CSLB C-10 Electrical ContractorPlanning and EstimatingHard
A commercial client requires a new 208V, 3-phase, 40A feeder for a motor load. The feeder will run 150 feet from the panel to the motor. The conductor insulation type is THHN, and the maximum allowable voltage drop is 3%. What is the minimum AWG copper conductor size required?
- ANo. 6 AWG
- BNo. 4 AWG
- CNo. 8 AWG
- DNo. 2 AWG
Show answer & explanationAnswer & explanation
Correct answer: A. No. 6 AWG
First, calculate the maximum allowable voltage drop. Then, use the voltage drop formula for 3-phase circuits and the K-factor for copper THHN to determine the required conductor circular mil area. Finally, select the smallest AWG conductor with a circular mil area greater than or equal to the calculated value.
Why the other options are wrong
- B. Incorrect. This is oversized, leading to unnecessary material cost.
- C. Incorrect. This size will result in excessive voltage drop.
- D. Incorrect. This is significantly oversized for the application.
Three-Phase Voltage Drop Calculation
The process of determining the appropriate conductor size for a 3-phase circuit to ensure the voltage drop remains within acceptable limits, typically 3% for feeders, as per NEC recommendations.
- Uses a specific formula for 3-phase circuits.
- Requires the K-factor (resistivity) for the conductor material and temperature.
- Aims to prevent excessive voltage drop, which can lead to inefficient operation and equipment damage.
Memory trick: For '3-Phase' voltage drop, 'CALC'ulate with K, I, L, and VD.