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May 29, 2007 A Penton Media Publication Vol. V No. 10



CONTENTS
Cast Your Vote Now!

250.142 Use of Grounded Neutral Conductor for Equipment Grounding (Bonding)

What's Wrong Here?

Code Q&A

Code Quiz

EC&M Code Change Conferences

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


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    Top 50 NEC Rules
















    250.142 Use of Grounded Neutral Conductor for Equipment Grounding (Bonding)

    By Mike Holt
    To remove dangerous voltage on metal parts from a ground fault, the metal parts of electrical raceways, cables, enclosures, and equipment must be bonded to an effective ground-fault current path in accordance with 250.4(A)(3).

    Supply side of service equipment. A grounded neutral conductor can be used as the effective ground-fault current path for metal parts of equipment, raceways, and other enclosures. Because an equipment grounding (bonding) conductor isn't run from the utility to electrical services, the grounded neutral service conductor can serve as the effective ground-fault current path to the utility power source. The effective ground-fault current path for service equipment is provided by the installation of the main bonding jumper at service equipment in accordance with 250.24(B) [250.28].

    Where no equipment grounding (bonding) conductor is run to a separate building or structure disconnect, the grounded neutral conductor can serve as the effective ground-fault current path to the source. This is accomplished by bonding the grounded neutral conductor to the equipment grounding (bonding) conductor at the separate building or structure building disconnecting means in accordance with 250.32(B)(2).

    Caution: Using the grounded neutral conductor as effective ground-fault current path poses potentially dangerous consequences and should only be done after careful consideration. The safest practice is to install an equipment grounding (bonding) conductor with the feeder conductors to the building or structure to serve as the effective ground-fault current path, as provided by 250.32(B)(1).

    On a separately derived system, the effective ground-fault current path is established when the system bonding jumper is installed between the metal enclosure of the separately derived system and the grounded neutral terminal in accordance with 250.30(A)(1). Failure to install the system bonding jumper as required by 250.30(A)(1) will create a condition where dangerous touch voltage from a ground fault will remain on the metal parts of electrical equipment.

    Load side equipment. To prevent dangerous voltage on metal parts, the grounded neutral conductor must not be bonded to the equipment grounding (bonding) conductor on the load side of service equipment, except as permitted by 250.142(A).

    Exception No. 1: The grounded neutral conductor can serve as the effective ground-fault current path for existing ranges, dryers, and ovens [250.140 Ex].

    Exception No. 2: The grounded neutral conductor can be bonded to the meter enclosure on the load side of the service disconnecting means if:

    1. No service ground-fault protection is installed,
    2. Meter enclosures are located immediately adjacent to the service disconnecting means, and
    3. The grounded neutral conductor is sized no smaller than specified in Table 250.122.

    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: Power in a pinch


    Code Q&A
    By Mike Holt
    Q. Can I install conductors from different panels in the same raceway?

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


    Code Quiz
    By Steven Owen
    Q. What is the minimum size and metal thickness required for a junction box constructed of sheet steel, with one 4-inch RMC conduit (per wall) connected on opposite walls of the box (i.e., directly across from one another)? The conductors within the raceways are 600kcmil type XHHW. The outer diameter (O.D.) of each of these insulated conductors is 1.2 inches.

    1. 24 inch by 24 inch. There is no requirement for metal thickness.
    2. 32 inch by 24 inch. Metal thickness = 1.35 millimeters (0.053 inches).
    3. 32 inch by 32 inch. Metal thickness = 1.35 millimeters (0.053 inches).
    4. 43.2 inches by 43.2 inches. Metal thickness = 1.35 millimeters (0.053 inches).

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


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