text: Q:
  
  NEC Breaker Selection for Motor Loads
  
  In the case of motor loads connected to a long branch circuit, why does NEC Table 430.52 limit selection of an over current protection device (OCPD) to a smaller size than the branch conductor will permit?
  My hypothesis is a branch conductor that is made larger to compensate for voltage drop cannot supply a larger motor load where voltage drop was the constraining factor.  Therefore, the OCPD is limited in size to add protection against motor damage from an unacceptable voltage drop.
  This flies in the face of common reasoning that says branch OCPD are for protecting the branch conductors and are not for motor protection.  Separate motor overload protection is installed for that purpose.
  What gives?  Is the NEC actually providing motor protection against unacceptable branch voltage drop during motor startup by prescribing these size limits on circuit breakers?
  I would appreciate others' thoughts on this.
  
  A:
  
  Here is an excerpt from the code:
  
  430.51 General. Part IV specifies devices intended to protect the motor branch-circuit conductors, the motor control apparatus, and the motors against overcurrent due to short circuits or ground faults. These rules add to or amend the provisions of Article 240. The devices specified in Part IV do not include the types of devices required by 210.8, 230.95, and 590.6.
  
  Referring to that other problem we were nattering over; if you have 1200 feet of #4 at 240 volts during a dead short at the end of the line I calculated 623 amps of current. If it was a ground fault close to the motor and the current flowed through the motor this could be quite damaging. I don't know if a 30 amp breaker can trip much faster than a 70 but I believe the intent of the code, as is stated in 430.51, is to protect the motor and conductors from damage resulting from short circuits and ground faults regardless of what size conductor is used.
  So, the code limits the size of the OCPD based on the Table FLA of the motor not the ampacity of the conductors. 
  IMHO
  Happy Friday!
  
  