the same size conductors. But that doesn't give 73% more power.
    Assuming the same L-N voltage, it's only 50% more. Assuming the same
    L-L voltage, it's only 57% more. And if you dismiss the neutral in
    both cases, which forces assuming the same L-L voltage, then you are
    increasing the copper by 50% to go to three phase, with the 57% power
    increase, for a net gain of 15.47% efficiency for the same amount of
    copper.

    One can always increase the efficiency of "copper" by increasing the
    voltage. The conducting material doesn't really care. The insulating
    material sure does. Of course you have to weigh the comparitive costs
    and I do believe copper is pretty damned expensive. So is aluminum
    (which utility transmission and distribution tends to use).


    |> As a variation of choice #2, where more than one transformer is needed
    |> for a building with large floors, these transformers can be balanced
    |> as reasonably possible over the core subfeed phases. Also assume that
    |> additional subfeed circuits can be separately wired if a single feed
    |> would be inadequate, up to as many feeds as needed, such as one feed
    |> separately to each floor.
    |
    |> I'm not specifying a particular size for this building. Instead, what
    |> I want to focus on is the practicality of supplying 120/240 volts for
    |> single phase tenants (and generally residential will be single phase)
    |> instead of 120/208 volts.
    |
    | Talk to some building owners about what they are willing to pay for extra
    | transformers and wiring just so their tenants can have 240/120 volts instead
    | of 208/120, and I think you will see the practical problem. I have clients
    | (owners) trying to squeeze every last penny out of the cost of the
    | electrical systems in these buildings. They just aren't going to buy into
    | the extra $$, when there really isn't any problem. If appliances were
    | failing every week, that would be different.

    In most cases, appliances won't fail; they will just underperform. For
    example an electric water heater takes longer to recover on 208 volts
    than if on 240 volts. Penny pinching owners won't want to pay extra to
    get water heaters with special 208 volt elements.

    This is also wasteful on energy costs. Heating elements are generally
    thermostatically controlled (watch the red glow going on and off with
    a glass-top range). At 208 volts (compared to 240 volts) they will be
    drawing 86.6% of the current but only providing 75% of the heat. This
    means they will be switched on 33% more (133% total) time. That means
    there is a 15.47% extra energy loss in the wiring.

    So really, the penny pinching owners are shifting the costs to the
    tenants, even if both parties don't even realize it.

    So home much more does a 75 kVA single phase transformer cost over that
    of a 75 kVA three phase transformer? My first scenario is to run each
    floor on just one of the phases, not be split up three ways. If 75 kVA
    is enough for the floor (4 apartments, as an example), this would work.

    More likely, they would end up not running 480 volt feeds to transformers
    on each floor, especially if the building is small (say 3 to 6 floors).
    They'd put in a big 208Y/120 transformer coming into the building, maybe
    as a pad mount out back, and just run everything from that.

    They could run full three phase to each apartment and put in a three
    phase panel. Then just hook appliances within at various phases and
    put each apartment in different rotation offsets to diversify things
    well. But they don't. It's actually _more_ expensive to do that, at
    least for the cost of a branch breaker panel (home comsumer demand and
    competititon has certainly driven down the price of single phase panels
    in the 100 to 200 amp range).


    |> Another question: would this preference be any different if some or
    |> all of the tenants were light business use, such as lawyer offices,
    |> corporate branch sales offices, recruiters, etc, with no unusual
    |> electrical needs (but would have a small kitchen with normal cooking
    |> facilities for employee use such as lunch breaks)?
    |
    | Nope. Many of those buildings also use 208/120 four wire. It is all about
    | the cost of distributing the power through the building. There is no
    | compelling reason to add cost or complexity.
    |
    | Ben Miller

    When I was around age 7 or 8 or so, I remember that my grandfather had a
    brownout problem. The power company was unable to deliver full voltage
    for a day or so. The voltage was something like 15% or 20% less. While
    lights did work, the electric stove just didn't even work at all. His
    freezer compressor (ran on what was supposed to be 120 volts) burned up.

    One big issue in this scenario was that he actually had three phase power
    at presumably 208Y/120 volts. He got that because he powered his wood
    shop in the back of the house with it since several of his big machines
    specifically used three phase (and presumably were designed for 208 volts).

