Article 430-Motors, Motor Circuits and Controllers

Published on 7 October 2024 at 21:40

 

In the National Electrical Code (NEC), 430.52(B) states that a motor branch-circuit short-circuit and ground-fault protective device must be able to carry the motor's starting current. This is because the starting current for typical AC induction motors can be six to eight times the full-load current. 
 
 
Here are some other requirements for motor branch-circuit short-circuit and ground-fault protection: 
 
 
  • 430.52(C)
    The protective device's rating or setting must not exceed the value calculated using the percentages in Table 430.52. 
     
     
  • Exception 1
    If the value from the table doesn't match a standard overcurrent device rating, you can use the next higher standard rating. 
     
     
  • Exception 2
    If the motor won't start without tripping the OCPD, you can use greater variations from the percentages in the table. 
     
     
  • Torque motors
    If the motor is a torque motor, you must protect the circuit at the motor nameplate current rating. 
     

     

    (3) Instantaneous-Trip Circuit Breaker

     

    An instantaneous-trip circuit breaker shall be permitted if the conditions of 430.52(C)(3) (a) and (C)(3) (b) are met.

    1. Application. Instantaneous-trip circuit breakers shall be adjustable and part of a listed combination motor controller having coordinated motor overload and short-circuit and ground-fault protection in each conductor.
      Informational Note No. 1: Instantaneous-trip circuit breakers are also known as motor-circuit protectors (MCPs).
      Informational Note No. 2: For the purpose of this article, instantaneous-trip circuit breakers could include a damping means to accommodate a transient motor inrush current without nuisance tripping of the circuit breaker.
    2. Setting. The instantaneous-trip circuit breaker shall be adjusted to a setting in accordance with one of the following:
      1. No greater than the value specified in Table 430.52(C)(1)
      2. Where the value specified in Table 430.52(C)(1) is not sufficient for the starting current of the motor, one of the following settings shall be permitted:
        1. Motors other than design B energy-efficient and Design B premium efficiency motors shall be permitted to be increased but shall in no case exceed 1300 percent of the motor full-load current.
        2. Design B energy-efficient and Design B premium efficiency motors shall be permitted to be increased but shall in no case exceed 1700 percent of the motor full-load current.
        3. Where an engineering analysis determines the value is not sufficient for the starting current of the motor, it shall not be necessary to first apply the value specified in Table 430.52(C)(1).
          Informational Note No. 3: See NEMA MG 1-2016, Motors and Generators, Part 12.59 for additional information on the requirements for a motor to be classified "energy efficient."
      3. Where the motor full-load current is 8 amperes or less, the setting of the instantaneous-trip circuit breaker with a continuous current rating of 15 amperes or less in a listed combination motor controller that provides coordinated motor branch-circuit overload and short-circuit and ground-fault protection shall be permitted to be increased to the value marked on the motor controller.

     

 

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