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July 25, 2006 A Prism Business Media Publication Vol. IV No. 14



CONTENTS
240.4 Protection of Conductors

What's Wrong Here?

Code Q&A

Code Quiz


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    240.4 Protection of Conductors

    By Mike Holt
    Except as permitted by (A) through (G), conductors must be protected against overcurrent in accordance with their ampacity after ampacity adjustment, as specified in 310.15.

    (A) Power Loss Hazard. Conductor overload protection is not required, but short-circuit protection is required where the interruption of the circuit would create a hazard, such as in a material-handling electromagnet circuit or fire pump circuit.

    (B) Overcurrent Protection Not Over 800A. The next higher standard rating overcurrent device (above the ampacity of the ungrounded conductors being protected) is permitted, provided all of the following conditions are met:

    1. The conductors do not supply multi-outlet receptacle branch circuits.
    2. The ampacity of a conductor, after ampacity adjustment and/or correction, doesn't correspond with the standard rating of a fuse or circuit breaker in 240.6(A).
    3. The protection device rating doesn't exceed 800A.
    This "next size up rule" doesn't apply to feeder tap conductors [240.21(B)], or secondary transformer conductors [240.21(C)].

    (C) Overcurrent Protection Over 800A. If the circuit's overcurrent protection device exceeds 800A, the conductor ampacity, after ampacity adjustment and/or correction, must have a rating not less than the rating of the overcurrent device.

    (D) Small Conductors. Unless specifically permitted in 240.4(E) or (G), overcurrent protection must not exceed 15A for 14AWG, 20A for 12 AWG, and 30A for 10 AWG copper, or 15A for 12 AWG and 25A for 10 AWG aluminum, after ampacity adjustment and/or correction.

    (E) Tap Conductors. Tap conductors must be protected against overcurrent as follows:

    1. Household Ranges and Cooking Appliances and Other Loads -- 210.19(A)(3) and (4)
    2. Fixture Wire -- 240.5(B)(2)
    3. Location in Circuit -- 240.21
    4. Reduction in Ampacity Size of Busway -- 368.17(B)
    5. Feeder or Branch Circuits (busway taps) -- 368.17(C)
    6. Single Motor Taps -- 430.53(D)

    (F) Transformer Secondary Conductors. The primary overcurrent protection device sized in accordance with 450.3(B) can protect the secondary conductors of a 2-wire system or a 3-wire, 3-phase, delta/delta connected system, provided the primary protection device does not exceed the value determined by multiplying the secondary conductor ampacity by the secondary-to-primary transformer voltage ratio.

    (G) Overcurrent for Specific Applications. Overcurrent protection for specific equipment and conductors must comply with that referenced in Table 240.4(G).

    • Air-conditioning and refrigeration equipment and circuit conductors must be protected against overcurrent in accordance with 440.22. Typically, the branch-circuit conductor and protection size is marked on the equipment nameplate [440.4(A)].
    • Motor circuit conductors must be protected against short circuits and ground faults in accordance with 430.52 and 430.62 [430.51].
    • Motor control circuit conductors must be sized and protected in accordance with 430.72.
    • Remote-control, signaling, and power-limited circuit conductors must be protected against overcurrent according to 725.23 and 725.41.
    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: Disturbed by this mix and match arrangement.

    Code Q&A
    By Mike Holt
    Q. Is liquidtight flexible metal conduit (LFMC) permitted under the raised floor of a computer room? If yes, must it be secured?

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

    Code Quiz
    By Steven Owen
    Q. What is the minimum size NM cable required for an electric space-heating branch circuit installed 3 inches above a heated ceiling (with no thermal insulation) in a dwelling unit? The branch circuit will supply a calculated load of 12A.

    A) 14 AWG NM
    B) 12 AWG NM
    C) 10 AWG NM
    D) 12 AWG Type TC

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


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