Motors

All free sections·Study notes·20-question practice test

1. 1 · Motors · NEC Knowledge

   A motor branch circuit has both an overload device and a short-circuit and ground-fault device. Which statement matches their different jobs?

   1. A. They are the same device under two names.
   2. B. Overload protection watches running current that stays too high. Short-circuit protection opens a fault.Keyed answer
   3. C. The overload device is only there to start the motor.
   4. D. Short-circuit protection is chosen from nameplate horsepower and ignores fault current.

   Motors draw more than their running current while starting, so the branch-circuit short-circuit device is allowed to sit above the running current. A separate overload device watches current that stays too high after the start. Both ideas are Article 430. Confirm the sections in NEC 2026.

   Open NEC 2026 Article 430

2. 2 · Motors · NEC Knowledge

   A general-use snap switch rated 15 A is proposed as the controller for a 5 hp motor. The helper says any switch that can start and stop the motor is a controller. What has to be true of that switch?

   1. A. It has to be a type the motor article allows as a controller, with a horsepower rating or a stated exception that fits this motor. A lighting snap switch is not assumed to be that device.Keyed answer
   2. B. Any 15 A switch is a motor controller up to 10 hp.
   3. C. Controllers are required only for motors over 100 hp.
   4. D. The overload relay replaces the controller.

   Article 430 requires the controller to be suitable for the motor, including the horsepower rules for a switch used as a controller. A general-use snap switch has a limited permission, and 5 hp is not a guess you make from the ampere rating alone. Confirm the controller section in NEC 2026.

   Open NEC 2026 Article 430

3. 3 · Motors · NEC Knowledge

   The motor controller is out of sight from the motor, so a second disconnect is installed within sight of the motor. What is that second disconnect for, in plain language?

   1. A. To replace the overload relay
   2. B. To raise the starting current
   3. C. To bond the motor frame to the neutral
   4. D. To let a person at the motor shut it off and keep it off while working on itKeyed answer

   The within-sight disconnect isolates the motor where the work is happening. It does not take the place of the overload device, and it is not a bonding jumper. Article 430 covers the controller disconnect and the additional disconnect within sight of the motor. Confirm any exception in NEC 2026.

   Open NEC 2026 Article 430

4. 4 · Motors · NEC Knowledge

   A motor problem gives you both a table full-load current and a nameplate current. Which statement matches how those two numbers are used?

   1. A. Both devices are sized from the nameplate, and the table is only for transformers.
   2. B. Branch-circuit conductors are sized from the table full-load current. The overload device is sized from the nameplate.Keyed answer
   3. C. The overload device is sized from the grounding electrode resistance.
   4. D. The table full-load current is used only for the disconnect height above the floor.

   Article 430 sends the branch-circuit conductors and the short-circuit device to the table full-load current. The overload device watches the nameplate. Mixing those two numbers is a common miss. Confirm the split in NEC 2026 Article 430.

   Open NEC 2026 Article 430

5. 5 · Motors · NEC Knowledge

   The equipment is a hermetic refrigerant motor-compressor. The helper sizes the branch circuit from the open-motor horsepower table and ignores the nameplate rated-load amperes. Which current do you start from?

   1. A. The compressor nameplate rated-load current, under the refrigeration article. The open-motor horsepower table is the wrong starting point for a hermetic unit.Keyed answer
   2. B. Always the locked-rotor amperes, and no other current.
   3. C. The efficiency number divided by 10.
   4. D. Hermetic compressors are not motors and have no conductor rules.

   Article 440 sizes the branch circuit of hermetic refrigerant motor-compressors from the nameplate rated-load current, not from the Article 430 horsepower table for an ordinary open motor. Confirm the nameplate rules in NEC 2026 before you open the motor table.

   Open NEC 2026 Article 440

6. 6 · Motors · NEC Knowledge

   You are choosing the branch short-circuit device for a motor. The table full-load current and the nameplate current are different. Which current does the percentage get applied to?

   1. A. The nameplate current is always larger, so it is the safer number for the breaker.
   2. B. The short-circuit device percentage is applied to the table full-load current, not to a nameplate you were not told to use.Keyed answer
   3. C. The breaker is sized at 100% of the nameplate so the motor cannot start.
   4. D. The table is used only for the overload heater.

   The branch-circuit short-circuit and ground-fault device is based on the table full-load current. The nameplate is the overload number. Article 430 keeps those jobs apart. Confirm the section in NEC 2026.

   Open NEC 2026 Article 430

7. 7 · Motors · NEC Knowledge

   A 5 hp motor controller is installed on a 10 hp motor. What is wrong with that controller?

