For a 3-phase, 125-horsepower, 460-volt motor with terminals rated at 75°C, what is the minimum conductor ampacity and conductor size required?

For a 3-phase, 125-horsepower, 460-volt motor with terminals rated at 75°C, what is the minimum conductor ampacity and conductor size required?

• A. 195 A / 3/0 AWG
• B. 210 A / 2/0 AWG
• C. 180 A / 1 AWG
• D. 205 A / 4/0 AWG

Answer: (A)

Sizing motor conductors uses 125% of the motor’s full-load current, with the 75°C ampacity rating used for the conductor. Start by estimating the motor’s full-load current from its horsepower, voltage, and typical efficiency and power factor: I_FLC ≈ (746 W/hp × 125 hp) / (√3 × 460 V × PF × η). Using PF ≈ 0.85 and η ≈ 0.88 gives I_FLC ≈ 156 A. Multiply by 1.25 to account for the conductor sizing rule: 156 A × 1.25 ≈ 195 A. So the conductors must have an ampacity of at least 195 A. Checking copper conductors at the 75°C rating, the smallest size that meets or exceeds 195 A is 3/0 AWG (about 200 A). Thus, the minimum conductor ampacity is about 195 A, and the appropriate conductor size is 3/0 AWG.

Sizing motor conductors uses 125% of the motor’s full-load current, with the 75°C ampacity rating used for the conductor. Start by estimating the motor’s full-load current from its horsepower, voltage, and typical efficiency and power factor: I_FLC ≈ (746 W/hp × 125 hp) / (√3 × 460 V × PF × η). Using PF ≈ 0.85 and η ≈ 0.88 gives I_FLC ≈ 156 A. Multiply by 1.25 to account for the conductor sizing rule: 156 A × 1.25 ≈ 195 A. So the conductors must have an ampacity of at least 195 A. Checking copper conductors at the 75°C rating, the smallest size that meets or exceeds 195 A is 3/0 AWG (about 200 A). Thus, the minimum conductor ampacity is about 195 A, and the appropriate conductor size is 3/0 AWG.