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Thread: 3 Phase 4 wire Vs 3 wire

  1. #1
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    3 Phase 4 wire Vs 3 wire

    Hi All. I'm a little new to the electrical design world and am not school trained to do what I'm doing. My background is in Biomedical Instrumentaton. So it's 'Trial by fire.' I have one question that concerns me as I dont want contractors to get PO'd with me.
    In 480V three phase systems with a generator to run parallel with utility; do I have to specify 4-wire if no 277V loads are going to be used?
    There is a considerable diffence in the cost of a 3 Wire ATS Vs a 4 wire ATS and this is my reason for asking.

  2. #2
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    Quote Originally Posted by montericci View Post
    Hi All. I'm a little new to the electrical design world and am not school trained to do what I'm doing. My background is in Biomedical Instrumentaton. So it's 'Trial by fire.' I have one question that concerns me as I dont want contractors to get PO'd with me.
    In 480V three phase systems with a generator to run parallel with utility; do I have to specify 4-wire if no 277V loads are going to be used?
    There is a considerable diffence in the cost of a 3 Wire ATS Vs a 4 wire ATS and this is my reason for asking.
    Do you mean "parallel with utility" as in interconnected or is this simply standby power?

    If standby, will the transfer switch be service equipment?

    Not likely that you need to switch the grounded conductor.

    Wecome to the forum.

  3. #3
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    If no line to neutral loads are being served it's not really necessary to bring a grounded conductor. This is a generalization though, more detail about the system would help.

    Welcome to the forum and good luck.

  4. #4
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    Not enough info to answer your question. It all depends if you have 277 loads off the service or not, and OCPD topology.

  5. #5
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    There may be a misunderstasnding regarding the difference between a 3ph w/o a switched neutral and one with a switched neutral.
    It would depend upon how the system neutral is grounded.
    If the ATS was switching between a normal source and a generator the neutral most likely is grounded at the source of service entrance only.
    If the ATS was switching between 2 sources where each source was grounded separately then it would require a switched neutral to prevent the neutral from being gorunded in two separate locations.

    As such the big difference in cost would be that switched 4th pole for the neutral.

  6. #6
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    Ottawa, IL
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    Added info

    I apoligize for being vauge but please excuse my ignorance. Let me rephrase the question a bit.
    The standby generators that I've been asked to specify will run all the loads that the utility is feeding. Sewer lift staions or Well pumps or an entire plant or facillity. I spec the outlets, motor control centers, panelboards, transformers,starters, VFDs, lighting, etc. for most of the these jobs. I typically spec a Service entrance rated ATS or a service entrance rated Disconnect ahead of the ATS.
    Can I ground the neutral at the genset and use a 3 wire ATS for these jobs if there is no 277V loads in the plans when using 480v 3 phase power for the equipment at the facillity?

  7. #7
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    If you don't have any 277 volt loads, you don't need a neutral. If you don't have a neutral, you don't need to switch it.

    If you have a neutral conductor and ground it at the generator, then you have a separately derived source and you need a 4-pole ATS. Usually, you ground the neutral only at the service and you use a 3-pole ATS. There are cases where the distance from the service to the generator is such that grounding only at the service makes ground fault current too low when running on the generators (fault current has to flow through the EGC to the service neutral bond and then through the neutral to the generator).

    If you have no neutral conductor, you can ground the generator neutral and still not have a separately derived source.

  8. #8
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    You probably have 3 choices:

    1. Spec. a 3 pole ATS, and don't run the generator's neutral wire to the ATS. You can do this since you don't have any 277V loads. However, there are many times when I have needed to add equipment that needs a neutral, only to find out someone saved a few bucks by not running one. Then I have to install a 1 to 1 delta to wye transformer just to get the neutral back. So before you do this, make sure you won't need any 277 V loads in the future.

    2. Spec a 3 pole ATS, and run the generators neutral wire to the ATS, and make a solid connection between it and the utility neutral. Then you still have a neutral available. If you do this, you want to make sure the neutral is bonded to the ground only at the utility, and not at the generator.

    3. Spec a 4 pole ATS, and switch the neutral wire along with the phase wires.

    In both cases 2 and 3, the neutral wire wouldn't have to be the full size as the phase conductors.

    Steve

  9. #9
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    If your utility service is 480Y/277 then a neutral should have been installed as far as the service disconnect and bonded, etc.
    If that is the case, then I would say the generator neutral should be brought to you 3 pole ATS and bonded.
    At my age, I'm accustomed to restaurants asking me to pay in advance, but now my bank has started sending me their calendar one month at a time.

  10. #10
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    Quote Originally Posted by montericci View Post
    I spec the outlets, motor control centers, panelboards, transformers,starters, VFDs, lighting, etc. for most of the these jobs.
    What voltage are your receptacles and lighting?

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