   1. A. The controller must be rated in kilovolt-amperes only, never horsepower.
   2. B. The controller is fine because any starter can run any motor.
   3. C. A larger motor makes the overload device unnecessary.
   4. D. The controller horsepower is below the motor horsepower.Keyed answer

   The controller has to be suitable for the motor it starts, including horsepower. A 5 hp controller on a 10 hp motor is below the motor. Article 430 covers controller ratings. Confirm the section in NEC 2026.

   Open NEC 2026 Article 430

8. 8 · Motors · NEC Knowledge

   A motor is supplied through an adjustable-speed drive. The helper sizes the load-side conductors from the drive's input breaker alone and says there is no further rule. What do you read before you accept that?

   1. A. The motor article has rules for the conductors on the load side of an adjustable-speed drive, including ampacity. The input breaker does not finish that run by itself.Keyed answer
   2. B. Output conductors of a drive are always 14 AWG.
   3. C. Adjustable-speed drives are covered only as luminaires.
   4. D. The load side of a drive is utility wiring and leaves the NEC.

   Article 430 includes the conductors between an adjustable-speed drive and the motor. Those conductors are not sized by copying the input breaker with no further look. Confirm the power-conversion part of the article in NEC 2026.

   Open NEC 2026 Article 430

9. 9 · Motors · NEC Knowledge

   A motor disconnect problem mentions locked-rotor current. In plain language, what is that current?

   1. A. Locked-rotor current is always lower than full-load current.
   2. B. Locked-rotor current is the grounding electrode current.
   3. C. Locked-rotor current is another name for the overload setting.
   4. D. Locked-rotor current is the high current with the shaft not turning, and some disconnect ratings are tied to it.Keyed answer

   Locked-rotor current is the current the motor draws when the shaft is not turning, and it is much higher than running current. Article 430 uses it for some disconnect horsepower relationships. Confirm the term and the table in NEC 2026.

   Open NEC 2026 Article 430

10. 10 · Motors · NEC Knowledge

    A motor nameplate shows a service factor. Where do you read whether that marking changes the overload setting?

    1. A. Article 430. A marked service factor can change the overload percentage you are allowed to use.Keyed answer
    2. B. Service factor is the voltage drop of the feeder, and it does not affect overload sizing.
    3. C. Service factor is read from the grounding electrode resistance.
    4. D. A service factor always lets you omit overload protection.

    Article 430 uses the service factor and the temperature rise, when they are marked, to pick the overload percentage. The percentage is in the book, not in this item. Confirm the overload section in NEC 2026 before you set the heater.

    Open NEC 2026 Article 430

11. 11 · Motors · NEC Knowledge

    Several motors are on one feeder. You already sized each branch short-circuit device. Where do you read the protection for the feeder itself?

    1. A. A group of motors is protected only by the grounding electrode.
    2. B. Feeder protection for motors is Article 500.
    3. C. Article 430 includes feeder short-circuit and ground-fault protection for a group of motors.Keyed answer
    4. D. The largest branch breaker automatically protects the feeder, with no feeder rule.

    Article 430 has a feeder rule for the short-circuit and ground-fault device ahead of a group of motors. Sizing the branch devices does not finish that step. Confirm the feeder protection section in NEC 2026.

    Open NEC 2026 Article 430

12. 12 · Motors · NEC Knowledge

    The nameplate full-load current is lower than the table value for this horsepower. The helper sizes the branch-circuit conductors from the nameplate because he says the motor in front of him is the fact. Which current sizes the conductors?

    1. A. The nameplate current, because a table is only for motors with no nameplate.
    2. B. Locked-rotor current, because the conductors must start the motor.
    3. C. The table full-load current, at 125 percent. The lower nameplate does not shrink the branch-circuit conductors.Keyed answer
    4. D. The breaker handle, because the wire ampacity is whatever the breaker says.

    Article 430 sizes motor branch-circuit conductors from the table full-load current and then applies 125 percent. The nameplate is the overload number, not the wire number. Confirm the table rule in NEC 2026.

    Open NEC 2026 Article 430

13. 13 · Motors · NEC Knowledge

    The inverse-time breaker is held at the conductor ampacity, and it trips when the motor starts. The helper says a breaker larger than the wire is always illegal, so he will not use the motor breaker rule. What is that breaker allowed to be?

    1. A. Larger than the conductor ampacity, because it is the short-circuit and ground-fault device and Article 430 selects it to let the motor start. The wire stays at 125 percent of table full-load current.Keyed answer
    2. B. No larger than the conductor ampacity, because motor breakers are sized like lighting breakers.
    3. C. Equal to locked-rotor current, which is also the required conductor size.
    4. D. Omitted, because the overload relay is the only protective device a motor gets.

