Free CSLB C-10 Electrical Contractor Exam Practice Test

Sample C-10 Electrical practice questions

Try a few representative questions below. Each includes the correct answer and a short explanation — the same style you'll see in the full practice test.

1. Question 1Planning & Estimating

   You are estimating the cost for a new commercial installation that requires a 480V, 3-phase, 100A feeder supplying a bank of transformers. The feeder conductors will be installed in rigid metal conduit (RMC) with a maximum ambient temperature of 40°C. Based on NEC Table 310.16, what is the minimum size copper conductor required for the ungrounded feeder conductors, assuming standard 75°C terminations and no derating factors other than ambient temperature?

   • A.1 AWG
   • B.1/0 AWG(Correct)
   • C.2 AWG
   • D.1/0 AWG

   Explanation

   According to NEC Table 310.16, for 100A at 75°C, a 1/0 AWG copper conductor has an allowable ampacity of 150A. This is the smallest conductor with an ampacity equal to or greater than the 100A feeder rating. Using 1 AWG (135A) would be undersized. 2 AWG (125A) is also undersized. The question specifies 75°C termination, aligning with the typical rating for transformer lugs, and the ambient temperature of 40°C does not require derating for this conductor size as its base ampacity is significantly higher than the 100A load. For example, when installing a 100A feeder to a step-down transformer for machinery, selecting 1/0 AWG ensures proper operation and code compliance.

2. Question 2NEC & Electrical Theory

   You are installing a 480V, 3-phase, 60Hz motor with a nameplate FLA of 80A and a service factor of 1.25. The motor will operate continuously and is protected by inverse-time circuit breakers. According to NEC Article 430, what is the minimum size copper conductor required for this branch circuit, considering THHN insulation and a maximum ambient temperature of 30°C?

   • A.3 AWG
   • B.1/0 AWG(Correct)
   • C.4 AWG
   • D.1 AWG

   Explanation

   According to NEC Table 310.16, THHN conductors at 75°C have an ampacity of 110A. For continuous loads, the conductors must be sized at 125% of the FLA (80A * 1.25 = 100A). For motors with a service factor of 1.25 or greater, the minimum conductor ampacity is 125% of the FLA, which is 100A. Looking at Table 310.16, 1/0 AWG copper THHN conductors are rated for 150A at 75°C, which satisfies the 100A minimum requirement. 1 AWG is rated for 130A, 3 AWG for 100A, and 4 AWG for 85A. Therefore, 1/0 AWG is the minimum size required. Incorrect options: 1 AWG is insufficient if the 100A requirement dictates a higher ampacity, 3 AWG is exactly 100A but the table often requires a step up due to rounding and specific conditions, and 4 AWG is too small. This is a common calculation for motor feeders on job sites.

3. Question 3NEC & Electrical Theory

   You are installing a 480V, 3-phase, 60Hz motor with a nameplate Full Load Amperes (FLA) of 70A and a Service Factor (SF) of 1.15. The motor will operate continuously and is protected by inverse-time circuit breakers. According to NEC Article 430, what is the minimum size copper conductor required for this motor branch circuit?

   • A.1 AWG(Correct)
   • B.3 AWG
   • C.1/0 AWG
   • D.2 AWG

   Explanation

   According to NEC Table 430.150, the minimum conductor ampacity for a 70A FLA motor is 87.5A (70A * 125%). For continuous duty, conductors must be sized at not less than 125% of the motor FLA. Using NEC Table 310.16, a 1 AWG copper conductor has an ampacity of 130A (assuming THHN, 75°C column, not more than three conductors in a raceway, ambient temperature 30°C), which is sufficient. Option A (2 AWG) is rated for 115A and is too small. Option C (1/0 AWG) is rated for 150A and is oversized. Option D (3 AWG) is rated for 100A and is too small. This ensures the conductors do not overheat under continuous load.

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