Journeyman/Master Electrician License Exam Motors, HVAC, and Special Equipment Flashcards

24 sample cards from the Motors, HVAC, and Special Equipment deck

Sampled from the end of the deck, so these are different cards from the ones shown on the syllabus page.

1. If air-conditioning equipment is marked with a “maximum overcurrent protective device” (MOCP) rating, how must it be applied?

   The branch-circuit OCPD must not exceed the marked maximum value, even if calculations would otherwise permit a larger device.

2. Per NEC 440.14, where must the disconnecting means for air-conditioning or refrigerating equipment be located?

   Within sight from and readily accessible from the air-conditioning or refrigerating equipment (it may be installed on or within the equipment).

3. What minimum conductor ampacity is required for a single hermetic motor-compressor (NEC 440.32)?

   At least 125% of the RLC or BCSC, whichever is greater: $I_{conductor} \geq 1.25 \times I_{RLC/BCSC}$.

4. How are conductors sized for a combination (multimotor) air-conditioning load per NEC 440.33?

   Ampacity at least 125% of the largest motor-compressor (or motor) load plus the sum of the rated currents of the other loads in the group.

5. What does the nameplate “minimum circuit ampacity” (MCA) on HVAC equipment determine, per NEC 440.35?

   The minimum conductor ampacity required to supply the equipment; conductors are selected so their ampacity is at least the marked MCA.

6. On a packaged HVAC unit nameplate marked with MCA and MOCP, which value sizes the conductors and which sizes the overcurrent device?

   MCA (minimum circuit ampacity) sizes the conductors; MOCP (maximum overcurrent protection) sets the maximum breaker/fuse rating.

7. For multimotor HVAC equipment, what current value is used for each motor-compressor when computing the combination conductor and protection sizes?

   The rated-load current or branch-circuit selection current (whichever is greater) of each motor-compressor, plus the full-load currents of any other associated motors/loads.

8. For a transformer over 600 V is excluded — for a transformer 600 V or less protected on the primary only with a primary current of $9$ A or more, what is the maximum primary OCPD percentage (Table 450.3(B))?

   125% of the rated primary current (with the next-higher-standard-size rule permitted when 125% does not correspond to a standard rating).

9. Per Table 450.3(B), what primary-only OCPD percentages apply when primary current is less than $2$ A and when it is $2$ A to less than $9$ A?

   Less than $2$ A: up to 300%; $2$ A to less than $9$ A: up to 167% of rated primary current.

10. For a 600 V-or-less transformer with both primary and secondary protection, what are the maximum percentages when the secondary current is $9$ A or more (Table 450.3(B))?

    Primary OCPD up to 250% of rated primary current and secondary OCPD up to 125% of rated secondary current.

11. What does the “next higher standard rating” note for Table 450.3(B) allow?

    When the calculated percentage (e.g., 125%) of rated current does not correspond to a standard OCPD rating, the next higher standard rating is permitted.

12. Does NEC 450.3 transformer overcurrent protection also protect the secondary conductors?

    No. Section 450.3 protects the transformer itself; the secondary (and primary) conductors require separate overcurrent/tap protection per 240.21 and 240.4.

13. What is the formula for the full-load primary current of a single-phase transformer?

    $I = \dfrac{kVA \times 1000}{V}$, where $V$ is the primary line voltage.

14. What is the formula for the line current of a three-phase transformer?

    $I = \dfrac{kVA \times 1000}{\sqrt{3} \times V_{line}}$.

15. Calculate the primary full-load current of a 45 kVA, 480 V, three-phase transformer.

    $I = \dfrac{45000}{\sqrt{3} \times 480} \approx 54.1$ A.

16. Calculate the secondary full-load current of a 45 kVA, three-phase transformer with a 208 V secondary.

    $I = \dfrac{45000}{\sqrt{3} \times 208} \approx 124.9$ A (about 125 A).

17. Which NEC provisions govern sizing and protecting transformer secondary conductors, given that primary-side protection may not protect them?

    The transformer secondary conductor tap rules of NEC 240.21(C); primary OCPD generally does not protect secondary conductors (except a 2-wire single-phase or balanced 3-wire arrangement).

18. What minimum conductor ampacity is required for the conductors supplying a capacitor (NEC 460.8(A))?

    At least 135% of the rated current of the capacitor: $I_{conductor} \geq 1.35 \times I_{cap}$.

19. How must the overcurrent device for a capacitor be rated per NEC 460.8(B)?

    As low as practicable — an individual overcurrent device is required for each capacitor bank, except where the capacitor is connected on the load side of the motor running overcurrent (overload) device.

20. What is the minimum ampere rating of a capacitor disconnecting means (NEC 460.8(C))?

    At least 135% of the rated current of the capacitor.

21. When a capacitor is connected on the load side of a motor's running overload device, what are the conductor requirements (NEC 460.8(A))?

    A separate overcurrent device is not required, but the capacitor circuit conductors must have an ampacity at least 135% of the capacitor rated current and at least one-third the ampacity of the motor circuit conductors.

22. Give the formula for the rated line current of a three-phase capacitor bank rated in kVAR.

    $I_{C} = \dfrac{kVAR \times 1000}{\sqrt{3} \times V_{line}}$.

23. A three-phase capacitor draws a rated current of 18 A. What is the minimum required conductor ampacity?

    $1.35 \times 18 = 24.3$ A minimum conductor ampacity (NEC 460.8(A)).

24. Compare how conductors are sized for a standard motor (Art. 430) versus a hermetic motor-compressor (Art. 440).

    Both use 125% of the governing current, but Article 430 bases it on the table FLC (from hp/voltage), while Article 440 bases it on the nameplate RLC or BCSC, whichever is greater.

What this deck covers

Motors, HVAC, and Special Equipment flashcards FAQ