CSLB General Building (B) Trade 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. Branch Circuits & Feeders

   A 208-volt, single-phase branch circuit supplies a continuous load of 24 amperes. What is the minimum standard overcurrent device rating permitted?

   • a.30 amperes
   • b.25 amperes
   • c.24 amperes
   • d.40 amperes

   Đáp án: a

   Giải thích: An overcurrent device supplying a continuous load must be rated at not less than 125 percent of that load: 24 A x 1.25 = 30 A, which is a standard rating in 240.6(A). The 24 A and 25 A choices ignore or under-apply the 125 percent factor, and 40 A jumps past the next standard size above the calculated value.

   Nguồn: 2023 NEC §210.20(A)

2. 2. Branch Circuits & Feeders

   A feeder supplies a 100-ampere continuous load. Using 75 degrees C terminations and copper THWN, what is the minimum feeder conductor size?

   • a.1 AWG
   • b.2 AWG
   • c.1/0 AWG
   • d.3 AWG

   Đáp án: a

   Giải thích: The feeder conductor must have an ampacity of at least 100 x 1.25 = 125 A. In the 75 degrees C column, 2 AWG copper is 115 A and falls short, while 1 AWG copper is 130 A and satisfies the requirement. 1/0 AWG at 150 A is larger than necessary, and 3 AWG at 100 A does not cover the continuous-load multiplier.

   Nguồn: 2023 NEC §215.2(A)(1)

3. 3. Branch Circuits & Feeders

   Using the dwelling service and feeder conductor table, what is the minimum copper conductor size for a 200-ampere single-phase dwelling service?

   • a.1/0 AWG
   • b.3/0 AWG
   • c.4/0 AWG
   • d.2/0 AWG

   Đáp án: d

   Giải thích: For single-phase dwelling services and the main power feeder, conductors are permitted to be sized at 83 percent of the service rating, and the dwelling table lists 2/0 copper for a 200-ampere service. The 3/0 answer is what Table 310.16 alone would require without the dwelling allowance, and 1/0 is below what the dwelling table permits.

   Nguồn: 2023 NEC §310.12

4. 4. Services, Grounding & Bonding

   What is the minimum size copper equipment grounding conductor for a 400-ampere feeder?

   • a.4 AWG
   • b.3 AWG
   • c.2 AWG
   • d.1 AWG

   Đáp án: b

   Giải thích: Table 250.122 lists 3 AWG copper at 400 amperes. 4 AWG is the 300-ampere value, 2 AWG the 500-ampere value and 1 AWG the 600-ampere value, so all three are one row away from the correct entry.

   Nguồn: 2023 NEC Table 250.122

5. 5. Services, Grounding & Bonding

   A concrete-encased electrode may be formed by at least 20 feet of which of the following?

   • a.Bare 4 AWG copper conductor encased in the concrete footing
   • b.Bare 6 AWG copper conductor encased in the concrete footing
   • c.Insulated 4 AWG copper conductor run through the footing
   • d.Bare 8 AWG copper conductor encased in the concrete footing

   Đáp án: a

   Giải thích: A concrete-encased electrode consists of at least 20 feet of bare copper not smaller than 4 AWG, or at least 20 feet of one-half inch or larger electrically conductive reinforcing bar, encased in at least 2 inches of concrete near the bottom of a footing in direct contact with earth. Smaller conductors do not qualify, and the copper must be bare so it can couple to the concrete.

   Nguồn: 2023 NEC §250.52(A)(3)

6. 6. Wiring Methods & Materials

   What is the minimum trade size of EMT required for nine 12 AWG THHN conductors?

   • a.One-half inch
   • b.Three-quarter inch
   • c.One inch
   • d.One and one-quarter inch

   Đáp án: a

   Giải thích: Annex C, Table C1 shows that one-half inch EMT accepts nine 12 AWG THHN conductors, exactly meeting the requirement. Three-quarter inch EMT holds sixteen and one inch holds twenty-six, so both are larger than the minimum. Annex C may be used directly whenever all conductors are the same size and insulation type.

   Nguồn: 2023 NEC Annex C, Table C1

7. 7. Wiring Methods & Materials

   What is the maximum support spacing for a 1-inch trade size rigid PVC conduit?

   • a.6 feet
   • b.5 feet
   • c.3 feet
   • d.10 feet

   Đáp án: c

   Giải thích: PVC expands and sags with temperature, so support spacing depends on trade size: one-half through 1 inch is supported every 3 feet, 1 and 1/4 through 2 inch every 5 feet, and larger sizes at progressively longer intervals. The 10-foot spacing belongs to EMT and rigid metal conduit, which are far stiffer.

   Nguồn: 2023 NEC §352.30(B)

8. 8. Motors & Equipment

   That same motor has a nameplate full-load current of 32 amperes and a marked service factor of 1.15. What is the maximum separate overload device rating?

   • a.36.8 amperes
   • b.40 amperes
   • c.42.5 amperes
   • d.32 amperes

   Đáp án: b

   Giải thích: Overload protection is based on the nameplate current, not the table value, and a service factor of 1.15 or greater permits 125 percent: 32 x 1.25 = 40 A. The 36.8-ampere answer applies the 115 percent factor used for motors with no marked service factor or temperature rise, and 42.5 amperes wrongly applies 125 percent to the table value of 34 A.

   Nguồn: 2023 NEC §430.32(A)(1)

9. 9. Motors & Equipment

   A hermetic refrigerant motor-compressor has a rated-load current of 20 amperes. What is the maximum branch-circuit short-circuit and ground-fault protective device at the standard 175 percent figure?

   • a.60 amperes
   • b.25 amperes
   • c.45 amperes
   • d.35 amperes

   Đáp án: d

   Giải thích: Protection for a motor-compressor is sized at not more than 175 percent of the rated-load current: 20 x 1.75 = 35 A, which is a standard rating. Where that device still will not carry the starting current it may be increased, but never above 225 percent, which here would be 45 amperes. A 25-ampere device is below the calculated value and would nuisance trip on start.

   Nguồn: 2023 NEC §440.22(A)

10. 10. Theory & Safety

    Three resistors of 10, 15 and 25 ohms are connected in series across 120 volts. What is the circuit current?

    • a.24 amperes
    • b.2.4 amperes
    • c.4.8 amperes
    • d.0.42 ampere

    Đáp án: b

    Giải thích: Series resistances add: 10 + 15 + 25 = 50 ohms, so I = 120 / 50 = 2.4 amperes. The 24-ampere answer misplaces the decimal, 4.8 amperes would result from a 25-ohm total, and 0.42 ampere inverts the division.