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June 27, 2007 A Penton Media Publication Vol. V No. 12

300.5 Underground Installations

What's Wrong Here?

Code Q&A

Code Quiz

Code Committee Call-Up

Let's Go Racing! Win a Free Road America Race Weekend for Two

EC&M Code Change Conferences

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    Top 50 NEC Rules

    300.5 Underground Installations

    By Mike Holt
    When cables and raceways are run underground, they must have a minimum "cover" in accordance with Table 300.5. Note 1 to Table 300.5 defines cover as the distance to the top of the underground cable or raceway to the surface of finish grade. The cover requirements contained in 300.5 do not apply to signaling or communications wiring. These requirements are found in other sections of the Code.

    Cables and insulated conductors installed in enclosures or raceways underground must be listed for use in wet locations. Cables run under a building must be installed in a raceway that extends past the outside walls of the building.

    Direct buried conductors and cables must be protected from damage in accordance with (1) through (4).

    1. Emerging from grade. Direct buried cables or conductors that emerge from the ground must be installed in an enclosure or raceway to protect against physical damage. Protection isn't required to extend more than 18 inches below grade, and protection above grade must extend to a height not less than 18 feet.
    2. Conductors entering buildings. Conductors that enter a building must be protected to the point of entrance.
    3. Service conductors. Service conductors that aren't under the exclusive control of the electric utility, and are buried 18 inches or more below grade, must have their location identified by a warning ribbon in the trench not less than 1 foot above the underground installation. Note: It's impossible to comply with the service conductor identification location requirement when service conductors are installed using directional boring equipment.
    4. Enclosure or raceway damage. Where direct buried cables, enclosures, or raceways are subject to physical damage, the conductors must be installed in rigid metal conduit, intermediate metal conduit, or Schedule 80 rigid nonmetallic conduit.
    Direct buried conductors or cables can be spliced or tapped underground without a splice box [300.15(G)], if the splice or tap is made in accordance with 110.14(B).

    Backfill material for underground wiring must not damage the underground cable raceway or contribute to the corrosion of the metal raceway. Large rocks, chunks of concrete, steel rods, mesh, or other sharp-edged objects cannot be used for backfill material because they can damage the underground conductors, cables, or raceways.

    Where moisture could enter a raceway and contact energized live parts, seals must be installed at one or both ends of the raceway. This is a common problem for equipment located physically downhill from the supply or in underground equipment rooms. See 230.8 for service raceway seals and 300.7(A) for different temperature area seals. A Fine Print Note in 300.5(G) explains that hazardous explosive gases or vapors make it necessary to seal underground conduit or raceways that enter the building in accordance with 501.15. It isn't the intent of this FPN to imply that seal-offs of the types required in hazardous (classified) locations must be installed in unclassified locations, except as required in Chapter 5. This also doesn't imply that the sealing material provides a watertight seal, but only that it prevents moisture from entering.

    Raceways that terminate underground must have a bushing or fitting at the end of the raceway to protect emerging cables or conductors.

    All conductors of the same circuit, including the equipment-grounding (bonding) conductor, must be inside the same raceway or in close proximity to each other. See 300.3(B). There are two Exceptions associated with this rule. The first notes that conductors can be installed in parallel in accordance with 310.4. The second exception states individual sets of parallel circuit conductors can be installed in underground nonmetallic raceways, if inductive heating at raceway terminations is reduced by complying with 300.20(B) [300.3(B)(1) Ex. No. 1]. Installing phase and neutral wires in different nonmetallic raceways makes it easier to terminate larger parallel sets of conductors, but it will result in higher levels of electromagnetic fields (EMF), which can cause computer monitors to flicker in a distracting manner.

    Direct buried conductors, cables, or raceways subject to movement by settlement of frost, must be arranged to prevent damage to conductors or equipment connected to the wiring.

    Cables or raceways installed using directional boring equipment must be approved by the authority having jurisdiction (AHJ) for this purpose.

    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: Count the prongs.

    Code Q&A
    By Mike Holt
    Q. What are the requirements for supporting outlet boxes above/to suspended ceilings?

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

    Code Quiz
    By Steven Owen
    Q. When installing a high-impedance grounded neutral system, for a 480V, 3-phase system, the following question has arisen. Which of the following choices best describes the proper routing for the grounded conductor of the system?

    1. It shall be run (installed) in the same raceway with the other phase conductors. This is the only choice. No other options are available.
    2. It shall be run (installed) in a non-metallic raceway with the other phase conductors, following the requirements of 300.5(I)(Exception No. 2).
    3. It shall be permitted to be run (installed) in the same raceway with the other phase conductors if desired; however, it shall also be permitted to be installed in a separate raceway by itself with any routing chosen by the installer.
    4. It must be insulated, bare, or covered, and installed so as not to be easily damaged, with or without a raceway for protection.

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

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    Code News Update
    Code Committee Call-Up
    Got some extra time on your hands? Looking to put some of your vast knowledge of the electrical field to use? NFPA is looking for new members for several of its committees, including the following:
    • Committee on Electrical Equipment in Chemical Atmospheres. This committee is responsible for NFPA 496, Standard for Purged and Pressurized Enclosures for Electrical Equipment; NFPA 497, Recommended Practice for the Classification of Flammable Liquids, Gases, or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas; and NFPA 499, Recommended Practice for the Classification of Combustible Dusts and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas
    • Committee on Electrical of Industrial Machinery (seeking members in all categories except end users or manufacturers). This committee is responsible for NFPA 79, Electrical Standard for Industrial Machinery.
    • Committee on Electrical Equipment Evaluation (seeking members in all categories)
    • Committee on Electrical Systems Maintenance (special experts excluded). This committee is responsible for NFPA 73, Electrical Inspection Code for Existing Dwellings.
    • Committee on Health Care Facilities -- Electrical Systems. This committee is responsible for chapters within NFPA 99, Standard for Health Care Facilities.
    Anyone interested in serving can download the application form at NFPA's Web site.

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    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.

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

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