A magnetic across-the-line starter is being selected for a 25-horsepower, 460-volt motor. Under the general rule, how must that controller be rated?
- Its horsepower rating at the applied voltage shall be at least the horsepower rating of the motor
- BIts ampere rating shall be at least 250 percent of the motor full-load current
That percentage belongs to the branch-circuit short-circuit and ground-fault protective device tables, not to controller selection. Controllers are chosen on horsepower at the applied voltage.
- CIts horsepower rating shall correspond to the locked-rotor code letter marked on the motor nameplate
The code letter describes starting inrush and is used to look up locked-rotor current. It is not a horsepower rating and cannot be used to select a controller.
- DIts ampere rating shall equal the ampere rating of the branch-circuit short-circuit protective device
The protective device is sized well above the motor current to allow starting inrush. Matching a controller to it would badly oversize the contactor and its overload block.
Why A is correct
Controllers are selected on horsepower at the applied voltage, and that rating must not be lower than the motor horsepower. Rating by horsepower rather than amperes builds the switching duty and the inrush withstand into the selection, which a plain ampere rating would not capture.
What this question is testing
This tests whether you know the unit a controller is rated in and that the rating is voltage-specific. It also checks that you do not carry motor protection percentages over into controller selection.
On the job
Undersized starters are one of the more common causes of repeat motor troubles on process equipment. A contactor asked to break more than its rated horsepower welds its tips or erodes them until the motor single-phases, and the symptom that reaches the electrician is a hot motor and a tripping overload, which sends people looking at the motor rather than at the starter. When replacing a failed starter, always check the horsepower rating at the actual system voltage rather than assuming the physical size tells you the rating.
Memory technique
Controllers speak horsepower, at a stated voltage.
Exam tip
Motor questions each have their own basis: horsepower for controllers, table full-load current for conductors, nameplate current for overloads.
Where to look it up
Article 430 Part VII, motor controllers. The horsepower ratings themselves come from the motor full-load current tables at the end of the article.