Feeder Size Calculator

The Feeder Size Calculator helps electricians, engineers, and contractors determine the correct wire size for electrical feeders based on load demand, system voltage, and power factor. It ensures compliance with national and international wiring codes, reduces energy losses, and enhances electrical safety.

As a core tool in the Electrical Engineering and Load Planning Calculators category, this calculator simplifies the process of selecting the appropriate wire gauge or cross-sectional area (mm²), especially for long-distance or high-load circuits.

formula of Feeder Size Calculator

Step 1: Calculate Feeder Current

Feeder Current (I) = Load Power / (Voltage × Power Factor)

Where:

• I = current (amperes)
• Load Power = total power in watts (W) or volt-amperes (VA)
• Voltage = supply voltage (in volts)
• Power Factor = unitless decimal (e.g., 0.9)

Step 2: Determine Feeder Size

Feeder Size (AWG or mm²) is selected based on:

• Calculated current
• Ambient temperature
• Conduit fill and correction factors
• Distance (for voltage drop)

Use NEC Table 310.16 (US) or IEC 60287 (International) to match wire ampacity with calculated current. Consider voltage drop for distances over 100 ft (30 m).

Voltage Drop Rule of Thumb:
Voltage Drop (%) = (2 × Length × Current × Resistance per unit length) / Voltage × 100
Try to keep it under 3% for feeders.

Reference Table: Common Feeder Sizes and Ampacity Ratings (Copper, 75°C insulation)

Values vary with insulation type and ambient temperature.

Example

Let’s say:

• Load Power = 15,000 W
• Voltage = 240 V
• Power Factor = 0.9

Step 1:
I = 15,000 / (240 × 0.9) = 15,000 / 216 = 69.44 A

Step 2:
A 4 AWG copper wire (ampacity 85A) would work. But if the feeder length exceeds 100 feet, voltage drop should be considered, and 3 AWG or 2 AWG may be required to maintain efficiency and code compliance.

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