What minimum size conductors are required to connect a 75 hp motor to a 400-amp feeder with 600 kcmil copper wire?

To determine the minimum size conductors required to connect a 75 hp motor to a 400-amp feeder, one needs to consult the National Electrical Code (NEC) standards that guide conductor sizing based on the ampacity required for the application.

In this scenario, a 75 hp motor generally has a full-load current that can be calculated using the formula: Full Load Amperes (FLA) = (Horsepower × 746) / (Voltage × Efficiency × Power Factor). For a typical motor voltage of 480V, the FLA of a 75 hp motor is approximately 90 amps (though conditions such as efficiency and power factor can affect this slightly).

Once the FLA is known, it’s essential to consider the feeder's role and any potential voltage drop, especially over longer distances. The conductors must be sized adequately to handle both the operational load and any additional load due to starting inrush currents that motors experience.

According to NEC tables, for copper conductors, 1/0 AWG is adequate for carrying currents well over 100 amps safely, and in most installations, it provides an appropriate safety margin to handle up to around 150 amps, depending on the application and conditions.

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