CSLB C-10 Electrical ContractorWiring Methods and MaterialsHard
A three-phase, 480V feeder circuit supplies a 75 kVA continuous load located 150 feet from the panel. The conductors are copper THHN, and the voltage drop must not exceed 3%. What is the minimum required conductor size?
- A3/0 AWG
- B1 AWG
- C2/0 AWG
- D1/0 AWG
Show answer & explanationAnswer & explanation
Correct answer: C. 2/0 AWG
First, calculate the continuous load current. Then, apply the 125% rule for continuous loads. Finally, calculate the maximum allowable voltage drop and use the voltage drop formula to determine the minimum conductor size.
Why the other options are wrong
- A. Incorrect; while this size meets the voltage drop requirement, it is larger than necessary, leading to increased material costs.
- B. Incorrect; this size results in a voltage drop exceeding 3%.
- D. Incorrect; this size results in a voltage drop exceeding 3%.
Three-Phase Voltage Drop Calculation
The process of determining the appropriate conductor size for a three-phase circuit to ensure voltage drop remains within acceptable limits, typically 3% for feeders, considering continuous load adjustments.
- Voltage drop calculation is critical for efficient and reliable electrical systems.
- Continuous loads require conductors to be sized at 125% of the load current before applying voltage drop calculations.
- The three-phase voltage drop formula differs from single-phase, using a √3 multiplier.
- NEC 215.2(A)(1)(b) recommends a maximum 3% voltage drop for feeders.
Memory trick: Volt Drop: Continuous Current, Then Size Right, Keep It Tight!