You've highlighted a common point of confusion when dealing with motors and VFDs. Let's break it down

Yes, your calculation is correct for the conductors from the overload relays to the motors based on 430.22 and 430.
250.
You correctly identified the 75 HP motor FLA from Table 430.250 (96 amps). You correctly applied the 125% factor (96 amps 1.25 = 120 amps). You correctly determined that 120 amps requires a 1 AWG conductor (based on typical ampacity charts like those in the NEC, assuming 75°C insulation).

Why the Discrepancy?

The reason for the difference in wire size between your calculated conductor size (1 AWG) and the motor's factory leads (4 AWG) comes down to where those conductors are installed and what code articles apply. Factory Motor Leads

The 4 AWG leads coming directly from the motor winding termination points

are not subject to NEC 430.22 requirements. They are considered part of the motor's internal construction and are sized by the motor manufacturer to withstand the motor's FLA plus a safety factor. They have a specific thermal rating and construction that allows them to carry that current within the motor's enclosure. Conductors from Overload to Motor

These conductors are subject to NEC 430.22. The code requires you to size these conductors at 125% of the motor's full-load current (FLA) from Table 430.250 or the motor nameplate FLA, whichever is greater. The purpose of this requirement is to provide adequate capacity for starting currents and short-term overloads, and to reduce voltage drop in the feeder circuit, which can damage the motor.

Key Considerations and Explanations1. Code Compliance vs. Engineering Optimization

Motor manufacturers optimize the internal wiring based on specific insulation types, motor cooling characteristics, and duty cycles. The NEC provides a minimum safety standard for external wiring, intended to protect the motor and the overall electrical system.2.

Starting Current

Motors draw significantly higher current during starting (locked rotor current). While the motor's internal leads are designed to handle this briefly, the external conductors need to be sized to accommodate this in addition to the normal running current.3.

Thermal Considerations

The NEC's sizing requirements account for the potential for heat build-up in the conductors. Larger conductors have lower resistance, which reduces heat generation.4.

Voltage Drop

Undersized conductors can cause excessive voltage drop, especially during motor starting. This can reduce motor torque and efficiency, and in extreme cases, damage the motor.5.

Nameplate vs. Table Value

If the motor's nameplate FLA (88.5A) is higher than the value from Table 430.250 (96A), you must use the nameplate value for your calculations. In this specific case, 125% of 88.5 is 110.625A. You could use a #2 AWG if your termination is rated to 75 deg C.

Summary

It is perfectly normal to see a difference in size between the motor's internal leads and the conductors you install from the overload relays to the motor connection box. You are following the correct NEC procedure for sizing the external conductors. Adhering to these codes ensures that your installation meets minimum safety standards and provides adequate performance and protection for the motor.

Important Reminders



Consult the NEC

Always refer to the latest edition of the NEC for the most up-to-date requirements.

Local Codes

Be sure to follow any local electrical codes that may be more stringent than the NEC.

Qualified Electrician

When in doubt, consult with a qualified and licensed electrician. They can assess your specific situation and ensure a safe and code-compliant installation.

VFD Considerations

Since you are using a VFD, ensure that the motor and cable are rated for VFD applications (e.g., inverter-duty motor, shielded cable to minimize harmonic interference). The VFD manual should provide specific cable recommendations.*

Derating: Consider derating the conductor ampacity if the ambient temperature is high, or if multiple conductors are bundled together.By carefully considering these factors, you can ensure that your motor installation is both safe and efficient. Flag for review