NEC Motor Calculations Overview — Flashcards

NEC motor calculations, primarily covered in Article 430, establish guidelines for safely sizing motor branch-circuit conductors, overcurrent protection (short-circuit/ground-fault), and motor overloads. Key steps include determining Full Load Current (FLC) from NEC tables, sizing conductors at 125% of FLC, and setting overload devices. 

Key Aspects of NEC Motor Calculations (Article 430):
Determine FLC (Full Load Current): Use NEC tables 430.247–430.250 based on horsepower, voltage, and phase to find FLC, rather than using the nameplate rating for circuit sizing.
Branch Circuit Conductors (NEC 430.22): Conductors must be sized to have an ampacity of at least 125% of the motor's full-load current (FLC).
Overload Protection (NEC 430.32): Designed to protect the motor, control apparatus, and conductors from excessive heating.
Typically set at 125% of the motor's nameplate FLA for motors with a service factor (SF) of greater than or equal to 1.15 or temperature rise less than or equal to 40°C.
Set at 115% for other motors.
Short-Circuit & Ground-Fault Protection (NEC 430.52): Protects against high-level faults. Typically calculated using table percentages of FLC (e.g., 250% for inverse time breakers, 300% for time-delay fuses).
Feeder Calculations (NEC 430.24): Feeders supplying multiple motors are sized at 125% of the highest rated motor FLC, plus 100% of the FLC of all other motors in the group.
Voltage Drop: The NEC recommends a maximum of 5% voltage drop from the source to the motor. 

For accurate calculation, always verify the motor's service factor, speed, and voltage from its nameplate.