Overcurrent Protection for Motor Installations, Part II

This month’s column will continue with questions and answers relating to overcurrent protection for conductors in motor and controller circuits and overload protection for motors. Requests to NECA’s “Code Question of the Day” online feature for answers about NEC requirements relating to motors and controllers indicate great subscriber interest in this subject. QUESTION: “Is a motor controller disconnecting means required to be within sight of the controller?” ANSWER: Section 430-102(a) requires, in installations of motor circuits under 600 volts, each motor controller to have an individual disconnecting means and it must be in sight from the motor controller. A recently added Exception No. 2 permits a single disconnecting means for a group of coordinated controllers that drive several parts of a machine or apparatus. The disconnecting means and controllers must be located in sight from the machine or apparatus. QUESTION: “How is the overcurrent protective device sized in the motor disconnecting means?” ANSWER: Overcurrent protection is not required in the motor disconnecting means. This means of disconnect is required solely for the protection of persons maintaining the motor or its driven machinery. This disconnecting means is not required if the controller disconnecting means is capable of being locked in the open position, Section 430-102(b). QUESTION: “How is the motor disconnecting means sized?” ANSWER: Section 430-110(a) requires the disconnecting means for motor circuits rated 600 volts or less to have an ampere rating of at least 115 percent of the motor full load current (FLC). There is a new exception permitting a listed, non-fused motor-circuit switch having a horsepower rating equal to or greater than the motor horsepower to have an ampere rating less than 115 percent of the full-load current rating of the motor. The substantiation for this new exception demonstrated that it is common for non-fused switches with horsepower ratings to have nameplate current ratings less than 115 percent of the motor full-load current. The standards for listing these switches are such that the 115 percent sizing requirement is taken care of by the manufacturer. This means it is not necessary for the electrical contractor to oversize a non-fused disconnect. Instead, he can select one whose nameplate horsepower rating matches the motor. QUESTION: “Is the motor disconnecting means required to be within sight of the motor?” ANSWER: If a motor disconnecting means is required, then it must be within sight from the motor and the driven machinery location. A motor disconnecting means is not required if the disconnecting means for the controller is individually capable of being locked in the open position. QUESTION: “Can I use a single-pole toggle switch in the coil circuit of a motor controller as the motor disconnecting means?” ANSWER: No. The intent of the requirements for a motor disconnecting means is to open all motor circuit conductors through the motor switch. Using a switch in the manner you are describing is using a coil circuit switch to de-energize a coil circuit. This would defeat any lock-out/tag-out procedure. The controller could be manually started, resulting in injury to persons working on the motor or its driven machinery. QUESTION: “Explain how, when using Column “C” in Table 430-72(b), a 60-ampere overcurrent device can protect a No. 12 copper control circuit conductor?” ANSWER: The motor control circuit conductors are protected from physical damage by the raceway in which they are enclosed. The coil circuit current is small and overload is not a problem. Short-circuit and ground-fault currents rise rapidly and will open the motor branch circuit overcurrent protective devices before any damage could occur. QUESTION: “Are motor control circuits feeding remote control devices required to have overcurrent protection?” ANSWER: Overcurrent protection for motor control circuits is covered in Section 430-72(b). The requirements for conductors that extend beyond the motor controller enclosure can be found in Table 430-72(b) in Column “C.” For example, if your motor branch circuit overcurrent protective device is rated at 60 amperes and you are using copper control circuit conductors, then you find 60 in the copper column and move to the left to control circuit conductor size where you find 12. This means that you need to install control circuit conductors not smaller than No. 12 copper. Using a smaller conductor would require supplemental overcurrent protection to protect those conductors. QUESTION: “If I tap a feeder supplying a group of motors, how far can I run that tap without overcurrent protection for that tap?” ANSWER: If you are tapping to a single motor and are not reducing conductor size, overcurrent is not a problem. If you are reducing the conductor size, Section 430-53(d)(2) requires the tap conductors to have protection from physical damage. These tap conductors are not to be more than 25 feet in length, and must have an ampacity of not less than one-third that of the feeder conductor with a minimum size in accordance with Section 430-22. This means the tap conductors must be sized at 125 percent of the motor full load current rating, and may not be less than one-third the ampacity of the feeder conductor. TROUT is a technical consultant for Maron Electric Company of Chicago, and chairman of the National Electrical Code-Making Panel 12, representing NECA. He is also the principal author of Electrical Contractor magazine’s “Code Question of the Day.”

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