Voltage drop in a motor circuit is governed by conductor resistance and current, and sizing the conductors should be based on ensuring the voltage at the motor remains within acceptable limits under full-load conditions. Direct answer

• Use a voltage-drop/conductor-sizing method that accounts for the circuit length, load current, voltage to be delivered, conductor material (typically copper), and allowable voltage drop. The standard approach is to size conductors so that the voltage drop along the run does not exceed a specified percentage (commonly 3% for branch circuits, 5% total including feeders) of the circuit voltage.

Key method (high level)

• For a single-phase motor circuit: approximate conductor sizing uses the formula for voltage drop:
  • CM (circular mils) ≈ (2 × K × I × D) / Vdrop
  • Where:
    • I = load current (A)
    • D = one-way distance (feet or meters, consistent with K units)
    • K = 12.9 for copper (in appropriate units)
    • Vdrop = allowable voltage drop (volts)
  • Then select a conductor with a circular mil area equal to or larger than the calculated CM.
• For three-phase circuits, the factor differs (approximately CM ≈ (1.732 × K × I × D) / Vdrop), and you still compare against standard conductor sizes.

Practical guidance

• Gather these data: motor full-load current (A), one-way length from supply to motor (ft or m), supply voltage (V), and desired maximum voltage drop percentage (typically 3%).
• Look up the conductor resistances or circular-mil areas from NEC-approved tables (or manufacturer data) to confirm the closest standard conductor size that meets the calculated CM.
• Account for NEC considerations beyond voltage drop:
  • The conductors must also handle the motor start-up surge, and you may need to use a larger size to limit inrush impact.
  • Use appropriate overcurrent protection and ensure short-circuit current ratings are adequate.
  • If the motor is on a constant-duty load or has soft-start controls, voltage-drop sensitivity can differ.

Common practice notes

• Shorter runs and higher supply voltages reduce the impact of voltage drop, allowing smaller conductors.
• Copper conductors are preferred for lower resistance; aluminum conductors require larger sizes for the same drop.
• In motor circuits, voltage drop can affect not only operation voltage but control circuits and protection settings, so verify both motor voltage and any control circuits (e.g., starters, VFDs) meet required tolerances.

If you can share the specific values (motor full-load current, run length, supply voltage, and allowed voltage drop), the exact conductor size can be estimated.