    Article 430 separates the inverse-time breaker from the conductor size. The breaker is short-circuit and ground-fault protection and may exceed the conductor ampacity. Confirm the device selection in NEC 2026. Do not invent a percentage the breaker marking does not show.

    Open NEC 2026 Article 430

14. 14 · Motors · NEC Knowledge

    Three motors share a feeder, and the helper puts 125 percent on every motor's table current. He says each motor can be the largest one at some point in the day. Which current actually gets the 125 percent?

    1. A. Every motor, because each one is largest while it is starting.
    2. B. The smallest motor, because it needs the most help starting.
    3. C. The largest motor's table full-load current, once. The other motors go in at their table currents without another 125 percent.Keyed answer
    4. D. None of them. Feeder motors are added at nameplate current with no increase.

    Article 430 increases the feeder for the largest motor and adds the other motors at their table full-load currents. Putting 125 percent on all of them overstates the feeder. Confirm the feeder rule in NEC 2026.

    Open NEC 2026 Article 430

15. 15 · Motors · Calculations

    Two motors share one feeder. The table full-load currents are 22 A and 14 A. For this drill the feeder conductor ampacity is 125% of the largest motor plus 100% of the other motor. What minimum ampacity do you need?

    1. A. 22 A
    2. B. 36 A
    3. C. 41.5 AKeyed answer
    4. D. 45 A

    125% of 22 A is 27.5 A. Add the 14 A motor at 100% and the feeder is 41.5 A. The sum with no extra 25% is 36 A. Taking 125% of both motors gives 45 A. Confirm the several-motor feeder rule in NEC 2026 Article 430.

    Open NEC 2026 430.24

16. 16 · Motors · Calculations

    A motor full-load current is 40 A. This drill requires the motor disconnect ampere rating to be at least 115% of that current and also not less than the branch-circuit overcurrent device, which is 70 A. What minimum ampere rating do you need?

    1. A. 40 A
    2. B. 46 A
    3. C. 70 AKeyed answer
    4. D. 80.5 A

    115% of 40 A is 46 A, but the disconnect also has to be at least the 70 A branch-circuit device. The larger of 46 A and 70 A is 70 A. Taking 115% of the 70 A device instead gives 80.5 A, which this stem did not ask for. Confirm the disconnect rating in NEC 2026 Article 430.

    Open NEC 2026 430.110

17. 17 · Motors · Calculations

    A motor full-load current is 15 A. For this drill, an inverse-time breaker used as the branch short-circuit device may be 250% of that current, and you may go up to the next standard size. Standard ratings you may use: 30, 35, 40, and 45. What is the smallest rating that is at least the calculated value?

    1. A. 35 A
    2. B. 45 A
    3. C. 40 AKeyed answer
    4. D. 30 A

    15 × 2.50 = 37.5 A. The smallest listed rating that is at least 37.5 A is 40 A. A 35 A breaker is under the calculated value. Article 430 is where the permitted percentage is confirmed. The 250% figure is printed in the stem.

    Open NEC 2026 Article 430

18. 18 · Motors · Calculations

    A motor full-load current is 10 A. For this drill, an instantaneous-trip breaker used as the branch short-circuit device may be 800% of that current. What is that calculated value?

    1. A. 100 A
    2. B. 80 AKeyed answer
    3. C. 50 A
    4. D. 40 A

    10 × 8 = 80 A. Instantaneous-trip settings sit well above full-load current so the motor can start, and Article 430 limits how high they may go. The 800% figure is printed in the stem. Confirm the section in NEC 2026.

    Open NEC 2026 Article 430

19. 19 · Motors · Calculations

    Three motors have full-load currents of 30 A, 10 A, and 10 A. For this drill, feeder ampacity is 125% of the largest plus the other two at 100%. What do you need?

    1. A. 70 A
    2. B. 57.5 AKeyed answer
    3. C. 62.5 A
    4. D. 50 A

    30 × 1.25 = 37.5 A. The other two motors add 20 A. The total is 57.5 A. Applying 125% to all three is the larger number. Article 430 is the feeder method.

    Open NEC 2026 Article 430

20. 20 · Motors · Calculations

    A shop feeder has motor full-load currents of 32 A, 18 A, and 12 A, plus a continuous heater of 8 A. This drill takes 125% of the largest motor, 100% of the other motors, and 125% of the heater. What minimum feeder ampacity do you need?

    1. A. 80 AKeyed answer
    2. B. 70 A
    3. C. 87.5 A
    4. D. 100 A

    125% of 32 A is 40 A. The other motors add 30 A. The heater is 8 × 1.25 = 10 A. Total 80 A. Leaving every load at 100% gives 70 A. Article 430 is the motor share. The heater's 125% is the continuous-load rule and is printed in the stem.

    Open NEC 2026 Article 430