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October 11, 2006 A Prism Business Media Publication Vol. IV No. 19



CONTENTS
250.30 Grounding and Bonding of Separately Derived AC Systems

What's Wrong Here?

Code Q&A

Code Quiz

Report on Proposals for 2008 NEC

Visit the 2006 EC&M E-TradeShow, a Year-Long Virtual Business Event

NFPA World Safety Conference & Exposition


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    250.30 Grounding and Bonding of Separately Derived AC Systems

    By Mike Holt
    A separately derived system is a premises wiring system with no direct electrical connection to conductors originating from another system [Art. 100 definition and 250.20(D)]. All transformers, except autotransformers, are separately derived because the primary circuit conductors do not have any direct electrical connection to the secondary circuit conductors. Generators that supply a transfer switch that opens the grounded neutral conductor would be considered separately derived [250.20(D) FPN 1].

    250.30(A) Grounded Systems. Separately derived systems must be system bonded and grounded in accordance with the following:

    A neutral-to-case bond must not be on the load side of the system bonding jumper, except as permitted by 250.142(B).

    (1) System Bonding Jumper. Bonding the metal parts of the separately derived system to the secondary grounded neutral terminal by the installation of a system bonding jumper ensures that dangerous voltage from a secondary ground fault can be quickly removed by opening the secondary circuit's overcurrent protection device [250.2(A)(3)].

    The system bonding jumper must be sized in accordance with Table 250.66, based on the area of the largest ungrounded secondary conductor [250.28(D)]. The system bonding jumper can be installed at the separately derived system, the first system disconnecting means, or any point in between the separately derived system and the first disconnecting means -- but not at both locations.

    In addition, the system bonding jumper must be installed at the same location where the grounding electrode conductor terminates to the grounded neutral terminal of the separately derived system, which can be at the separately derived system, the first system disconnecting means, or any point in between -- but not at more than one location [250.30(A)(3)].

    Exception 2: A system bonding jumper can be installed at both the separately derived system and the secondary system disconnecting means where doing so doesn't establish a parallel path for neutral current.

    For all practical purposes, this isn't possible except in a wood frame building that doesn't have any conductive metal.

    Click here to read the rest of this article.



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    Code Challenge
    What's Wrong Here?
    By Joe Tedesco
    Think you know how this installation violates the NEC? Visit EC&M's Web site to see the answer.

    Hint: New math: Addition = Violation


    Code Q&A
    By Mike Holt
    Q. If an outdoor generator has a disconnect, do I need an additional disconnect where the conductors enter the building?

    Visit EC&M's Web site to see the answer.


    Code Quiz
    By Steven Owen
    Q. What is the proper size branch-circuit, short-circuit, ground-fault protective device required for a hermetic compressor with a nameplate rated load of 40A? The branch-circuit selection current on the nameplate is rated at 38A. Note: Do not exceed the values obtained from the lower multiplier.
    A) 50A
    B) 60A
    C) 70A
    D) 90A

    Visit EC&M's Web site for the answer and explanation.


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    Code News Update
    Report on Proposals for 2008 NEC
    The Code is the definition of a "living document." Constantly changing, it's the subject of thousands of proposals for revision throughout each cycle. Earlier this year, the Code Making Panels acted on 3,688 proposals associated with the 2008 NEC. Now the deadline for comments is fast approaching.

    If you're interested in submitting comments on any of the proposed changes, you better get moving. All comments must be submitted to the NFPA by 5 p.m. EST on October 20, 2006. You can submit your comments online or download a comment form.


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    Shows and Events
    Visit the 2006 EC&M E-TradeShow, a Year-Long Virtual Business Event

    Don't miss these scheduled seminars on October 18 in the EC&M e-Tradeshow:
    • "The Basics of Insulation Resistance Testing," by John Olobri, Dir. Sales and Marketing, AEMC Instruments (9:00 am EST and PST, Conference Room A)
    • "Good Project Management, Enhancing the Bottom Line," by John DeDad, Senior Director, Editorial and EC&M Development (11:00 am EST and PST, Conference Room A)
    • "Presenting AEMC's 3-Phase PQ Analyzer," by Ed Cunie, Eastern Regional Manager, AEMC Instruments (10:00 am EST and PST, Conference Room A)
    • "Electrical Market News Update," by Jim Lucy, Chief Editor, Electrical Wholesaling, and Dale Funk, Chief Editor, Electrical Marketing (10:00 am EST and PST, Conference Room B)
    Before attending these events, make sure you visit the 2006 EC&M e-TradeShow, a year long virtual business event. In addition to attending live activities at conference sessions scheduled throughout the year, you can meet with exhibitors in virtual exhibit halls. You can also access past presentations that are archived in the e-TradeShow. Employing the latest interactive 3D technology, sponsors use online trade-show booths to generate leads on a continuous basis throughout the year, while interacting live with customers and prospects during scheduled events. Free access and all the information you need are available at the event's Web site.


    NFPA World Safety Conference & Exposition It's not too early to start making plans for the NFPA World Safety Conference & Exposition, to be held at the Boston Convention & Exhibition Center June 3-7, 2007, in Boston. A popular segment of this show is the necforum™, a track emphasizing the most important issues relating to the National Electrical Code (NEC). This coverage includes seminars on new electrical design issues, successful maintenance programs, best practices in electrical contracting, effective inspection techniques, and practical electrical safety programs. Make sure you save the date for this upcoming event. For more information, visit the show's Web site.



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