    The problem with the stove, though, was that it was dealt a double blow
    in brownout effects. It was a normal home model presumably intended for
    240 volts. It was being run on 208 volts. With the brownout at say 15%
    it was now getting only 177 volts.

    The point here is this. Things designed for 240 volts might well work
    fine over the whole range of voltage they could get when connected at
    240 volts, and work fine when getting the true nominal voltage when
    connected at 208 volts. But run them at the lower end of the voltage
    range when also connected at 208 volts, and this is pushing things to
    the extreme. If the 240 volt appliance can in fact operate OK all the
    way from 200 volts to 280 volts actual, putting it on what is nominally
    at eaither far end could be a problem when that supply voltage swings
    away from that nominal voltage the more extreme way.

    If the appliance can operate at + or - 20% around its designed nominal
    voltage, and the utility supply can vary by + or - 10% long term and
    an additional + or - 10% short term, AND if you intentionally connect
    the appliance to only 86.6% of the voltage, then it really can end up
    being out of range at least sometimes ... when that 86.6% voltage ends
    up actually being really just 73% of the voltage.

    How much of a range would an appliance need to support to be able to
    handle the FULL high swing from a 240 volt connection to the FULL LOW
    swing of a 208 volt connection.
     
  12. I lived in two apartment complexes prior to buying my first home. Both were
    larger complexes and both had 120/240 distribution. The utility provided
    multiple services from single phase pad mount transformers. I'm assuming
    that given the size of the complex, the primary was three phase with the
    transformers distributed among the phases.
     
  13. I guess I didn't finish my point, so here goes:

    A distribution transformer has about the same cost and efficiency going from
    primary voltage to either 480V or 240V. If you add the extra intermediate
    voltage of 480V, there is the extra cost of transformers and the extra loss
    of transformers to go from 480V to 240V. This is offset by the lower losses
    and lower cost in SOME of the distribution equipment, mostly the wires.
    480V panels tend to be more expensive than 240V for the same power at the
    lower power end of things - like an apartment would be. In an industrial /
    commercial application, much of the load is at 480V. The lighting,
    elevators and HVAC can all be run at 277Y/480V. Only a small fraction of
    the load ends up being double transformed. I have seen buildings designed
    with both 480V and 208V service from the utility. I expect that the loss
    with this type of service would be the lowest. In an apartment complex,
    only a small amount of the load could be utilized at 277Y/480V. Most would
    be double transformed, so the extra transformer loss would be more
    significant.

    The point I made above is that a complex could be feed by a multiple of
    three single phase services. That would give 120/240 to everyone. That
    would give the utility reasonably balanced load.

    One more thing to consider is the dislike many utilities have for delta
    connected transformers. I talked to a utility engineer since our least
    thread on this subject about the common 480V to 120Y/208V in commercial and
    industrial buildings. He indicated that he would actually prefer that they
    were wired Y-Y, but realizes that won't happen. He said that they don't
    amount to too much of a problem because they are such a small fraction of
    the load - usually 25% or less. In a shorted phase or ferroresonance
    incident, the primary breaker of the transformer often trips because it's so
    much smaller than the large transformer feeding the service. The other
    common line to line loads are motors. In a phase loss or power loss
    situation, they usually trip off line so they are not a problem. Motors
    also don't cause ferroresonance like a transformer does.
     
  14. I major gripe I have with many of the apartment complex developers is the
    choice of resistance heat. The owner pays for the equipment, but the tenant
    pays the power bill. That usually leads to the cheapest equipment and
    maintenance cost but highest operating cost choice of electric heat. I'm a
    fairly hands off the market person, but I'm almost willing to support
    legislation on more efficient heat for apartments.
     
  15. Guest

    |> Talk to some building owners about what they are willing to pay for extra
    |> transformers and wiring just so their tenants can have 240/120 volts
    |> instead of 208/120, and I think you will see the practical problem. I have
    |> clients (owners) trying to squeeze every last penny out of the cost of the
    |> electrical systems in these buildings. They just aren't going to buy into
    |> the extra $$, when there really isn't any problem. If appliances were
    |> failing every week, that would be different.
    |
    | I major gripe I have with many of the apartment complex developers is the
    | choice of resistance heat. The owner pays for the equipment, but the tenant
    | pays the power bill. That usually leads to the cheapest equipment and
    | maintenance cost but highest operating cost choice of electric heat. I'm a
    | fairly hands off the market person, but I'm almost willing to support
    | legislation on more efficient heat for apartments.

