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Quizzes on the Code

Feb 1, 2001 12:00 PM, By James Stallcup, Jr., NEC and OSHA Consultant

Choose the best answer:

1. What size nonfused disconnect are you required to install for an air-conditioning unit with a compressor rated at 29A and a condenser rated at 2.5A?

(a) 20A (b) 30A (c) 60A (d) 100A

2. What size horsepower-rated disconnect are you required to install for an air-conditioning unit with a compressor rated at 29A and a condenser rated at 2.5A with a locked-rotor current rating (LRA) of 180A? (Note that the supply is 480V, 3-phase.)

(a) 25 hp (b) 30 hp (c) 40 hp (d) 50 hp

3. What size horsepower-rated disconnect are you required to install for the following loads on a 3-phase, 230V system?

• A/C unit with a compressor rated at 29A with a 200A LRA

• A/C unit with a compressor rated at 24A with a 150A LRA

• A/C unit with a compressor rated at 20A with a 140A LRA

(a) 20 hp

(b) 30 hp

(c) 35 hp

(d) 40 hp

4. What size disconnecting means (in horsepower) is required to disconnect the following loads on a 3-phase, 230V system?

• Motor rated at 38A with a 212A LRA

• A/C unit with a compressor rated at 28A with a 160A LRA

• A/C unit with a compressor rated at 24A with a 160A LRA

(a) 15 hp

(b) 20 hp

(c) 30 hp

(d) 35 hp

5. A cord-and-plug device may serve as the disconnecting means for a room air conditioner if it operates at:

(a) 120V or less

(b) 150V or less

(c) 250V or less

(d) 300V or less

Answers and Discussion:

1. (c), 60A. You must size the disconnect based on the full-load current rating of the nameplate or the nameplate branch-circuit selection current of the compressor, whichever is greater. The multiplier is 115%.

[29A + 2.5A] × 115 = 436.2A

36.2A requires a 60A disconnect

Stallcup’s Code Loop: Secs. 440-12(a)(1) and 240-6(a).

2. (a), 25 hp. When working with nameplate amperage ratings or branch-circuit current ratings instead of horsepower ratings, you must select the horsepower amperage rating from Tables 430-147 through 430-150. If the nameplate fails to list the LRA rating of the motor, you must select the horsepower amperage rating from Tables 430-151(A) and (B).

31.5 FLA requires 25 hp

180 LRA requires 25 hp

Stallcup’s Code Loop: Sec. 440-12(a)(2), Tables 430-150 and 430-151(B).

3. (c), 35 hp. Same explanation as in answer 2.

The disconnecting means must be rated at 35 hp.

Stallcup’s Code Loop: Sec. 440-12(b), Tables 430-150 and 430-151(B).

4. (d), 35 hp. When working with two or more hermetic motors or combination loads (such as hermetic motor loads, standard motor loads, and other loads), you must separate your totals to determine the rating of a single disconnecting means.

Therefore, the disconnecting means is required to be rated at 35 hp.

Stallcup’s Code Loop: Sec. 440-12(b)(2), Tables 430-150 and 430-151(B).

5. (c), 250V or less. A cord-and-plug device may serve as the disconnecting means for a room air conditioner if it operates at 250V or less, its controls are manually operated, it’s located within 6 ft of the floor, and it’s accessible to the user.

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