There are many misconceptions in the field when dealing with motor circuits. Can the overcurrent protection device in the motor circuit be sized significantly larger than what is normally used to protect the circuit conductors?
Section 240-3 of the National Electrical Code (NEC) requires conductors, other than flexible cords and fixture wires, to be protected against overcurrent in accordance with their ampacities as specified in Section 310-15.
Section 240-3(g) also permits specialty circuits to be sized and protected based upon individual specific Articles in the NEC. One such article is Article 430 for motor circuits.
Motor circuits can be protected in accordance with Parts C, D, E, F, and H of Article 430. Part C of Article 430 provides information on motor and branch-circuit overload protection. Part D covers motor branch-circuit short-circuit and ground-fault protection and Part E covers motor short-circuit and ground-fault protection for feeders. Part F covers overcurrent protection for motor control circuits and Part H covers overcurrent protection for motor control centers.
Section 430-6(a)(1) requires the use of Tables 430-147 through 430-150, based upon the motor horsepower rating, for determining the ampacity of circuit conductors and for sizing any switches or branch-circuit short-circuit and ground-fault devices for the circuit. The current rating marked on the motor nameplate is only used for sizing of overloads for the circuit and if the motor nameplate ampacity is larger than the table values given.
Use of the term “full load current (FLC) rating” indicates the table rating while use of the term “full load ampere (FLA) rating” indicates the actual nameplate rating. This makes it easier to clarify whether the table ampacity or the nameplate ampacity is being used.
Because motors are required to have overload protection, either within the motor itself or somewhere in close proximity to the line side of the motor, this overload protection is actually protecting the motor, the conductors, and much of the circuit ahead of the overloads. An overload in the circuit would trip the circuit overload devices, thus protecting the circuit from overload conditions.
Overload protection of the motor circuit permits the overcurrent protection device of the circuit to function solely as a short circuit or a ground-fault device hence the term “branch-circuit short-circuit and ground-fault device.” The primary function of the short-circuit and ground-fault device is to react during a short or a ground fault in the circuit.
If a single motor is being installed for a continuous duty application, Section 430-22 requires the branch-circuit conductors to have an ampacity of not less than 125 percent of the motor’s full load current rating based upon Tables 430-147 through 430-150.
For example, a three-phase 40 horsepower motor operating at 480 volts would have an FLC of 52 amperes. Section 430-22 requires the conductors supplying the motor to be sized at least 125 percent of the FLC. This calculation would be 125 percent of 52 amperes = 65 amperes.
Section 310-15 and accompanying Table 310-16 provides the allowable ampacities for conductors. Using a 60 degrees/75 degrees or 75 degrees Celsius inverse time circuit breaker to feed the 40-horsepower motor with 75 degrees Celsius terminations at the motor, the 75 degrees Celsius copper column of Table 310-16 can be used for sizing the conductors for the motor. A No. 6 AWG conductor with at least 75 degrees Celsius insulation will safely carry 65 amperes, based upon the table, and would be the minimum-size conductor used for that motor circuit.
Section 430-109 provides a list of acceptable types of disconnecting means for motor circuits. The motor disconnecting means can be a Listed motor circuit switch, a Listed molded case circuit breaker, or an instantaneous trip circuit breaker, among other disconnects.
Section 430-110(a) requires the disconnecting means for 600-volt-or-less systems tabulated in Section 430-109 to have an ampere rating of at least 115 percent of the FLC of the motor. Using the 40-horsepower motor, this calculation would be 52 amperes times 115 percent = 59.8 amps or at least a 60-ampere disconnect device. This sizing at 115 percent permits the disconnecting means to open or close the circuit with little or no damage to the disconnecting means.
Finally, the nameplate full load ampere (FLA) rating of the motor is used for sizing the overloads for the motor. Part C of Article 430 provides this information. If the 40 horsepower motor has a marked service factor of not less than 1.15 or is marked with a temperature rise of not over 40 degrees C, or both, an overload device would be sized at 125 percent of 52 amperes or 65 amperes. This overload device protects the circuit upstream from its location from any overload that exceeds the 125 percent value.
ODE is a staff engineering associate at Underwriters Laboratories, Inc., in Research Triangle Park, N.C. He can be reached at (919) 549-1726 or by e-mail at email@example.com.