A disconnect for a motor branch circuit must be capable of interrupting the locked rotor current of the motor. This disconnecting means must disconnect both the motor and the controller from all ungrounded supply conductors, NEC Section 430 103. The disconnecting means must be located within sight from the motor controller, NEC Section 430 102. The definition of” in sigh from” in NEC Article 100 means that the controller i: not more than 50 ft (15. 24 m) from the disconnect, an’ that the controller is actually visible when standing at tht disconnect location
A fusible switch serving as the motor circuit discon necting means must be rated in horsepower, NEC Sec tion 430 109. A circuit breaker serving as a disconnect ing means is not required to be rated in horsepower but instead will be rated in amperes, NEC Section 430 109. This circuit breaker must have an ampere rating not less than 115% of the motor full load current, NEC Section 430 110(a)
There are exceptions to these rules for motor disconnects. Motors with a horsepower rating not greater than 1/s hp may use the branch circuit overcurrent device as the disconnect, NEC Section 430 109, Exception No. 1. If the motor is not larger than 2 hp, and operates at not more than 300 V, a general use switch may serve as the disconnect, NEC Section 430 109, Exception No.2. The switch must have an ampere rating at least twice that of the full load current of the motor. An ac switch used on an ac circuit need only be rated at 1. 25 times the fullload current of the motor
Cord and plug connected motors may use an attachment plug and receptacle as the motor disconnect. This type of disconnect would be used for motors on portable or movable equipment used on the farm. The plug and receptacle should have a horsepower rating not smaller than the rating of the motor if it is intended to break power while operating, NEC Section 430 109, Exception No. 5. The current rating of the attachment plug and receptacle must be not less than 115% of the full load current of the motor
A fusible switch or a circuit breaker is permitted to serve as both the controiler and disconnect, NEC Section 430 111. However, the requirements for motor controllers discussed previously must be met
Assume that the motor of Figure 17 13 will have a fusible switch as the disconnect. The disconnect switch must have a rating marked on the switch of at least 71/2 hp. However, if a circuit breaker serves as the disconnect, the rating will be in amperes. The minimum ampere rating of the circuit breaker is 1. 15 times the full load current, or 25 A. A 25 A or 30 A circuit breaker would be chosen. A circuit breaker rated at 30 A or more would most likely be chosen, because a 25 A breaker would probably trip due to the high starting current. The value of current from NEC Table 430 150 is used, rather than the nameplate current of 19.2 A.
1. 15 X 22 A = 25 A
BRANCH-CIRCUIT WIRES
The wires supplying a single motor shall have an ampere rating not less than 1.25 times the full load current of the motor. For a multispeed motor, the highest normal full load current shall be used, NEC Section 430 22(a).
The minimum ampere rating of the circuit wires for the motor of Figure 17 13 is 28 A
1 .25 X 22 A = 28 A
The minimum size copper branch circuit wire is found in NEC Table 310 16 (included in Unit 9 of this text). If the wire is THWN copper, the minimum size is No. 10 AWG. The footnote at the bottom of NEC Table 310 16 which specifies the maximum rating overcurrent device for wire sizes No. 14, 12, and 10 AWG does not apply in the case of electric motor circuits, NEC Section 210 19(a). Aluminum wire is seldom used for motor circuits. Most terminals in motor starters are rate only for copper wire.
Problem 17-1
A single phase, 5 hp, 230 V electric motor has a nameplate current rating of 26 A. Determine the minimum size copper THWN branch circuit wire
Solution
Look up the full load current of a 5 hp single phase, 230 V motor in NEC Table 430 148. The current value is 28 A. The minimum ampere rating of the wire is 35 A
1.25 X 28 A = 35 A