CSLB C-10 Electrical ContractorPlanning and EstimatingHard
A commercial building renovation requires a new 480V, 3-phase, 100A feeder for a motor control center. The feeder will run 150 feet from the main distribution panel. What is the minimum size of THHN copper conductor required, assuming a maximum 3% voltage drop and considering an ambient temperature of 30°C?
- A3/0 AWG
- B2/0 AWG
- C1 AWG
- D1/0 AWG
Show answer & explanationAnswer & explanation
Correct answer: D. 1/0 AWG
For a 480V, 3-phase, 100A circuit over 150 feet, a 1/0 AWG THHN copper conductor is required to maintain voltage drop within 3%. This accounts for the three-phase calculation and conductor ampacity.
Why the other options are wrong
- A. 3/0 AWG copper is significantly oversized, resulting in unnecessary expense and installation difficulty.
- B. 2/0 AWG copper is larger than necessary, leading to increased material costs.
- C. 1 AWG copper will exceed the 3% voltage drop limit for this circuit.
Three-Phase Voltage Drop
Calculating voltage drop in three-phase systems requires using a specific formula that accounts for the three-phase nature of the power, ensuring stable voltage delivery.
- The three-phase voltage drop formula includes the square root of 3 (√3).
- K-factor for copper is 12.9, for aluminum is 21.2.
- Conductor ampacity must also be checked against NEC Table 310.16.
Memory trick: Three phases, three steps: Ampacity, Voltage Drop, and then you're set!