    Don't forget to also include a requirement for an abundance of high R factor
    insulation in the exterior walls, and even between apartments (FYI, I set my
    thermostat to 15C in winter, which is well below what most people set it at,
    so I'm effectively taking heat from neighbors that are usually at 20C).

    Those willing to be less intrusive on how to construct the buildings could
    instead simply mandate disclosure of things like the total insulation value
    or the heat loss rate for the unit. But most consumers won't understand
    these figures.

    How many of you "smart" engineers know how much your credit card companies
    are ripping you off for? Businesses make money off of consumer ignorance
    and actively fight legislation to inform consumers, even when most of them
    wouldn't even understand the information.

    I'm all for mandating savings ... except I'll keep (and pay for) incandescent
    lights in my kitchen. As long as I'm the one paying for the energy, I should
    have that right. If I build an apartment building, I would be expected to
    put energy saving lights in, however.
     
  16. Guest

    | I lived in two apartment complexes prior to buying my first home. Both were
    | larger complexes and both had 120/240 distribution. The utility provided
    | multiple services from single phase pad mount transformers. I'm assuming
    | that given the size of the complex, the primary was three phase with the
    | transformers distributed among the phases.

    That's not really hard to do. The last apartment I lived in was 120/240
    (little single phase pads for each building). The one before that (the
    one with the master metering mentioned in another newsgroup) was a big
    tall three phase cabinet pad mount with a rather deep reservoir section,
    serving a cluster of several buildings (about 18 buildings of 16 units
    each). Good money says it was 120/208, but I never checked. The range
    and A/C at least worked, and the hot water was centrally supplied.
    I never had occaision to see what kind of fault current it provided.

    But say you are building a 16 floor apartment building with 8 units per
    each of the 15 floors above the ground floor. Would you want to power
    that whole thing from a single 208Y/120 transformer and potentially have
    to pay for all the extra interruption series ratings because such a beast
    might put you too close to, or over, the typical 22 kVA interrupt level?
    My inclination would be 480 up the core and rotating the taps on it and
    put a single phase 480 to 120/240 dry transformer on each floor. Three
    phase loads, like elevators, could run on the 480. Exterior HID lights
    could run on 277. The lobby floor might well end up with 208Y/120 from
    it's own dry transformer to service a small restaurant or two (which
    generally does have 208 volt rated equipment available).

    If someone does want to do the load calcs on a building like that for fun,
    pick some apartment size for each and have a go at typical values and see
    what you really come up with given the diversity. Of course it would all
    be in fun since there are no real specifics on a hypothetical building.

    How big of a building would you say it would have to be to make even 480
    not enough voltage for the core feed?
     
  17. Guest

    | I guess I didn't finish my point, so here goes:
    |
    | A distribution transformer has about the same cost and efficiency going from
    | primary voltage to either 480V or 240V. If you add the extra intermediate
    | voltage of 480V, there is the extra cost of transformers and the extra loss
    | of transformers to go from 480V to 240V. This is offset by the lower losses
    | and lower cost in SOME of the distribution equipment, mostly the wires.
    | 480V panels tend to be more expensive than 240V for the same power at the
    | lower power end of things - like an apartment would be. In an industrial /
    | commercial application, much of the load is at 480V. The lighting,
    | elevators and HVAC can all be run at 277Y/480V. Only a small fraction of
    | the load ends up being double transformed. I have seen buildings designed
    | with both 480V and 208V service from the utility. I expect that the loss
    | with this type of service would be the lowest. In an apartment complex,
    | only a small amount of the load could be utilized at 277Y/480V. Most would
    | be double transformed, so the extra transformer loss would be more
    | significant.

    It's going to be double transformed whether the tenants get 120/240 or
    120/208 or 208Y/120, unless the main transformer outputs 208Y/120.


    | The point I made above is that a complex could be feed by a multiple of
    | three single phase services. That would give 120/240 to everyone. That
    | would give the utility reasonably balanced load.

