4.1 Motor Conductor & Breaker Sizing
Calculate conductor ampacity from NEC tables (125% FLC) and determine maximum inverse-time circuit breaker ratings under Table 430.52.
Key Blueprint Takeaways
- Motor branch-circuit conductors must be sized at 125% of NEC Table Full-Load Current (FLC).
- Inverse-time circuit breakers default to 250% of motor FLC for standard polyphase squirrel-cage motors.
- Motor overload devices protect against sustained mechanical overloads, whereas circuit breakers protect against short circuits and ground faults.
Motors draw significant starting inrush currents (up to 6 times full-load current). Because of this inrush, motor circuits separate short-circuit protection from running overload protection.
Branch conductors must be sized at 125% of Table FLC under 430.22, while the branch circuit overcurrent device is sized substantially larger under Table 430.52 to prevent nuisance tripping during motor acceleration.
Knowledge Checkpoint • Section 4.1
When sizing branch-circuit conductors supplying a single continuous-duty motor, what percentage of the motor full-load current (FLC) from NEC Tables 430.247–430.250 must be used under NEC 430.22?