A three-phase, 480V motor branch circuit is being installed. The disconnecting means for the motor must be capable of opening the circuit to the motor and its controller. According to NEC 430.109(A)(1), what is the minimum horsepower rating required for the disconnecting means that is a general-use switch?
- AIt must have an ampere rating not less than 115% of the motor's full-load current.
- BIt can be an ampere-rated general-use switch if the circuit has an instantaneous trip circuit breaker.
- CIt must be rated in horsepower at least 150% of the motor horsepower.
- DIt must be rated in horsepower equal to or greater than the motor horsepower.
Show answer & explanationAnswer & explanation
Correct answer: D. It must be rated in horsepower equal to or greater than the motor horsepower.
NEC 430.109(A)(1) states that a disconnecting means for a motor circuit, if it is a horsepower-rated switch, must have a horsepower rating not less than the motor horsepower. Other types of switches, such as general-use switches, can be used if they also meet specific ampere or horsepower requirements. However, the direct question asks about a general-use switch *rated in horsepower*. If a general-use switch is *rated in horsepower*, it must be equal to or greater than the motor horsepower. If it's *not* horsepower rated, then other rules apply (like having an ampere rating of at least 115% of FLA and being suitable for the current it must interrupt). The question specifically asks about a general-use switch with a horsepower rating.
Why the other options are wrong
- A. This rule applies to ampere-rated general-use switches or circuit breakers used as disconnecting means, not necessarily horsepower-rated switches.
- B. While an ampere-rated general-use switch can be used, the question specifically asks about a general-use switch *rated in horsepower* and its minimum horsepower rating.
- C. This is incorrect; 150% is not a standard requirement for horsepower-rated disconnects.
Motor Disconnecting Means Sizing (NEC 430.109)
NEC 430.109 specifies the requirements for the rating and type of disconnecting means used for motor and motor controller circuits, ensuring safe and effective isolation of power.
- Disconnecting means must be horsepower-rated for the motor, or meet specific ampere ratings for non-horsepower-rated devices.
- It must be capable of interrupting the locked-rotor current of the motor.
- Different rules apply for general-use switches, isolating switches, and circuit breakers.
Memory trick: Disconnects: Horsepower matches or Amps exceed FLA, always be ready to stop the show.