Motor Cable Gauge Calculator – NEC

Determining the correct motor cable gauge is critical for electrical safety and performance. The NEC provides precise guidelines to ensure proper sizing.

This article explores the NEC motor cable gauge calculation, including formulas, tables, and real-world examples for accurate sizing.

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• Calculate cable gauge for a 15 HP motor at 230V, 3-phase, 60Hz.
• Determine wire size for a 10 HP motor with 480V supply and 75°C insulation.
• Find motor cable gauge for a 25 HP motor running at 460V, 3-phase.
• Calculate conductor size for a 5 HP motor with a 230V single-phase supply.

Comprehensive Tables for Motor Cable Gauge Selection According to NEC

Below are detailed tables correlating motor horsepower, voltage, and NEC recommended cable gauge sizes. These tables consider conductor insulation ratings, ambient temperature, and typical installation conditions.

For 460V motors, the full load current approximately halves, affecting wire gauge selection accordingly.

Essential Formulas for Motor Cable Gauge Calculation According to NEC

Accurate motor cable sizing requires understanding and applying NEC formulas, including adjustments for temperature, voltage drop, and conductor material.

1. Full Load Current (FLC) Calculation

NEC Table 430.250 provides standard FLC values for motors based on horsepower and voltage. However, if unknown, FLC can be estimated by:

• Motor Power (W): Power in watts (HP × 746 W/HP)
• Voltage (V): Line-to-line voltage
• Power Factor: Typically 0.85 for induction motors
• Efficiency: Usually 0.9 to 0.95

2. Conductor Ampacity Selection

According to NEC 430.22, conductors must have an ampacity not less than 125% of the motor full load current:

This ensures the conductor can handle starting currents and thermal stresses.

3. Voltage Drop Calculation

Voltage drop must be limited to 3% for motor feeders to maintain performance. The voltage drop is calculated by:

• Vd: Voltage drop (volts)
• K: Resistivity constant (12.9 for copper, 21.2 for aluminum at 75°C, in ohm-cmil/ft)
• I: Load current (amps)
• L: One-way conductor length (feet)
• CM: Circular mil area of conductor

For three-phase circuits, the formula adjusts to:

4. Circular Mil Area (CM) from AWG

To find CM from AWG size:

Common CM values for standard AWG sizes:

5. Temperature Correction Factor

NEC Table 310.15(B)(16) requires adjusting ampacity for ambient temperature:

Example correction factors for THHN insulation:

Real-World Application Examples of Motor Cable Gauge Calculation

Example 1: Sizing Cable for a 15 HP, 230V, 3-Phase Motor

Determine the appropriate copper conductor size for a 15 HP motor operating at 230V, 3-phase, with a conductor insulation rating of 75°C. The motor is located 150 feet from the power source, and voltage drop must not exceed 3%.

• Step 1: Find Full Load Current (FLC)
  From NEC Table 430.250, a 15 HP, 230V, 3-phase motor has an FLC of 45 A.
• Step 2: Calculate Required Ampacity
  Required Ampacity = 45 A × 1.25 = 56.25 A
• Step 3: Select Wire Gauge Based on Ampacity
  From NEC Table 310.16 (75°C column), 6 AWG copper wire has an ampacity of 65 A, which is sufficient.
• Step 4: Check Voltage Drop
  Use the three-phase voltage drop formula:
  Vd = (√3 × K × I × L) / CM

Where:

• K = 12.9 ohm-cmil/ft (copper)
• I = 45 A (FLC)
• L = 150 ft (one-way length)
• CM for 6 AWG = 26,240

Calculate:

Calculate numerator:

Voltage drop:

Calculate percentage voltage drop:

The voltage drop is within the 3% limit, so 6 AWG copper wire is acceptable.

Example 2: Motor Cable Sizing for a 10 HP, 460V, 3-Phase Motor in a Hot Environment

A 10 HP motor operates at 460V, 3-phase, located 200 feet from the power source. The ambient temperature is 40°C, and the conductor insulation is rated for 75°C. Determine the minimum conductor size.

• Step 1: Find Full Load Current (FLC)
  From NEC Table 430.250, FLC for 10 HP, 460V, 3-phase motor is 16 A.
• Step 2: Calculate Required Ampacity
  Required Ampacity = 16 A × 1.25 = 20 A
• Step 3: Apply Temperature Correction Factor
  At 40°C ambient, correction factor = 0.88 (from NEC Table 310.15(B)(16))
• Step 4: Calculate Base Ampacity Required
  Base Ampacity = Required Ampacity / Correction Factor = 20 A / 0.88 ≈ 22.7 A
• Step 5: Select Wire Gauge
  From NEC Table 310.16 (75°C column), 12 AWG copper wire has ampacity 25 A, which is sufficient.
• Step 6: Check Voltage Drop
  Using three-phase voltage drop formula:
  Vd = (√3 × K × I × L) / CM

Where:

• K = 12.9 ohm-cmil/ft (copper)
• I = 16 A (FLC)
• L = 200 ft
• CM for 12 AWG = 6,530

Calculate:

Calculate numerator:

Voltage drop:

Percentage voltage drop:

The voltage drop is acceptable, confirming 12 AWG copper wire is suitable.

Additional Technical Considerations for Motor Cable Sizing

• Conductor Material: Copper is preferred for its conductivity and durability; aluminum requires larger sizes.
• Insulation Type: THHN, XHHW, and other insulation types affect ampacity and temperature ratings.
• Ambient Temperature: Higher temperatures reduce ampacity; always apply correction factors.
• Conduit Fill: Multiple conductors in conduit require adjustment per NEC 310.15(C).
• Motor Starting Current: Motors draw 5-7 times FLC at startup; conductors and protection devices must accommodate this.
• Grounding Conductors: NEC 250.122 specifies minimum sizes for equipment grounding conductors.
• Voltage Drop Limits: NEC recommends limiting voltage drop to 3% for feeders and branch circuits to ensure motor efficiency.

For more detailed NEC guidelines, consult the official NEC Handbook or visit the National Fire Protection Association website at nfpa.org.

Summary

Proper motor cable gauge calculation per NEC standards ensures safety, efficiency, and longevity of motor installations. Using NEC tables, formulas, and correction factors is essential for accurate sizing.

Always verify calculations with local codes and consider environmental factors for optimal conductor selection.