Based on the 2017 NEC.
Q. Where is the motor current found for use in sizing the various components of a motor circuit?
See answer below.
A. (A) General Requirements [Sec. 430.6(A)]
(1) Table Full-Load Current (FLC)
The motor full-load current ratings listed in Tables 430.247, 430.248, and 430.250 are used to determine the conductor ampacity [Sec. 430.22], the branch-circuit short-circuit and ground-fault overcurrent protection device size [Sec. 430.52 and Sec. 430.62], and the ampere rating of disconnecting switches [Sec. 430.110].
The actual current rating on the motor nameplate full-load amperes (FLA) [Sec. 430.6(A)(2)] isn’t permitted to be used when determining the conductor ampacity, the branch-circuit short-circuit and ground-fault overcurrent protection device size, nor the ampere rating of disconnecting switches.
Motors built to operate at less than 1,200 RPM, or that have high torques, may have higher full-load currents, and multispeed motors have full-load current varying with speed, in which case the nameplate current ratings must be used [Sec. 430.6(A)(1)].
Exception No. 3: For a listed motor-operated appliance, the actual current marked on the nameplate of the appliance must be used instead of the horsepower rating on the appliance nameplate to determine the ampacity or rating of the disconnect, the branch-circuit conductors, the controller, and the branch-circuit short-circuit and ground-fault protection.
(2) Motor Nameplate Current Rating (FLA)
Overload devices must be sized based on the motor nameplate current rating in accordance with Sec. 430.31.
The motor nameplate full-load ampere rating is identified as full-load amperes (FLA). The FLA rating is the current in amperes the motor draws while producing its rated horsepower load at its rated voltage, based on its rated efficiency and power factor.
The actual current drawn by the motor depends upon the load on the motor and on the actual operating voltage at the motor terminals. That is, if the load increases, the current also increases, or if the motor operates at a voltage below its nameplate rating, the operating current will increase.
CAUTION: To prevent damage to motor windings from excessive heat (caused by excessive current), never load a motor above its horsepower rating, and be sure the voltage source matches the motor’s voltage rating.
These materials are provided to us by Mike Holt Enterprises in Leesburg, Fla. To view Code training materials offered by this company, visit www.mikeholt.com/code.
About the Author
Mike Holt
Mike Holt is the owner of Mike Holt Enterprises (www.MikeHolt.com), one of the largest electrical publishers in the United States. He earned a master's degree in the Business Administration Program (MBA) from the University of Miami. He earned his reputation as a National Electrical Code (NEC) expert by working his way up through the electrical trade. Formally a construction editor for two different trade publications, Mike started his career as an apprentice electrician and eventually became a master electrician, an electrical inspector, a contractor, and an educator. Mike has taught more than 1,000 classes on 30 different electrical-related subjects — ranging from alarm installations to exam preparation and voltage drop calculations. He continues to produce seminars, videos, books, and online training for the trade as well as contribute monthly Code content to EC&M magazine.