Motors & Generators — Texas Journeyman Practice Questions

10 questions · 9% of the NEC Knowledge exam

Worked questions

1. A required motor disconnect is hidden around a corner and cannot be seen from the motor. What must be checked?

AOnly conductor color
BWhether the START button is nearby
COnly motor horsepower
The applicable within-sight rule or a permitted lockable exception

Why D is correct

Local disconnect rules protect service personnel from unexpected energization. Visibility, distance, lockability, and the specific exception control; a control button is not normally the required disconnecting means.

2. A running three-phase motor loses one phase. What condition is likely?

Severe current imbalance and overheating
BHigher insulation resistance with no load effect
CBalanced current at lower temperature
DAutomatic creation of a neutral

Why A is correct

Single phasing can leave the motor turning while the remaining phases carry damaging current and torque falls. Proper overload and phase protection plus prompt diagnosis limit winding damage.

3. A three-phase motor has seriously unbalanced currents. What should be checked before increasing overload settings?

ARaise settings until tripping stops
Voltage, load, and motor condition
CRemove one phase
DDisconnect equipment grounding

Why B is correct

Imbalance may indicate voltage imbalance, a high-resistance connection, mechanical load trouble, or winding damage. Raising protection masks the symptom and can permit destructive overheating.

4. What hazard is a motor overload device primarily intended to address?

ALoss of equipment grounding continuity
BAll bolted short circuits by itself
Sustained overcurrent and overheating
DLow building water pressure

Why C is correct

Overload protection responds to sustained excessive motor current that can overheat windings. Branch short-circuit and ground-fault protection has a separate role and can be sized under different motor rules.

5. Why can motor branch-circuit conductor sizing use a table full-load current rather than the motor nameplate current?

ATable current is always lower
BMotor conductors do not need overcurrent protection
CNameplates are never used for any motor purpose
Use NEC table current where specified

Why D is correct

Motor rules deliberately use different current values for different tasks. Table full-load current commonly sizes branch conductors and short-circuit protection, while nameplate current is used for overload protection; the applicable article must be followed.

6. Does a motor overload relay replace branch-circuit short-circuit and ground-fault protection?

AYes, when the motor is three-phase
BNo, because overload relays protect feeders only for the motor circuit described in the question
CYes, for every motor circuit
No; the two devices protect against different conditions

Why D is correct

Overload devices protect motors and conductors from sustained overload, while branch short-circuit and ground-fault devices address high-current faults. A complete motor circuit generally needs both functions.

7. What rating must a motor controller have for the motor it controls?

A horsepower rating suitable for the connected motor
BA voltage rating below the circuit
COnly a matching enclosure color
DOnly the same conductor temperature for the motor circuit described in the question

Why A is correct

A controller must be suitable for the motor horsepower, voltage, current, duty, and application. General ampere rating alone may not establish motor switching suitability.

8. Where is a motor disconnect generally required relative to the motor and driven machinery?

AOnly at the service equipment for the motor circuit described in the question
Within sight from the motor location
CInside the utility vault
DAt any unmarked remote panel

Why B is correct

The motor disconnect is generally within sight from the motor and driven machinery, with limited exceptions involving lockable disconnects and qualified servicing conditions.

9. Which current is generally used to size motor branch-circuit conductors for a typical motor covered by the NEC tables?

AThe overload relay trip current for the motor circuit described in the question
BThe utility service demand
The applicable table full-load current
DThe measured no-load current

Why C is correct

Motor branch-circuit conductor and short-circuit protection calculations generally use the applicable NEC table full-load current. Nameplate current is used for specified items such as overload protection.

10. Which current is generally used when selecting motor overload protection?

AThe feeder conductor ampacity
BThe transformer primary current
CThe panelboard bus rating
The motor nameplate current

Why D is correct

Motor overload protection responds to sustained motor overload and is generally based on nameplate current and service-factor or temperature-rise conditions, not the table value used for branch conductors.

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Motors & Generators — Texas Journeyman Practice Questions

Worked questions

1. A required motor disconnect is hidden around a corner and cannot be seen from the motor. What must be checked?

2. A running three-phase motor loses one phase. What condition is likely?

3. A three-phase motor has seriously unbalanced currents. What should be checked before increasing overload settings?

4. What hazard is a motor overload device primarily intended to address?

5. Why can motor branch-circuit conductor sizing use a table full-load current rather than the motor nameplate current?

6. Does a motor overload relay replace branch-circuit short-circuit and ground-fault protection?

7. What rating must a motor controller have for the motor it controls?

8. Where is a motor disconnect generally required relative to the motor and driven machinery?

9. Which current is generally used to size motor branch-circuit conductors for a typical motor covered by the NEC tables?

10. Which current is generally used when selecting motor overload protection?

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