Published In January 2001
"What is the full load current of that motor over there?” “Which current rating do you want? Each motor has three current ratings: nameplate, NEC Table, and locked rotor.” And so it is, each current rating needed to select the various elements of a motor branch circuit.The following information applies to about 97 percent of the motors you will encounter. The nameplate rating is used only in selecting the running overload protection. Overload devices protect the controller, the motor, and the branch circuit conductors against excessive heating due to motor overload or failure to start. Separate overload protection is not required for a continuous duty motor of 1 hp or less that is neither permanently installed nor started automatically, and is within sight of the controller location. Some motors are inherently protected by built-in thermal protectors; others are inherently protected by the impedance of the motor windings. In both cases, the nameplate will carry a marking indicating the type of protection. Most other continuous duty motors having a marked service factor of not less than 1.15 or, with a marked temperature rise not over 40 degrees Celsius, are required to have overload protection rated at 125 percent of the motor nameplate full-load current rating. Fuses can provide this protection, but heaters in the motor controller most commonly provide it. Most controllers will have a paper table pasted inside the cover, which indicates the proper heater for various motor nameplate full-load current ratings. In this case, the added 25 percent has been accounted for in the table, so no math is necessary in selecting the correct overload heaters. Overload protection is generally unsuitable for protection against short circuits or ground faults. Some overload devices will be accompanied by information regarding the largest fuse or circuit breaker permitted ahead of them in order to provide short-circuit and ground-fault protection. For other than overload protection, the table value of full-load running current is used, rather than the nameplate rating. In most cases the table rating (Table 430-148 for single-phase, Table 430-150 for three-phase) will be somewhat higher than the nameplate rating. This is because the branch circuit elements selected on the basis of the table values will probably accommodate a replacement motor of the same horsepower. Thus the wire size, the rating of the disconnecting means, and the rating of the branch-circuit short-circuit and ground-fault overcurrent device need not be changed when a replacement motor of the same horsepower is installed. A motor circuit disconnecting means shall have a current rating of 115 percent of the motor full-load current rating based on the table value. It may also be a listed motor circuit switch, either fused or non-fused, a listed molded case circuit breaker, a listed molded case switch, or, but only in a listed combination motor controller, an instantaneous trip circuit breaker or a motor circuit protector. Switches shall also be rated in horsepower not smaller than the motor horsepower. Due to the required tests performed on circuit breakers, they don’t have to be rated in horsepower. Wire size is required to be not smaller than 125 percent of the (Table) motor full-load running current. It is apparent, then, that the wire has adequate overload protection. The motor branch-circuit, short-circuit, and ground-fault protection must be large enough to allow the motor to start without opening the fuse or circuit breaker. Taken from Table 430-152 and depending on the type of overcurrent device used, the following percentages of full-load current based on the table amperes for the motor, are permitted: for a nontime delay fuse, 300 percent; for a dual element (Time Delay) fuse, 175 percent; for an instantaneous trip breaker, 800 percent; and for an inverse time breaker, 250 percent. If the values calculated using these percentages do not match the sizes of standard overcurrent devices, the next-higher standard size may be used. Note that the instantaneous trip breaker may only be used in a listed combination motor controller. The third rating, of locked rotor amperes, is taken care of mainly by product standards. In other words, if the disconnect and controller are horsepower rated, they will handle the inrush current each time the motor is started and will safely interrupt the circuit if the motor is stalled. The motor at standstill, before it starts to turn, is consuming locked rotor current. Tables 430-151(A) and 430-151(B) show the locked rotor currents for various horsepower motors. These tables are used when combining several motors on one disconnecting means, as required in Section 430-110, and for the selection of disconnecting means and controllers for hermetic refrigerant motor-compressors, Secs. 440-12 and 440-41. These are the three current ratings of a motor: nameplate, code table, and locked rotor. Although the wire is not protected against overcurrent as prescribed in Article 240, it is nevertheless protected, against overload by the heaters in the starter, and against ground faults and short circuits, by the motor branch-circuit short-circuit and ground-fault protection. SCHWAN is an electrical code consultant in Hayward, Calif. He can be reached at firstname.lastname@example.org.