CME Electrical Systems Design & Installation 2 — Questions and Answers
Question 1: When sizing a feeder for a 480V, 3-phase motor load totaling 200A, what minimum ampacity must the feeder conductors have per NEC 215.2?
- 200A
- 210A
- 220A
- 250A (Correct answer)
Correct answer: 250A
NEC 215.2 requires feeders supplying motor loads to be sized at 125% of the largest motor's FLC plus 100% of all other loads, often resulting in 125% of total current.
Question 2: A commercial building requires a 400A, 208Y/120V, 3-phase service. Per NEC 230.79, what is the minimum service entrance conductor ampacity required?
- 300A
- 350A
- 400A (Correct answer)
- 450A
Correct answer: 400A
NEC 230.79(D) requires service entrance conductors to have an ampacity not less than the load to be served, so 400A service requires minimum 400A conductors.
Question 3: In a high-rise building, a 277V fluorescent lighting branch circuit serves continuous loads. What is the maximum load allowed on a 20A breaker per NEC 210.20?
- 16A (Correct answer)
- 20A
- 24A
- 25A
Correct answer: 16A
NEC 210.20(A) limits continuous loads on a branch circuit to 80% of the overcurrent device rating, so a 20A breaker allows maximum 16A continuous load.
Question 4: What NEC table is used to determine the maximum number of current-carrying conductors permitted in a conduit before derating is required?
- Table 310.15(B)(3)(a) (Correct answer)
- Table 310.16
- Table 314.16(A)
- Table 250.66
Correct answer: Table 310.15(B)(3)(a)
NEC Table 310.15(B)(3)(a) provides adjustment factors for more than three current-carrying conductors in a raceway, requiring derating when exceeded.
Question 5: A 3-phase, 480V system has an available fault current of 65,000A at the service entrance. What must all downstream equipment be rated for?
- 10,000A AIC minimum
- 22,000A AIC minimum
- At least equal to the available fault current (Correct answer)
- 50% of available fault current
Correct answer: At least equal to the available fault current
NEC 110.9 and 110.10 require all equipment to have an interrupting rating sufficient for the available fault current at the point of installation.
Question 6: When designing a hospital essential electrical system, which NEC article governs the branch circuit and feeder requirements?
- Article 517 (Correct answer)
- Article 700
- Article 701
- Article 702
Correct answer: Article 517
NEC Article 517 covers health care facilities including essential electrical systems, emergency systems, and critical branch requirements for hospitals.
Question 7: A 200A residential service uses 4/0 AWG copper conductors. What is the minimum burial depth required for a direct-buried service lateral per NEC Table 300.5?
- 12 inches
- 18 inches
- 24 inches (Correct answer)
- 36 inches
Correct answer: 24 inches
NEC Table 300.5 requires direct-buried service laterals (not in conduit) to be buried a minimum of 24 inches under general conditions.
CME Electrical Systems Design & Installation 2
When sizing a feeder for a 480V, 3-phase motor load totaling 200A, what minimum ampacity must the feeder conductors have per NEC 215.2?