Answer:
D. 4
Explanation:
To determine the correct size of conductors for the motor, we need to follow a few steps and consider some key factors, including the current draw of the motor, the insulation type of the wires, and any applicable code requirements.
Step 1: Find the Full Load Current (FLC) of the Motor
First, we need to determine the Full Load Current (FLC) of the motor. Since the motor's NEMA design code is not provided, we can use the National Electrical Code (NEC) Table 430.250 to find the FLC for a 25-HP, three-phase motor operating at 240 volts. According to the table, a 25-HP motor at 240 volts typically has an FLC of about 74 amperes.
Step 2: Check Insulation and Temperature Ratings
You mentioned the conductors have THWN insulation, but the termination temperature rating of the devices is not known. For THWN, the highest ampacity value allowed by the NEC is based on a temperature rating of 75°C, if the equipment's termination rating is not known.
Step 3: Determine the Correct Wire Size Based on Ampacity
Next, we consult the NEC ampacity tables to find the appropriate wire size that can handle the calculated current, considering the insulation type. We'll look at Table 310.15(B)(16) for THWN insulated conductors, considering a 75°C column since the termination temperature rating is unknown.
For a current of 74 amperes, we find that:
#4 AWG copper wire is rated for up to 85 amperes when used in a 75°C environment.
#6 AWG copper wire is rated for up to 65 amperes, which is not sufficient for our application.
Therefore, #4 AWG copper wire would be the appropriate choice to safely handle the motor’s current under these conditions.
The answer is D. 4
This means you should use #4 AWG copper conductors with THWN insulation for the motor connection. This wire size adequately supports the motor's current requirements while complying with the NEC and ensuring safe operation under continuous duty.
Hope this helps!
