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Thread: 4160V to 480V 1000KVA Unit Substation Disconnect

  1. #1
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    4160V to 480V 1000KVA Unit Substation Disconnect

    For a chiller replacement project, we are removing 4,160V chillers and replacing them with 480V chillers. (the new chillers are smaller making 480V a better choice)

    I will be specing around a Square-D Model III unit substation. This contains the 1,000KVA transformer and switchboard in one package.

    The unit substation will be 40 feet and out of site from the 4,160V switchgear section that will feed it.

    The 4,160V switchgear is fused and has a disconnect.

    My question is, does the unit substation require a main disconnect ahead of the transformer within the substation itself?

    Thanks

  2. #2
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    Re: 4160V to 480V 1000KVA Unit Substation Disconnect

    No.
    Just because you can, doesn't mean you should.

  3. #3
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    Re: 4160V to 480V 1000KVA Unit Substation Disconnect

    Could the answer please expanded on?

    Or, point me to the article in the NEC that indicates that a disconnect is not required in this situation.

    thanks

    [ January 31, 2006, 02:53 PM: Message edited by: MO EE ]

  4. #4
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    Re: 4160V to 480V 1000KVA Unit Substation Disconnect

    Motors and appliances may require a disconnect within site, but Transformers don't. Look through Article 450 and you won't find any requirements for a disconnect to be within site.

    I assume both the substation and the 4160 stuff is in the same building, or both are outside. Or you might need a disconnect where you pass through or into a building.

  5. #5
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    Re: 4160V to 480V 1000KVA Unit Substation Disconnect

    It is impossible to prove a negative. How can I show you a code reference that does not exist?

    450.3(A) requires your transformer to have overcurrent protection but does not tell you where it needs to be located. You have provided this in your feeder equipment, so you also meet the requirements of 240.100.

    Other than for motors and motor driven equipment the NEC rarely requires a "local" disconnect.
    Just because you can, doesn't mean you should.

  6. #6
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    Re: 4160V to 480V 1000KVA Unit Substation Disconnect

    Lest someone thinks it applies only to switchgear, I would read Section 490.22 in context of Section 490.1.

    For most Equipment over 600V applications, “In Sight From” as defined in Article 100 is often meaningless. This does not mean a disconnecting switch is the only solution, but “isolation” means more than simply disconnecting the transformer’s feeder at a remote location since 490.22 refers to an “item of equipment.” The feeder conductors are “equipment” themselves as defined in Article 100.

    This was one of the reasons for Exception 1 to 430.102(A). But remember it only applies to motors. There is nothing analogous in Article 450 for transformers.

    Depending on the AHJ, various taps installed per Section 240.21 may be considered incidental parts of the “item of equipment.”
    "Bob"
    Robert B. Alexander, P.E.
    "I know that you believe you understand what you think the NEC says, but I am not sure you realize that what you read is not what it means." (Corollary to Charlie's Rule)

  7. #7
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    Re: 4160V to 480V 1000KVA Unit Substation Disconnect

    Bob, good reference to 490.22 which the OP has complied with by providing a disconnect device with his overcurrent protection.

    490.32 also does not require a disconnect and is often met by the use of "tamper-resistant" hardware. One time on one of my projects, in Wisconsin, the transformer had a hinged door on the terminal compartment. To meet the local code at the time we simply provided a key interlock with the remote fused switch.
    Just because you can, doesn't mean you should.

  8. #8
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    Re: 4160V to 480V 1000KVA Unit Substation Disconnect

    As included under Part III, Section 490.32 ONLY applies to metal-enclosed switchgear and industrial control assemblies and not to transformers. (See 490.30)

    The problem with 490.22 is that "disconnecting means" is a defined term; "isolating means" is not. However, it is a more rigorous requirement as characterized by the statement "... to completely isolate an item [singular] of equipment.

    Obviously conductor connections have to be made and they may or may not be considered a single "item of equipment." I generally try to be fairly liberal with code interpretations, but the context and content of the rest of Section 490.22 strongly requires some form of local, not remote, isolating means.

    I confess I have rarely included a local isolating means on the secondary of a transformer even if it is over 600V, primarily because I keep the secondary conductors within the tap rules and consider them part of the transformer; but I almost always have a local primary isolating means.
    "Bob"
    Robert B. Alexander, P.E.
    "I know that you believe you understand what you think the NEC says, but I am not sure you realize that what you read is not what it means." (Corollary to Charlie's Rule)

  9. #9
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    Re: 4160V to 480V 1000KVA Unit Substation Disconnect

    rbalex-

    Thanks for the explanation. It's appreciated.

  10. #10
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    Re: 4160V to 480V 1000KVA Unit Substation Disconnect

    I beg to differ, 490.22 requires that an isolation means be provided somewhere, it does not require it to be local. In this case there is a remote disconnect switch which may be used for isolating purposes thereby meeting 490.22.

    490.32 and 490.35 may apply to transformers if they are part of a unit substation (see the last sentence of 490.30).
    Just because you can, doesn't mean you should.

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