Motors, HVAC & Appliance Circuits
Motor FLC, branch circuit conductor sizing at 125%, overcurrent protection at 250%, A/C equipment rules, fixed appliances, and heating equipment.
- Size motor branch circuit conductors at 125% of full-load current
- Calculate motor overcurrent protection using the 250% inverse time breaker rule
- Apply NEC Article 440 requirements for A/C equipment circuits
- Determine when a fixed appliance requires an individual branch circuit
- Size circuits for fixed electric space heating at 125%
Leçon 1
Motor Branch Circuit Sizing - 125% FLC & 250% OCPD
Motor Circuit Fundamentals
Motor circuits require special sizing rules because motors draw significantly more current during starting than during normal operation. A motor's inrush current (starting current) can be 6 to 8 times its full-load current. Standard overcurrent protection rules would cause nuisance tripping every time the motor starts.
Motor Full-Load Current (FLC)
Motor circuit sizing is based on the full-load current (FLC) from NEC Tables 430.247 through 430.250 - NOT from the motor nameplate. The NEC tables represent standardized values that account for motor design variations. Using the NEC table values rather than nameplate ratings provides consistent sizing across all motors of a given horsepower and voltage.
Conductor Sizing - 125% of FLC
Per NEC 430.22, motor branch circuit conductors must have an ampacity of at least 125% of the motor full-load current.
Example: A motor has an FLC of 10 amps.
- Minimum conductor ampacity: 10 x 1.25 = 12.5 amps
- Use #14 AWG minimum (15-amp ampacity at 60 degrees C)
Overcurrent Protection - 250% for Inverse Time Breakers
Per NEC 430.52, the maximum overcurrent protection for a motor circuit depends on the type of protection device:
| Protection Device | Maximum Size |
|---|---|
| Inverse time circuit breaker | 250% of FLC |
| Dual-element time-delay fuse | 175% of FLC |
| Instantaneous trip breaker | 800-1300% of FLC |
| Non-time-delay fuse | 300% of FLC |
Example: A motor with 10-amp FLC using an inverse time breaker:
- Maximum breaker: 10 x 2.50 = 25 amps
- Select a 25-amp breaker
Exam Tip - Next Standard Size
If the calculated overcurrent protection does not correspond to a standard breaker size, NEC 430.52(C)(1) permits rounding up to the next standard size. For example, if the calculation yields 22 amps, you can use a 25-amp breaker.
Motor branch circuit conductors must be sized at 125% of FLC per NEC 430.22. The maximum inverse time circuit breaker is 250% of FLC per NEC 430.52. Always use FLC from the NEC tables, not the motor nameplate.