The other day, A manufacturer inquired about switching the neutral in a branch circuit and whether the 2011 National Electrical Code (NEC) would permit a switched neutral for any application within the Code. My first reaction was no, and then I thought about the differences between neutrals and grounded conductors. I also had thoughts about overload protection in motor circuits and control circuits for motors as well as other applications where a switched neutral or grounded conductor could be used. After mulling all the possibilities, I decided to research this subject and provide as thorough an answer to this question as possible.
A starting point is the definition of a “grounded conductor” as a system or circuit conductor that is intentionally grounded. That definition certainly applies to a neutral and applies to a grounded conductor in a three-phase delta system where one of the phase conductors is intentionally grounded for stabilizing the voltage. A “neutral conductor” is defined as the conductor connected to the neutral point of a system that is intended to carry current under normal conditions. Since the definition of a neutral conductor also contains the phrase “neutral point,” the definition of a neutral point is necessary to help determine a neutral conductor. The “neutral point” is defined as the common point on a wye-connection in a polyphase (three-phase) system; the midpoint on a single--phase, 3-wire system; the midpoint of a single-phase portion of a three-phase delta system; or the midpoint of a 3-wire, direct-current system.
There also is an informational note that Panel 5 added to help explain the neutral point: “At the neutral point of the system, the vectorial sum of the nominal voltages from all other phases within the system that utilize the neutral, with respect to the neutral point, is zero potential.” The phrase “neutral point” and the word “neutral” were added to Article 100 and used throughout in the 2008 NEC.
Now that we have the basics for grounded conductors and neutral conductors, we must look within the NEC to find coverage of both of these phrases for permission or restriction of switching action for these conductors. Section-200.6 requires grounded conductors, of which neutrals are a part, to be identified using white, gray or colored conductors with three white stripes. Where a white, gray or colored-with-three-white-stripes conductor is used for another purpose other than as a neutral, 200.7(C)(1) requires the conductors to be re-identified. If used for single-pole, three-way or four-way switch loops, the re-identified conductor (not used as a neutral) shall be used only for the supply to the switch but not as a return conductor from the switch to the outlet.
Section 240.22 does not permit an overcurrent device to be connected in series with any conductor that is intentionally grounded, unless the overcurrent device opens all the conductors of the circuit, including the grounded conductor, and is designed so that no pole can be operated independently. An overcurrent device can be installed in a grounded conductor where required or permitted in accordance with Section 430.36, where fuses are used for motor overload protection, or in accordance with Section 430.37, where overload relays and trip coils are installed for overload protection.
Section 404.2(A) covers switch connections for three- and four-way switches. It specifically states that these combinations shall be wired so the switching is accomplished only in the ungrounded circuit conductors. Grounded conductors are specifically covered in 404.2(B), which states switches or circuit breakers shall not disconnect the grounded conductor of a circuit. In this case, there is an exception in 404.2(B) that states: A switch or circuit breaker shall be permitted to disconnect a grounded circuit conductor where all circuit conductors are disconnected simultaneously or where the device is arranged so that the grounded conductor cannot be disconnected until all the ungrounded conductors of the circuit have been disconnected.
There are two other locations in the NEC with similar text to the exception noted above. Section 430.85 states, “one pole of the controller shall be permitted to be placed in a permanently grounded conductor, provided the controller is designed so that the pole in the grounded conductor cannot be opened without simultaneously opening all conductors of the circuit.”
Similar wording is used in 430.105: “One pole of the disconnecting means shall be permitted to disconnect a permanently grounded conductor, provided the disconnecting means is designed so that the pole in the grounded conductor cannot be opened without simultaneously disconnecting all conductors of the circuit.” Grounded and neutral conductors generally must not be switched; doing so can be dangerous.
ODE is a staff engineering associate at Underwriters Laboratories Inc., based in Peoria, Ariz. He can be reached at 919.949.2576 and firstname.lastname@example.org.