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April 3, 2008 A Penton Media Publication Vol. VI No. 7

450.3 Overcurrent Protection

What's Wrong Here?

Code Q&A

Code Quiz

Comments Sought on Proposed TIA


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    Top 50 NEC Rules

    450.3 Overcurrent Protection

    By Mike Holt
    Overcurrent protection of the primary winding of a transformer not exceeding 600V must comply with (B). Fine print note No. 2 states that 4-wire, 3-phase 120/208V or 277/480V systems that supply nonlinear line-to-neutral loads can overheat because of triplen harmonics (3rd, 9th, 15th, 21st, etc.) [450.9 FPN 2].

    The primary winding of a transformer rated less than 600V must be protected against overcurrent in accordance with the percentages listed in Table 450.3(B) and all applicable notes.

    Let’s review an example to better explain these requirements.

    Question: What is the primary protection device rating and conductor size required for a 45kVA, 3-phase, 480V-120/208V transformer that is fully loaded? Terminals are rated 75°C.

    Answer: Step 1. Calculate the primary current.
    I = VA ÷ (E x 1.732) = 45,000VA ÷ (480V x 1.732) = 54A

    Step 2. Identify the primary protection device rating per 240.6(A).
    54A x 1.25 = 68A, next size up is 70A, Note 1

    Step 3. The primary conductor must be sized to carry 54A continuously (54A x 1.25 = 68A) [215.2(A)(1)] and be protected by a 70A protection device [240.4(A)]. A 4 AWG conductor rated 85A at 75°C meets all of the requirements [110.14(C)(1) and 310.16].

    Step 4. Calculate the secondary current.
    I = VA ÷ (E x 1.732) = 45,000VA ÷ (208V x 1.732) = 125A

    Secondary conductors having a maximum length of 25 feet that terminate in an overcurrent protection device that doesn’t exceed the ampacity of the conductors, must be sized at 125% of the continuous load [215.2(A)(1) and 240.21(C)(6)].

    Editor’s note: This information was extracted from Mike Holt’s textbook, Understanding the National Electrical Code

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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: The 72-hour shunt

    Code Q&A
    By Mike Holt
    Q. Does the NEC require a connection of a parking lot lighting pole to a ground rod?

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

    Code Quiz
    By Steven Owen
    Q. When installing wiring methods in ducts or plenums used for environmental air, all but which of the following wiring methods are permitted?

    1. Flexible metal conduit no greater than 4 feet in length
    2. EMT
    3. IMC
    4. MC cable with overall nonmetallic covering

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

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    Code News Update
    Comments Sought on Proposed TIA
    As published in the March issue of NFPA News, the following Tentative Interim Amendment (TIA) has been proposed to the NFPA.

    NFPA 70-2008, National Electrical Code
    TIA Log No. 904
    Reference: 645.17
    Submitter: William Burke, American Power Conversion
    1. Delete 645.17 as follows: “645.17 Power Distribution Units. Power distribution units that are used for information technology equipment shall be permitted to have multiple panelboards within a single cabinet, provided that each panelboard has no more than 42 overcurrent devices and the power distribution unit is utilization equipment listed for information technology application.”

    Substantiation: A longstanding requirement for the NEC has been the limitation that no more than 42 overcurrent devices of a lighting and appliance branch-circuit panelboard be installed in any one cabinet or cutout box (reference Section 408.35 of the 2004 NEC).

    Equipment intended for use in an information technology equipment (ITE) room (Article 645 installation) has traditionally been constructed using multiple 42-circuit panelboards within the same enclosure. To account for this reality, a proposal was processed for the 2005 Code that resulted in Section 645.17. This new section clarified that distribution units used for ITE are permitted to have multiple panelboards within a single cabinet, provided that each panelboard has no more than 42 overcurrent devices, and the unit is listed for information technology applications.

    This requirement was conceived with the traditional 42-circuit limitation in place and was intended as a relaxation of the limitation of no more than 42 circuits within a single enclosure.

    With the revisions to Section 408.36 in the 2008 NEC, the 42-circuit limitation has been removed. A companion proposal was not submitted to Code-making Panel 12, and the revision processed by Code-making Panel 9 was not correlated between technical committees. The end result is the unintended consequence that Section 645.17 is more restrictive than the general rule in Section 408.36, without any technical reasons for the differences.

    Emergency Nature: APC, as the original submitter of the proposal that results in Section 645.17 in the 2005 NEC (reference ROP – May 2004, Proposal 9-110), submit that the intent of the proposal for the 2005 NEC process was to include a provision for additional overcurrent devices in ITE power distribution units. New requirements in Section 408.36, which were not coordinated with existing requirements in 645.17, resulted in the unintended consequence of more restrictive requirements for ITE power distribution units. Approval of this request for a Tentative Interim Amendment will correct this error.

    This TIA has been published for public review and comment. Comments should be filed with the Secretary, Standards Council by May 2, 2008. You should identify the TIA number when submitting your comments. The Standards Council will review the technical committees’ ballot results, the public comments, and any other information that has been submitted when it considers the issuance of the TIA at its July 22-24, 2008 meeting.

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