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Journeyman Exam Prep Set 3: Motors and Control Circuits

Use this set as a timed drill: answer first, then check the key and explanation.

Questions

1. Which NEC article primarily covers motors, motor circuits, and controllers?
2. Are branch-circuit short-circuit and ground-fault devices for motors sized the same as overload devices?
3. What is a common starting point for motor full-load current values used in calculations?
4. Why is overload protection required for motors?
5. Can motor branch-circuit conductors be sized at 125% of motor FLC in many cases?
6. What is the role of a disconnecting means in a motor circuit?
7. Why are control circuit transformer protections important?
8. What happens if motor inrush is ignored when selecting OCPD type?
9. What field check reduces motor callback rates after install?
10. What documentation helps with motor inspection questions?

Answer Key + Explanations

1) Article 430.

Article 430 includes conductor sizing, overload protection, short-circuit protection, and controller rules.

2) No.

Overload protection and short-circuit/ground-fault protection serve different purposes and are sized by different rules.

3) NEC motor FLC tables.

Table values are used for many sizing calculations rather than nameplate current alone.

4) To protect motor windings from overheating due to overcurrent over time.

Overloads respond to sustained overcurrent that may not trip short-circuit protection devices quickly.

5) Yes.

NEC Article 430 commonly applies a 125% multiplier for branch-circuit conductor sizing.

6) To provide a means to safely isolate equipment for servicing.

Disconnect location and rating must meet NEC and equipment requirements.

7) They protect smaller control conductors/components from fault and overload conditions.

Undersized control wiring can fail quickly without properly coordinated protection.

8) Nuisance tripping or startup failures may occur.

Time-delay fuses/breaker characteristics may be required to accommodate inrush while preserving protection.

9) Verify rotation, voltage balance, and load current against expected values.

Commissioning checks catch phase/connection issues before turnover.

10) Conductor/OCPD calculations, equipment data sheets, and startup test records.

Traceable records justify selections and speed acceptance.

References

• NFPA 70, National Electrical Code (NEC), current adopted edition in your jurisdiction.
• NFPA 70E, Standard for Electrical Safety in the Workplace.
• OSHA 29 CFR 1910 Subpart S and 1926 Subpart K (as applicable).
• Local AHJ amendments and utility service requirements.

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Related internal guide

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