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Quizzes on the Code

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

Choose the best answer:

What is the minimum size overcurrent protection device (OCPD) required for an A/C unit with a compressor rated at 20A and a condenser rated at 2.5A?

(a) 30A
(b) 35A
(c) 40A
(d) 45A
What is the maximum size OCPD allowed for an A/C unit with a compressor rated at 20A and a condenser rated at 2.5A?

(a) 30A
(b) 35A
(c) 40A
(d) 45A
What is the minimum size OCPD required for a 230V, 3-phase feeder-circuit with the following loads:
- A/C unit with a compressor rated at 29A and a condenser rated at 2.5A
- A/C unit with a compressor rated at 26A and a condenser rated at 2.5A
- A/C unit with a compressor rated at 22A and a condenser rated at 2.5A
(a) 100A
(b) 110A
(c) 125A
(d) 150A
What is the maximum size OCPD required for a 230V, 3-phase feeder-circuit with the following loads:
- A/C unit with a compressor rated at 29A and a condenser rated at 2.5A
- A/C unit with a compressor rated at 26A and a condenser rated at 2.5A
- A/C unit with a compressor rated at 22A and a condenser rated at 2.5A
(a) 100A
(b) 110A
(c) 125A
(d) 150A
What size OCPD is required for a 230V, 3-phase feeder-circuit with the following loads:
- A/C unit with a compressor rated at 26A and the condenser rated at 2.5A
- A/C unit with a compressor rated at 24A and the condenser rated at 2.5A
- 10 hp, 230V, three-phase, Design letter B motor
(a) 100A
(b) 110A
(c) 125A
(d) 150A

answers and discussion

(b), 35A. When calculating the minimum size OCPD for hermetic seal compressors, use a factor of 175% of the compressor FLA rating or branch-circuit selection circuit current — whichever is greater.
Step 1: 20A x 175%+ 2.5A = 37.5A
Solution: 37.5A requires you to use a 35A OCPD.
Stallcup's Code Loop: Secs. 240-3(g), 240-6(a), and 440-22(a).
(d), 45A. When calculating the maximum size OCPD for hermetic seal compressors, use a factor of 225% of the compressor FLA rating or branch-circuit selection circuit current — whichever is greater.
Step 1: 20A x 225%+ 2.5A = 47.5A
Solution: 47.5A allows you to use a 45A OCPD.
Stallcup's Code Loop: Secs. 240-3(g), 240-6(a), and 440-22(a).
(a), 100A. The NEC requires you to size the OCPD for a feeder-circuit supplying two or more air-conditioning units to allow the largest unit to start at one time and the other units to start at different intervals. If there are two or more hermetically sealed motors installed on the same feeder-circuit, you must size them at 175% of the full-load current rating of the nameplate or branch-circuit selection current rating of the largest motor — whichever is greater.
Step 1: 29A x 175%+ 2.5A = 53.25A
Step 2: 53.25A + 26A + 2.5A + 22A + 2.5A = 106.25A.
Solution: 106.25A requires a 100A OCPD.
Stallcup's Code Loop: Secs. 240-3(g), 240-6(a), 440-22(a), and 440-22(b)(1).
(b), 110A. You may size the OCPD up to 225% of the motor-rated load current or branch-circuit selection current for hermetically sealed motors to allow the motor to start. You typically do this when the motor will not start and develop speed when applying a lower rating, which is normally selected at 175% or less.
Step 1: 29A x 225%+ 2.5A = 67.75A
Step 2: 67.75A + 26A + 2.5A + 22A + 2.5A = 120.75A
Solution: 120.75A requires a 110A OCPD.
Stallcup's Code Loop: Secs. 240-3(g), 240-6(a), 440-22(a), and 440-22(b)(1).
(c), 125A. When installing hermetically sealed motors and other loads (such as motors) on the same circuit, and the largest load in the group is a motor, you must size the OCPD based on the percentages shown in Table 430-152.
Step 1: 28A x 250% = 70A
Step 2: 70A+ 26A + 2.5A + 24A + 2.5A = 125A
Solution: 125A requires you to use a 125A OCPD.
Stallcup's Code Loop: Secs. 240-3(g), 240-6(a), 440-22(a), 440-22(b)(1), 430-52(c)(1), and Tables 430-150 and 430-152.

Note: A smaller OCPD can be used than the minimum calculated size of the unit.

Stallcup's Code Loops

Each quiz on the 1999 National Electrical Code is provided with Stallcup's Code Loops to show the direct correlation with other sections of the NEC. This allows the reader to review other sections of the NEC when determining if the correct answer to a problem is in one or multiple sections of the Code.