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

Discussion in 'Electrical Engineering' started by Beachcomber, Feb 11, 2004.

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  1. Beachcomber

    Beachcomber Guest

    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
     
  2. SQLit

    SQLit Guest

    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.
     
  3. Beachcomber

    Beachcomber Guest


    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
     
  4. Beachcomber

    Beachcomber Guest

    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
     
  5. Beachcomber

    Beachcomber Guest

    On 12 Feb 2004 08:51:03 GMT, 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
     
  6. Beachcomber

    Beachcomber Guest

    Thanks to all for a good discussion. The spa building is still in the
    planning stages (without existing service) so that will figure into
    the specifications. Much useful input was given from these
    discussions.

    Transformers are interesting, seemingly simply devices... In reality
    however, they are quite complex, like a game of chess, in terms of a
    theoretical analysis of the basis underlying principals.

    For those interested in a little bit of history...

    The inventors of the transformer (Lucien Gaulard & John Gibbs)
    experienced years of difficulty with their invention because they kept
    on hooking up their circuits with the primaries in series instead of
    parallel. The end result was that they could transmit power for long
    distances, but the practicality of the invention suffered due to poor
    load regulation. The parallel primary connection (for multiple
    transformers) that seems so obvious to us today, was not perfected
    until a theoretical understanding was applied by men like Tesla,
    Stanley, and Westinghouse.

    Lucien Gaulard was born in Paris in 1850. Gaulard was a French
    scientist, primarily interested in chemistry of explosives, but later
    he shifted his interests to electrotechnics. He developed a
    thermochemic battery and he is particularly known for his work with
    induction coils (transformers). In 1882, Goulard and his English
    colleague Gibbs patented a system of distributing power using
    alternating current and two-coil induction devices. They used devices
    (then known as secondary generators) of the Ruhmkorff type in the
    first alternating current distribution system and had a 1:1 ratio and
    were used with their primaries in series.

    A power transformer developed by Lucien Gaulard and John Gibbs was
    demonstrated in London in 1881, and attracted the interest of
    Westinghouse. Transformers were nothing new, but the Gaulard-Gibbs
    design was one of the first that could handle large amounts of power
    and promised to be easy to manufacture. In 1885, Westinghouse imported
    a number of Gaulard-Gibbs transformers and a Siemens AC generator to
    begin experimenting with AC networks in Pittsburgh. In hands of
    Westinghouse the transformers received really practical
    implementations.

    Gaulard's transformers were successfully presented in 1884 on the
    international exhibition in Turin. The farthest lamp fed, on the
    Torino-Lanzo railway line, was at 40 km distance from the 2,000 V
    generator with 133 Hz frequency. The series connection led to
    unsatisfactory regulation unless all the transformers were equally
    loaded. However, these transformers were in use until 1912.

    Gaulard was ingenious, but unlucky inventor. During his life his work
    was not recognized in France. Gaulard fell in depression state and
    seems he went mad, was sheltered in clinic where 1888 died the 26
    November. Now Gaulard fame is memorized with tombstone in the railway
    station of Lanzo, there is also a street with his name in Paris.

    This man was the co-inventor of one of the world's most revolutionary
    inventions that gave the world the ability to transmit electric power
    for long distances, but for some reason he just couldn't get the
    connections correct.

    Beachcomber

    Reference: (excerpts from)
    http://chem.ch.huji.ac.il/~eugeniik/history/gaulard.html
     
  7. Beachcomber

    Beachcomber Guest

    I thought I stated it earlier, but perhaps I did not make myself
    clear., these would be commercial units. Most, if not all of the
    commercial units available would still be rated for 240V. (same as
    residential).

    Beachcomber
     
  8. Louis Bybee

    Louis Bybee Guest

    That could be said about almost any installation of any product!

    The problem you've seen is crap installations not the Buck-Boost Xfmr
    proper. Properly specified, and installed, a Buck-Boost xfmr installation is
    neither complex or unreliable. Just the opposite is usually the case. Proper
    application can extend the life of the connected components due to operating
    within a proper voltage window.

    There are ample sources to guide the proper application of a Buck-Boost
    application. A Google search is just one of many.

    Louis--
    *********************************************
    Remove the two fish in address to respond

    Louis
     
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