    If it's a tall building, that could be done with the 2nd transformer on
    each floor or groups of 2 or 3 floors, depending on practical sizing.


    | One more thing to consider is the dislike many utilities have for delta
    | connected transformers. I talked to a utility engineer since our least
    | thread on this subject about the common 480V to 120Y/208V in commercial and
    | industrial buildings. He indicated that he would actually prefer that they
    | were wired Y-Y, but realizes that won't happen. He said that they don't
    | amount to too much of a problem because they are such a small fraction of
    | the load - usually 25% or less. In a shorted phase or ferroresonance
    | incident, the primary breaker of the transformer often trips because it's so
    | much smaller than the large transformer feeding the service. The other
    | common line to line loads are motors. In a phase loss or power loss
    | situation, they usually trip off line so they are not a problem. Motors
    | also don't cause ferroresonance like a transformer does.

    So how about running 832Y/480 up the core and make the utility happy, and
    you can still use the common 480 to 120/240 single phase transformers. If
    it means more reliable service to the tenants, and doesn't cost much more,
    I wouldn't care. But the big issue is getting the entrance transformer,
    wiring, and panels to support 832Y/480. For smaller buildings, 416Y/240
    might be an option where the single phase transormers are wired for a 240
    volt primary. The problem is the upper end of LV tends to be 600 volts
    L-L, and some cheaper stuff is only 480 L-L and 277 L-N. So maybe 416Y/240
    is about their only option (and now you have to special order elevators and
    HVAC for the unusual 416Y/240 or double transform that, too).
     
  18. Guest

    | One more thing to consider is the dislike many utilities have for delta
    | connected transformers. I talked to a utility engineer since our least
    | thread on this subject about the common 480V to 120Y/208V in commercial and
    | industrial buildings. He indicated that he would actually prefer that they
    | were wired Y-Y, but realizes that won't happen. He said that they don't
    | amount to too much of a problem because they are such a small fraction of
    | the load - usually 25% or less. In a shorted phase or ferroresonance
    | incident, the primary breaker of the transformer often trips because it's so
    | much smaller than the large transformer feeding the service. The other
    | common line to line loads are motors. In a phase loss or power loss
    | situation, they usually trip off line so they are not a problem. Motors
    | also don't cause ferroresonance like a transformer does.

    Still thinking about this ... if a building is fed by a MV -> 480Y/277 that
    is Y-Y as the utility likes for it to be, how much of a problem would exist
    if all the transformers fed by this 480Y/277 were single phase connected
    L-L at 480 volts in and 120/240 out? That's not a delta. If one phase is
    lost coming in to the main Y-Y transformer, that's simply going to be one
    phase dead on the 480Y/277 side, and 2/3 of the building w/o power. But
    what other problems could exist with that? What problems could happen if
    there was a major short on the secondary side of one of the single phase
    transformers (aside from hopefully tripping only the minimum breaker that
    is necessary)? Would there be any reason for excessive breaker trips if
    they are all coordinated correctly? Would making all the sub-transformers
    single phase effectively solve the delta connected transformer problems as
    the utility sees it?
     
  19. John Gilmer

    John Gilmer Guest

    They would prefer it over drawing 240 volts on one phase only.

    That's the "other reason" the 120/208 is popular: large loads at least draw
    from 2 phases. Seems like "around here" the power company just doesn't
    like "weird" 3 phase arrangements anymore. New service is Y or single
    phase. It may have something to do with deregulation: the local
    distribution company might have to pay a penalty for "unbalanced" draws from
    suppliers. Before de-reg it was all in the same company.
    |------------------------------------/-------------------------------------|
     
  20. Guest

    |> I wonder what the power company would think if I put big single phase
    | loads
    |> on that derived C-N connection :)
    |
    | They would prefer it over drawing 240 volts on one phase only.
    |
    | That's the "other reason" the 120/208 is popular: large loads at least draw
    | from 2 phases. Seems like "around here" the power company just doesn't
    | like "weird" 3 phase arrangements anymore. New service is Y or single
    | phase. It may have something to do with deregulation: the local
    | distribution company might have to pay a penalty for "unbalanced" draws from
    | suppliers. Before de-reg it was all in the same company.

    If the one load is given a voltage in the C-N phase angle, but the only
    connections are A and B, how does this affect the current phase angles?
     
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