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August 27, 2007 A Penton Media Publication Vol. V No. 16

300.20 Induced Currents in Metal Parts

What's Wrong Here?

Code Q&A

Code Quiz

Comments Sought on Tentative Interim Amendments

EC&M Code Change Conferences

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    Top 50 NEC Rules

    300.20 Induced Currents in Metal Parts

    By Mike Holt
    To minimize induction heating of ferrous metallic raceways and enclosures -- and to maintain an effective ground-fault current path -- all conductors of a circuit must be installed in the same raceway, cable, trench, cord, or cable tray. See 250.102(E), 300.3(B), 300.5(I), and 392.8(D).

    When alternating current (AC) flows through a conductor, a pulsating or varying magnetic field is created around the conductor. This magnetic field is constantly expanding and contracting with the amplitude of the AC current. In the United States, the frequency is 60 cycles per second. Since AC reverses polarity 120 times per second, the magnetic field that surrounds the conductor also reverses its direction 120 times per second. This expanding and collapsing magnetic field induces eddy currents in the ferrous metal parts that surround the conductors, causing the metal parts to heat up from hysteresis.

    Hysteresis heating affects only the ferrous metals with magnetic properties, such as steel and iron, but not aluminum. Simply put, the molecules of steel and iron align to the polarity of the magnetic field and when the magnetic field reverses, the molecules reverse their polarity as well. This back and forth alignment of the molecules heats up the metal -- the greater the current flow, the greater the heat rise in the ferrous metal parts.

    When conductors of the same circuit are grouped together, the magnetic fields of the different conductors tend to cancel each other out, resulting in a reduced magnetic field around the conductors. The lower magnetic field reduces induced currents in the ferrous metal raceways or enclosures, which reduces hysteresis heating of the surrounding metal enclosure.

    When single conductors are installed in nonmetallic raceways as permitted in 300.5(I) Ex. 2, the inductive heating of the metal enclosure can be minimized by the use of aluminum locknuts and by cutting a slot between the individual holes through which the conductors pass.

    Aluminum conduit, locknuts, and enclosures carry eddy currents, but because aluminum is nonferrous, it doesn't heat up.

    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: A case of the bends.

    Code Q&A
    By Mike Holt
    Q. What is the minimum service drop clearance above a roof?

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

    Code Quiz
    By Steven Owen
    Q. A construction site has 10 office trailers. Each unit is identical. The HVAC unit for each trailer has a motor-rated load current of 27A and a branch circuit selection current of 30A at 208VAC, 3-phase. What is the proper size for the ground-fault, short-circuit protective device for each HVAC unit?

    1. 30A
    2. 40A
    3. 45A
    4. 50A

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

    Streamline the implementation of your Arc Flash and Electrical Safety Program. What used to take hours, now takes mere seconds! EasyPower delivers Windows® -based tools for designing, analyzing, and monitoring electrical power systems. Register now for the EasyPower training coming to Orlando, Florida November 5 - 9, 2007! One-touch automated design and one-touch automated protective device coordination options, will also be introduced!

    Code News Update
    Comments Sought on Tentative Interim Amendments
    According to August/September 2007 issue of NFPA News, two Tentative Interim Amendments (TIAs) to the 2008 version of the National Electrical Code have been proposed to the NFPA. The TIAs are now available for public review and comment. Any comments on these TIAs should be filed with the Secretary Standards Council by September 17, 2007. The NEC technical committee will consider all comments prior to taking final action on these items. The Standards Council will then review the technical committees' ballot results, the public comments, and any other information that has been submitted to determine whether to issue the TIA at its meeting October 3-4, 2007.

    The two TIAs in question are as follows:

    TIA Log No. 881 -- "Add the following Exception to Articles 374.17, 366.23(A)(B), 372.11, 372.17, 374.5, 374.17, 376.22, 378.22, 384.22, 386.22, 388.22, 392.9, 392.10, and 392.11: Exception No. 1: Additional conductors shall not be added unless with a tested and listed or recognized method or device that maintains spacing for air to surround the conductors, and it is to be followed by its installation instructions with its limitations. Article 310.15(B)(a) spacing shall apply."

    TIA Log No. 891 -- "Revise the following section to read as follows: (374.17) The ampacity adjustment factors in Table 310.15(B)(2)(a) shall not apply to cellular metal floor raceways."

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    Shows and Events
    EC&M Code Change Conferences
    Where do you turn when you need accurate information on changes to the National Electrical Code? Acknowledged as the leaders in providing information on the NEC, EC&M magazine and EC&M Seminars have been the preferred sources of this information for more than 60 years. Seven Code change conferences have been scheduled in the fall of 2007. Host cities include: Atlanta, Boston, Dallas, Orlando, Philadelphia, St. Louis, and Seattle.

    As an approved provider with the National Council of Examiners for Engineering and Surveying (NCEES), through its Registered Continuing Education provider Program (RCEPP), professional engineers attending any of our 2008 Code change conferences will receive Professional Development Hours (PDHs), a requirement for re-licensing in many states. The conferences are also approved by every state that has a continuing education requirement for contractors and electricians.

    For additional information on the dates and locations of these events, click here.

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