Master Electrician (NEC) Practice Test

Câu hỏi luyện tập mẫu

Một vài câu hỏi thực tế từ ngân hàng miễn phí này, kèm giải thích đầy đủ. Dùng công cụ luyện tập ở trên cho toàn bộ.

1. 1. Calculations

   A single-family dwelling has 2,000 sq ft of habitable floor area. Using the standard calculation method at 3 VA per sq ft, what is the general lighting and general-use receptacle load before any demand factors?

   • a.3,000 VA
   • b.6,000 VA
   • c.4,000 VA
   • d.8,000 VA

   Đáp án: b

   Giải thích: General lighting is figured at 3 VA per sq ft: 2,000 sq ft x 3 VA = 6,000 VA. This value is calculated before applying the general-lighting demand factors.

2. 2. Services & Distribution

   What is the maximum number of service disconnecting means permitted for a single set of service-entrance conductors (each in a separate enclosure)?

   • a.Eight
   • b.Six
   • c.One
   • d.Four

   Đáp án: b

   Giải thích: A service is limited to a maximum of six disconnecting means. In the 2023 NEC each of the up-to-six disconnects must be in a separate enclosure or be part of listed equipment.

   Nguồn: 2023 NEC 230.71

3. 3. Advanced Wiring

   The equipotential bonding grid around a permanently installed pool must use a solid copper conductor no smaller than:

   • a.#4 AWG
   • b.#8 AWG
   • c.#6 AWG
   • d.#10 AWG

   Đáp án: b

   Giải thích: Pool equipotential bonding must use an insulated, covered, or bare solid copper conductor not smaller than #8 AWG.

   Nguồn: 2023 NEC 680.26(B)

4. 4. Theory & Design

   A transformer has a 4:1 turns ratio and a 10-A primary current. What is the secondary current (ideal)?

   • a.2.5 A
   • b.40 A
   • c.10 A
   • d.20 A

   Đáp án: b

   Giải thích: Current is inversely proportional to the turns ratio: Is = Ip x (Np/Ns) = 10 x 4 = 40 A.

5. 5. Calculations

   A 75-kVA transformer with a 208-A secondary (over 9 A) is protected on both the primary and the secondary. What is the maximum secondary overcurrent device (next higher standard permitted)?

   • a.300 A
   • b.260 A
   • c.250 A
   • d.225 A

   Đáp án: a

   Giải thích: Table 450.3(B) allows up to 125% on the secondary where both sides are protected: 208 x 1.25 = 260 A. Since 260 A is not a standard size, Note 1 permits the next higher standard rating, 300 A. Trap: 250 A is the next lower standard and would be too small to carry full secondary load.

   Nguồn: 2023 NEC 450.3(B)

6. 6. Calculations

   A one-family dwelling has 2,500 sq ft of habitable floor area. Using the standard method at 3 VA per sq ft, what is the general lighting and general-use receptacle load before demand factors?

   • a.9,000 VA
   • b.5,000 VA
   • c.6,000 VA
   • d.7,500 VA

   Đáp án: d

   Giải thích: 2023 NEC 220.41 figures dwelling general lighting at 3 VA/sq ft: 2,500 x 3 = 7,500 VA, calculated before the 220.42 demand factors are applied.

7. 7. Calculations

   A box contains five #12 AWG and three #10 AWG conductors and nothing else. Using the box-fill volume allowances, what minimum box volume is required?

   • a.16.5 cu in
   • b.20.0 cu in
   • c.17.25 cu in
   • d.18.75 cu in

   Đáp án: d

   Giải thích: 2023 NEC Table 314.16(B): #12 = 2.25, #10 = 2.5 cu in. (5 x 2.25) + (3 x 2.5) = 11.25 + 7.5 = 18.75 cu in.

8. 8. Services & Distribution

   Where two ground rods are used as the grounding electrode, what is the minimum required spacing between them?

   • a.8 ft
   • b.3 ft
   • c.10 ft
   • d.6 ft

   Đáp án: d

   Giải thích: 2023 NEC 250.53(A)(3): rod, pipe, or plate electrodes that supplement each other must be at least 6 ft apart.

9. 9. Advanced Wiring

   A hospital's essential electrical system for a Type 1 facility is divided into which branches?

   • a.isolated and grounded branches
   • b.life safety, critical, and equipment branches
   • c.emergency and legally required branches
   • d.normal and standby branches only

   Đáp án: b

   Giải thích: 2023 NEC 517.30: the Type 1 essential electrical system consists of the life safety branch, the critical branch, and the equipment branch.

10. 10. Theory & Design

    A transformer has a 10:1 turns ratio and a 5-A primary current. What is the ideal secondary current?

    • a.5 A
    • b.50 A
    • c.15 A
    • d.0.5 A

    Đáp án: b

    Giải thích: Current is inversely proportional to the turns ratio: Is = Ip x (Np/Ns) = 5 x 10 = 50 A.